![viii
Type of Inheritance .............................................................................................................................192
Multiple Inheritance ...........................................................................................................................193
25. OVERLOADING (OPERATOR & FUNCTION).......................................................................196
Function Overloading in C++ ...............................................................................................................196
Operators Overloading in C++ .............................................................................................................197
Overloadable/Non-overloadable Operators........................................................................................200
Operator Overloading Examples .........................................................................................................201
Unary Operators Overloading .............................................................................................................201
Increment (++) and Decrement (- -) Operators ....................................................................................203
Binary Operators Overloading.............................................................................................................205
Relational Operators Overloading.......................................................................................................208
Input/Output Operators Overloading..................................................................................................210
++ and - - Operators Overloading ........................................................................................................212
Assignment Operators Overloading ....................................................................................................214
Function Call () Operator Overloading.................................................................................................215
Subscripting [ ] Operator Overloading.................................................................................................217
Class Member Access Operator - > Overloading ..................................................................................219
26. POLYMORPHISM..............................................................................................................223
Virtual Function ..................................................................................................................................226
Pure Virtual Functions.........................................................................................................................226
27. DATA ABSTRACTION........................................................................................................227
Access Labels Enforce Abstraction.......................................................................................................228
Benefits of Data Abstraction ...............................................................................................................228
Data Abstraction Example...................................................................................................................228
Designing Strategy ..............................................................................................................................230
28. DATA ENCAPSULATION....................................................................................................231](https://image.slidesharecdn.com/c-250109035803-902aa064/85/c-pdf-programming-fundamentals-and-basics-if-IT-9-320.jpg)
![C++
10
??= #
??/
??' ^
??( [
??) ]
??! |
??< {
??> }
??- ~
All the compilers do not support trigraphs and they are not advised to be used
because of their confusing nature.
Whitespace in C++
A line containing only whitespace, possibly with a comment, is known as a blank
line, and C++ compiler totally ignores it.
Whitespace is the term used in C++ to describe blanks, tabs, newline characters
and comments. Whitespace separates one part of a statement from another and
enables the compiler to identify where one element in a statement, such as int,
ends and the next element begins. Statement 1:
int age;
In the above statement there must be at least one whitespace character (usually
a space) between int and age for the compiler to be able to distinguish them.
Statement 2:
fruit = apples + oranges; // Get the total fruit
In the above statement 2, no whitespace characters are necessary between fruit
and =, or between = and apples, although you are free to include some if you
wish for readability purpose.](https://image.slidesharecdn.com/c-250109035803-902aa064/85/c-pdf-programming-fundamentals-and-basics-if-IT-22-320.jpg)
![C++
48
Condition ? X : Y Conditional operator (?). If Condition is true then it
returns value of X otherwise returns value of Y.
, Comma operator causes a sequence of operations to
be performed. The value of the entire comma
expression is the value of the last expression of the
comma-separated list.
. (dot) and -> (arrow) Member operators are used to reference individual
members of classes, structures, and unions.
Cast Casting operators convert one data type to another.
For example, int(2.2000) would return 2.
& Pointer operator ‘&’ returns the address of a variable.
For example &a; will give actual address of the
variable.
* Pointer operator * is pointer to a variable. For example
*var; will pointer to a variable var.
Operators Precedence in C++
Operator precedence determines the grouping of terms in an expression. This
affects how an expression is evaluated. Certain operators have higher
precedence than others; for example, the multiplication operator has higher
precedence than the addition operator:
For example x = 7 + 3 * 2; here, x is assigned 13, not 20 because operator *
has higher precedence than +, so it first gets multiplied with 3*2 and then adds
into 7.
Here, operators with the highest precedence appear at the top of the table,
those with the lowest appear at the bottom. Within an expression, higher
precedence operators will be evaluated first.
Category Operator Associativity
Postfix () [] -> . ++ - - Left to right
Unary + - ! ~ ++ - - (type)* & sizeof Right to left
Multiplicative * / % Left to right](https://image.slidesharecdn.com/c-250109035803-902aa064/85/c-pdf-programming-fundamentals-and-basics-if-IT-60-320.jpg)
![C++
98
C++ provides a data structure, the array, which stores a fixed-size sequential
collection of elements of the same type. An array is used to store a collection of
data, but it is often more useful to think of an array as a collection of variables
of the same type.
Instead of declaring individual variables, such as number0, number1, ..., and
number99, you declare one array variable such as numbers and use
numbers[0], numbers[1], and ..., numbers[99] to represent individual variables.
A specific element in an array is accessed by an index.
All arrays consist of contiguous memory locations. The lowest address
corresponds to the first element and the highest address to the last element.
Declaring Arrays
To declare an array in C++, the programmer specifies the type of the elements
and the number of elements required by an array as follows:
type arrayName [ arraySize ];
This is called a single-dimension array. The arraySize must be an integer
constant greater than zero and type can be any valid C++ data type. For
example, to declare a 10-element array called balance of type double, use this
statement:
double balance[10];
Initializing Arrays
You can initialize C++ array elements either one by one or using a single
statement as follows:
double balance[5] = {1000.0, 2.0, 3.4, 17.0, 50.0};
The number of values between braces { } cannot be larger than the number of
elements that we declare for the array between square brackets [ ]. Following is
an example to assign a single element of the array:
If you omit the size of the array, an array just big enough to hold the
initialization is created. Therefore, if you write:
double balance[] = {1000.0, 2.0, 3.4, 17.0, 50.0};
You will create exactly the same array as you did in the previous example.
16. ARRAYS](https://image.slidesharecdn.com/c-250109035803-902aa064/85/c-pdf-programming-fundamentals-and-basics-if-IT-110-320.jpg)
![C++
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balance[4] = 50.0;
The above statement assigns element number 5th in the array a value of 50.0.
Array with 4th index will be 5th, i.e., last element because all arrays have 0 as
the index of their first element which is also called base index. Following is the
pictorial representation of the same array we discussed above:
Accessing Array Elements
An element is accessed by indexing the array name. This is done by placing the
index of the element within square brackets after the name of the array. For
example:
double salary = balance[9];
The above statement will take 10th element from the array and assign the value
to salary variable. Following is an example, which will use all the above-
mentioned three concepts viz. declaration, assignment and accessing arrays:
#include <iostream>
using namespace std;
#include <iomanip>
using std::setw;
int main ()
{
int n[ 10 ]; // n is an array of 10 integers
// initialize elements of array n to 0
for ( int i = 0; i < 10; i++ )
{
n[ i ] = i + 100; // set element at location i to i + 100
}
cout << "Element" << setw( 13 ) << "Value" << endl;
// output each array element's value](https://image.slidesharecdn.com/c-250109035803-902aa064/85/c-pdf-programming-fundamentals-and-basics-if-IT-111-320.jpg)
![C++
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for ( int j = 0; j < 10; j++ )
{
cout << setw( 7 )<< j << setw( 13 ) << n[ j ] << endl;
}
return 0;
}
This program makes use of setw() function to format the output. When the
above code is compiled and executed, it produces the following result:
Element Value
0 100
1 101
2 102
3 103
4 104
5 105
6 106
7 107
8 108
9 109
Arrays in C++
Arrays are important to C++ and should need lots of more detail. There are
following few important concepts, which should be clear to a C++ programmer:
Concept Description
Multi-dimensional arrays C++ supports multidimensional arrays. The
simplest form of the multidimensional array is
the two-dimensional array.
Pointer to an array You can generate a pointer to the first
element of an array by simply specifying the
array name, without any index.
Passing arrays to functions You can pass to the function a pointer to an](https://image.slidesharecdn.com/c-250109035803-902aa064/85/c-pdf-programming-fundamentals-and-basics-if-IT-112-320.jpg)
![C++
101
array by specifying the array's name without
an index.
Return array from functions C++ allows a function to return an array.
Multi-dimensional Arrays
C++ allows multidimensional arrays. Here is the general form of a
multidimensional array declaration:
type name[size1][size2]...[sizeN];
For example, the following declaration creates a three dimensional 5 . 10 . 4
integer array:
int threedim[5][10][4];
Two-Dimensional Arrays
The simplest form of the multidimensional array is the two-dimensional array. A
two-dimensional array is, in essence, a list of one-dimensional arrays. To declare
a two-dimensional integer array of size x,y, you would write something as
follows:
type arrayName [ x ][ y ];
Where type can be any valid C++ data type and arrayName will be a valid C++
identifier.
A two-dimensional array can be think as a table, which will have x number of
rows and y number of columns. A 2-dimensional array a, which contains three
rows and four columns can be shown as below:
Thus, every element in array a is identified by an element name of the form a[ i
][ j ], where a is the name of the array, and i and j are the subscripts that
uniquely identify each element in a.
Initializing Two-Dimensional Arrays](https://image.slidesharecdn.com/c-250109035803-902aa064/85/c-pdf-programming-fundamentals-and-basics-if-IT-113-320.jpg)
![C++
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Multidimensioned arrays may be initialized by specifying bracketed values for
each row. Following is an array with 3 rows and each row have 4 columns.
int a[3][4] = {
{0, 1, 2, 3} , /* initializers for row indexed by 0 */
{4, 5, 6, 7} , /* initializers for row indexed by 1 */
{8, 9, 10, 11} /* initializers for row indexed by 2 */
};
The nested braces, which indicate the intended row, are optional. The following
initialization is equivalent to previous example:
int a[3][4] = {0,1,2,3,4,5,6,7,8,9,10,11};
Accessing Two-Dimensional Array Elements
An element in 2-dimensional array is accessed by using the subscripts, i.e., row
index and column index of the array. For example:
int val = a[2][3];
The above statement will take 4th element from the 3rd row of the array. You
can verify it in the above digram.
#include <iostream>
using namespace std;
int main ()
{
// an array with 5 rows and 2 columns.
int a[5][2] = { {0,0}, {1,2}, {2,4}, {3,6},{4,8}};
// output each array element's value
for ( int i = 0; i < 5; i++ )
for ( int j = 0; j < 2; j++ )
{
cout << "a[" << i << "][" << j << "]: ";
cout << a[i][j]<< endl;
}](https://image.slidesharecdn.com/c-250109035803-902aa064/85/c-pdf-programming-fundamentals-and-basics-if-IT-114-320.jpg)
![C++
103
return 0;
}
When the above code is compiled and executed, it produces the following result:
a[0][0]: 0
a[0][1]: 0
a[1][0]: 1
a[1][1]: 2
a[2][0]: 2
a[2][1]: 4
a[3][0]: 3
a[3][1]: 6
a[4][0]: 4
a[4][1]: 8
As explained above, you can have arrays with any number of dimensions,
although it is likely that most of the arrays you create will be of one or two
dimensions.
Pointer to an Array
It is most likely that you would not understand this chapter until you go through
the chapter related C++ Pointers.
So assuming you have bit understanding on pointers in C++, let us start: An
array name is a constant pointer to the first element of the array. Therefore, in
the declaration:
double balance[50];
balance is a pointer to &balance[0], which is the address of the first element of
the array balance. Thus, the following program fragment assigns p the address
of the first element ofbalance:
double *p;
double balance[10];
p = balance;
It is legal to use array names as constant pointers, and vice versa. Therefore,
*(balance + 4) is a legitimate way of accessing the data at balance[4].](https://image.slidesharecdn.com/c-250109035803-902aa064/85/c-pdf-programming-fundamentals-and-basics-if-IT-115-320.jpg)
![C++
104
Once you store the address of first element in p, you can access array elements
using *p, *(p+1), *(p+2) and so on. Below is the example to show all the
concepts discussed above:
#include <iostream>
using namespace std;
int main ()
{
// an array with 5 elements.
double balance[5] = {1000.0, 2.0, 3.4, 17.0, 50.0};
double *p;
p = balance;
// output each array element's value
cout << "Array values using pointer " << endl;
for ( int i = 0; i < 5; i++ )
{
cout << "*(p + " << i << ") : ";
cout << *(p + i) << endl;
}
cout << "Array values using balance as address " << endl;
for ( int i = 0; i < 5; i++ )
{
cout << "*(balance + " << i << ") : ";
cout << *(balance + i) << endl;
}
return 0;
}
When the above code is compiled and executed, it produces the following result:
Array values using pointer
*(p + 0) : 1000](https://image.slidesharecdn.com/c-250109035803-902aa064/85/c-pdf-programming-fundamentals-and-basics-if-IT-116-320.jpg)
![C++
105
*(p + 1) : 2
*(p + 2) : 3.4
*(p + 3) : 17
*(p + 4) : 50
Array values using balance as address
*(balance + 0) : 1000
*(balance + 1) : 2
*(balance + 2) : 3.4
*(balance + 3) : 17
*(balance + 4) : 50
In the above example, p is a pointer to double which means it can store address
of a variable of double type. Once we have address in p, then *p will give us
value available at the address stored in p, as we have shown in the above
example.
Passing Arrays to Functions
C++ does not allow to pass an entire array as an argument to a function.
However, You can pass a pointer to an array by specifying the array's name
without an index.
If you want to pass a single-dimension array as an argument in a function, you
would have to declare function formal parameter in one of following three ways
and all three declaration methods produce similar results because each tells the
compiler that an integer pointer is going to be received.
Way-1
Formal parameters as a pointer as follows:
void myFunction(int *param)
{
.
.
.
}
Way-2
Formal parameters as a sized array as follows:
void myFunction(int param[10])](https://image.slidesharecdn.com/c-250109035803-902aa064/85/c-pdf-programming-fundamentals-and-basics-if-IT-117-320.jpg)
![C++
106
{
.
.
.
}
Way-3
Formal parameters as an unsized array as follows:
void myFunction(int param[])
{
.
.
.
}
Now, consider the following function, which will take an array as an argument
along with another argument and based on the passed arguments, it will return
average of the numbers passed through the array as follows:
double getAverage(int arr[], int size)
{
int i, sum = 0;
double avg;
for (i = 0; i < size; ++i)
{
sum += arr[i];
}
avg = double(sum) / size;
return avg;
}
Now, let us call the above function as follows:
#include <iostream>](https://image.slidesharecdn.com/c-250109035803-902aa064/85/c-pdf-programming-fundamentals-and-basics-if-IT-118-320.jpg)
![C++
107
using namespace std;
// function declaration:
double getAverage(int arr[], int size);
int main ()
{
// an int array with 5 elements.
int balance[5] = {1000, 2, 3, 17, 50};
double avg;
// pass pointer to the array as an argument.
avg = getAverage( balance, 5 ) ;
// output the returned value
cout << "Average value is: " << avg << endl;
return 0;
}
When the above code is compiled together and executed, it produces the
following result:
Average value is: 214.4
As you can see, the length of the array doesn't matter as far as the function is
concerned because C++ performs no bounds checking for the formal
parameters.
Return Array from Functions
C++ does not allow to return an entire array as an argument to a function.
However, you can return a pointer to an array by specifying the array's name
without an index.
If you want to return a single-dimension array from a function, you would have
to declare a function returning a pointer as in the following example:
int * myFunction()
{
.](https://image.slidesharecdn.com/c-250109035803-902aa064/85/c-pdf-programming-fundamentals-and-basics-if-IT-119-320.jpg)
![C++
108
.
.
}
Second point to remember is that C++ does not advocate to return the address
of a local variable to outside of the function so you would have to define the
local variable as staticvariable.
Now, consider the following function, which will generate 10 random numbers
and return them using an array and call this function as follows:
#include <iostream>
#include <ctime>
using namespace std;
// function to generate and retrun random numbers.
int * getRandom( )
{
static int r[10];
// set the seed
srand( (unsigned)time( NULL ) );
for (int i = 0; i < 10; ++i)
{
r[i] = rand();
cout << r[i] << endl;
}
return r;
}
// main function to call above defined function.
int main ()
{
// a pointer to an int.
int *p;](https://image.slidesharecdn.com/c-250109035803-902aa064/85/c-pdf-programming-fundamentals-and-basics-if-IT-120-320.jpg)
![C++
111
C++ provides following two types of string representations:
The C-style character string.
The string class type introduced with Standard C++.
The C-Style Character String
The C-style character string originated within the C language and continues to
be supported within C++. This string is actually a one-dimensional array of
characters which is terminated by a null character '0'. Thus a null-terminated
string contains the characters that comprise the string followed by a null.
The following declaration and initialization create a string consisting of the word
"Hello". To hold the null character at the end of the array, the size of the
character array containing the string is one more than the number of characters
in the word "Hello."
char greeting[6] = {'H', 'e', 'l', 'l', 'o', '0'};
If you follow the rule of array initialization, then you can write the above
statement as follows:
char greeting[] = "Hello";
Following is the memory presentation of above defined string in C/C++:
Actually, you do not place the null character at the end of a string constant. The
C++ compiler automatically places the '0' at the end of the string when it
initializes the array. Let us try to print above-mentioned string:
#include <iostream>
using namespace std;
17. STRINGS](https://image.slidesharecdn.com/c-250109035803-902aa064/85/c-pdf-programming-fundamentals-and-basics-if-IT-123-320.jpg)
![C++
112
int main ()
{
char greeting[6] = {'H', 'e', 'l', 'l', 'o', '0'};
cout << "Greeting message: ";
cout << greeting << endl;
return 0;
}
When the above code is compiled and executed, it produces the following result:
Greeting message: Hello
C++ supports a wide range of functions that manipulate null-terminated strings:
S.N. Function & Purpose
1 strcpy(s1, s2);
Copies string s2 into string s1.
2 strcat(s1, s2);
Concatenates string s2 onto the end of string s1.
3 strlen(s1);
Returns the length of string s1.
4 strcmp(s1, s2);
Returns 0 if s1 and s2 are the same; less than 0 if s1<s2; greater than
0 if s1>s2.
5 strchr(s1, ch);
Returns a pointer to the first occurrence of character ch in string s1.
6 strstr(s1, s2);](https://image.slidesharecdn.com/c-250109035803-902aa064/85/c-pdf-programming-fundamentals-and-basics-if-IT-124-320.jpg)
![C++
113
Returns a pointer to the first occurrence of string s2 in string s1.
Following example makes use of few of the above-mentioned functions:
#include <iostream>
#include <cstring>
using namespace std;
int main ()
{
char str1[10] = "Hello";
char str2[10] = "World";
char str3[10];
int len ;
// copy str1 into str3
strcpy( str3, str1);
cout << "strcpy( str3, str1) : " << str3 << endl;
// concatenates str1 and str2
strcat( str1, str2);
cout << "strcat( str1, str2): " << str1 << endl;
// total lenghth of str1 after concatenation
len = strlen(str1);
cout << "strlen(str1) : " << len << endl;
return 0;
}
When the above code is compiled and executed, it produces result something as
follows:
strcpy( str3, str1) : Hello](https://image.slidesharecdn.com/c-250109035803-902aa064/85/c-pdf-programming-fundamentals-and-basics-if-IT-125-320.jpg)
![C++
116
C++ pointers are easy and fun to learn. Some C++ tasks are performed more
easily with pointers, and other C++ tasks, such as dynamic memory allocation,
cannot be performed without them.
As you know every variable is a memory location and every memory location
has its address defined which can be accessed using ampersand (&) operator
which denotes an address in memory. Consider the following which will print the
address of the variables defined:
#include <iostream>
using namespace std;
int main ()
{
int var1;
char var2[10];
cout << "Address of var1 variable: ";
cout << &var1 << endl;
cout << "Address of var2 variable: ";
cout << &var2 << endl;
return 0;
}
When the above code is compiled and executed, it produces the following result:
Address of var1 variable: 0xbfebd5c0
Address of var2 variable: 0xbfebd5b6
What are Pointers?
A pointer is a variable whose value is the address of another variable. Like any
variable or constant, you must declare a pointer before you can work with it. The
general form of a pointer variable declaration is:
18. POINTERS](https://image.slidesharecdn.com/c-250109035803-902aa064/85/c-pdf-programming-fundamentals-and-basics-if-IT-128-320.jpg)
![C++
120
convention, if a pointer contains the null (zero) value, it is assumed to point to
nothing.
To check for a null pointer you can use an if statement as follows:
if(ptr) // succeeds if p is not null
if(!ptr) // succeeds if p is null
Thus, if all unused pointers are given the null value and you avoid the use of a
null pointer, you can avoid the accidental misuse of an uninitialized pointer.
Many times, uninitialized variables hold some junk values and it becomes
difficult to debug the program.
Pointer Arithmetic
As you understood pointer is an address which is a numeric value; therefore,
you can perform arithmetic operations on a pointer just as you can a numeric
value. There are four arithmetic operators that can be used on pointers: ++, --,
+, and -
To understand pointer arithmetic, let us consider that ptr is an integer pointer
which points to the address 1000. Assuming 32-bit integers, let us perform the
following arithmatic operation on the pointer:
ptr++
the ptr will point to the location 1004 because each time ptr is incremented, it
will point to the next integer. This operation will move the pointer to next
memory location without impacting actual value at the memory location. If ptr
points to a character whose address is 1000, then above operation will point to
the location 1001 because next character will be available at 1001.
Incrementing a Pointer
We prefer using a pointer in our program instead of an array because the
variable pointer can be incremented, unlike the array name which cannot be
incremented because it is a constant pointer. The following program increments
the variable pointer to access each succeeding element of the array:
#include <iostream>
using namespace std;
const int MAX = 3;
int main ()
{
int var[MAX] = {10, 100, 200};](https://image.slidesharecdn.com/c-250109035803-902aa064/85/c-pdf-programming-fundamentals-and-basics-if-IT-132-320.jpg)
![C++
121
int *ptr;
// let us have array address in pointer.
ptr = var;
for (int i = 0; i < MAX; i++)
{
cout << "Address of var[" << i << "] = ";
cout << ptr << endl;
cout << "Value of var[" << i << "] = ";
cout << *ptr << endl;
// point to the next location
ptr++;
}
return 0;
}
When the above code is compiled and executed, it produces result something as
follows:
Address of var[0] = 0xbfa088b0
Value of var[0] = 10
Address of var[1] = 0xbfa088b4
Value of var[1] = 100
Address of var[2] = 0xbfa088b8
Value of var[2] = 200
Decrementing a Pointer
The same considerations apply to decrementing a pointer, which decreases its
value by the number of bytes of its data type as shown below:
#include <iostream>
using namespace std;
const int MAX = 3;](https://image.slidesharecdn.com/c-250109035803-902aa064/85/c-pdf-programming-fundamentals-and-basics-if-IT-133-320.jpg)
![C++
122
int main ()
{
int var[MAX] = {10, 100, 200};
int *ptr;
// let us have address of the last element in pointer.
ptr = &var[MAX-1];
for (int i = MAX; i > 0; i--)
{
cout << "Address of var[" << i << "] = ";
cout << ptr << endl;
cout << "Value of var[" << i << "] = ";
cout << *ptr << endl;
// point to the previous location
ptr--;
}
return 0;
}
When the above code is compiled and executed, it produces result something as
follows:
Address of var[3] = 0xbfdb70f8
Value of var[3] = 200
Address of var[2] = 0xbfdb70f4
Value of var[2] = 100
Address of var[1] = 0xbfdb70f0
Value of var[1] = 10
Pointer Comparisons
Pointers may be compared by using relational operators, such as ==, <, and >.
If p1 and p2 point to variables that are related to each other, such as elements
of the same array, then p1 and p2 can be meaningfully compared.](https://image.slidesharecdn.com/c-250109035803-902aa064/85/c-pdf-programming-fundamentals-and-basics-if-IT-134-320.jpg)
![C++
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The following program modifies the previous example one by incrementing the
variable pointer so long as the address to which it points is either less than or
equal to the address of the last element of the array, which is &var[MAX - 1]:
#include <iostream>
using namespace std;
const int MAX = 3;
int main ()
{
int var[MAX] = {10, 100, 200};
int *ptr;
// let us have address of the first element in pointer.
ptr = var;
int i = 0;
while ( ptr <= &var[MAX - 1] )
{
cout << "Address of var[" << i << "] = ";
cout << ptr << endl;
cout << "Value of var[" << i << "] = ";
cout << *ptr << endl;
// point to the previous location
ptr++;
i++;
}
return 0;
}
When the above code is compiled and executed, it produces result something as
follows:
Address of var[0] = 0xbfce42d0
Value of var[0] = 10](https://image.slidesharecdn.com/c-250109035803-902aa064/85/c-pdf-programming-fundamentals-and-basics-if-IT-135-320.jpg)
![C++
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Address of var[1] = 0xbfce42d4
Value of var[1] = 100
Address of var[2] = 0xbfce42d8
Value of var[2] = 200
Pointers vs Arrays
Pointers and arrays are strongly related. In fact, pointers and arrays are
interchangeable in many cases. For example, a pointer that points to the
beginning of an array can access that array by using either pointer arithmetic or
array-style indexing. Consider the following program:
#include <iostream>
using namespace std;
const int MAX = 3;
int main ()
{
int var[MAX] = {10, 100, 200};
int *ptr;
// let us have array address in pointer.
ptr = var;
for (int i = 0; i < MAX; i++)
{
cout << "Address of var[" << i << "] = ";
cout << ptr << endl;
cout << "Value of var[" << i << "] = ";
cout << *ptr << endl;
// point to the next location
ptr++;
}
return 0;
}](https://image.slidesharecdn.com/c-250109035803-902aa064/85/c-pdf-programming-fundamentals-and-basics-if-IT-136-320.jpg)
![C++
125
When the above code is compiled and executed, it produces result something as
follows:
Address of var[0] = 0xbfa088b0
Value of var[0] = 10
Address of var[1] = 0xbfa088b4
Value of var[1] = 100
Address of var[2] = 0xbfa088b8
Value of var[2] = 200
However, pointers and arrays are not completely interchangeable. For example,
consider the following program:
#include <iostream>
using namespace std;
const int MAX = 3;
int main ()
{
int var[MAX] = {10, 100, 200};
for (int i = 0; i < MAX; i++)
{
*var = i; // This is a correct syntax
var++; // This is incorrect.
}
return 0;
}
It is perfectly acceptable to apply the pointer operator * to var but it is illegal to
modify var value. The reason for this is that var is a constant that points to the
beginning of an array and can not be used as l-value.
Because an array name generates a pointer constant, it can still be used in
pointer-style expressions, as long as it is not modified. For example, the
following is a valid statement that assigns var[2] the value 500:
*(var + 2) = 500;](https://image.slidesharecdn.com/c-250109035803-902aa064/85/c-pdf-programming-fundamentals-and-basics-if-IT-137-320.jpg)
![C++
126
Above statement is valid and will compile successfully because var is not
changed.
Array of Pointers
Before we understand the concept of array of pointers, let us consider the
following example, which makes use of an array of 3 integers:
#include <iostream>
using namespace std;
const int MAX = 3;
int main ()
{
int var[MAX] = {10, 100, 200};
for (int i = 0; i < MAX; i++)
{
cout << "Value of var[" << i << "] = ";
cout << var[i] << endl;
}
return 0;
}
When the above code is compiled and executed, it produces the following result:
Value of var[0] = 10
Value of var[1] = 100
Value of var[2] = 200
There may be a situation, when we want to maintain an array, which can store
pointers to an int or char or any other data type available. Following is the
declaration of an array of pointers to an integer:
int *ptr[MAX];
This declares ptr as an array of MAX integer pointers. Thus, each element in ptr,
now holds a pointer to an int value. Following example makes use of three
integers which will be stored in an array of pointers as follows:
#include <iostream>](https://image.slidesharecdn.com/c-250109035803-902aa064/85/c-pdf-programming-fundamentals-and-basics-if-IT-138-320.jpg)
![C++
127
using namespace std;
const int MAX = 3;
int main ()
{
int var[MAX] = {10, 100, 200};
int *ptr[MAX];
for (int i = 0; i < MAX; i++)
{
ptr[i] = &var[i]; // assign the address of integer.
}
for (int i = 0; i < MAX; i++)
{
cout << "Value of var[" << i << "] = ";
cout << *ptr[i] << endl;
}
return 0;
}
When the above code is compiled and executed, it produces the following result:
Value of var[0] = 10
Value of var[1] = 100
Value of var[2] = 200
You can also use an array of pointers to character to store a list of strings as
follows:
#include <iostream>
using namespace std;
const int MAX = 4;
int main ()
{](https://image.slidesharecdn.com/c-250109035803-902aa064/85/c-pdf-programming-fundamentals-and-basics-if-IT-139-320.jpg)
![C++
128
char *names[MAX] = {
"Zara Ali",
"Hina Ali",
"Nuha Ali",
"Sara Ali",
};
for (int i = 0; i < MAX; i++)
{
cout << "Value of names[" << i << "] = ";
cout << names[i] << endl;
}
return 0;
}
When the above code is compiled and executed, it produces the following result:
Value of names[0] = Zara Ali
Value of names[1] = Hina Ali
Value of names[2] = Nuha Ali
Value of names[3] = Sara Ali
Pointer to a Pointer
A pointer to a pointer is a form of multiple indirection or a chain of pointers.
Normally, a pointer contains the address of a variable. When we define a pointer
to a pointer, the first pointer contains the address of the second pointer, which
points to the location that contains the actual value as shown below.
A variable that is a pointer to a pointer must be declared as such. This is done
by placing an additional asterisk in front of its name. For example, following is
the declaration to declare a pointer to a pointer of type int:
int **var;](https://image.slidesharecdn.com/c-250109035803-902aa064/85/c-pdf-programming-fundamentals-and-basics-if-IT-140-320.jpg)
![C++
131
The function which can accept a pointer, can also accept an array as shown in
the following example:
#include <iostream>
using namespace std;
// function declaration:
double getAverage(int *arr, int size);
int main ()
{
// an int array with 5 elements.
int balance[5] = {1000, 2, 3, 17, 50};
double avg;
// pass pointer to the array as an argument.
avg = getAverage( balance, 5 ) ;
// output the returned value
cout << "Average value is: " << avg << endl;
return 0;
}
double getAverage(int *arr, int size)
{
int i, sum = 0;
double avg;
for (i = 0; i < size; ++i)
{
sum += arr[i];
}
avg = double(sum) / size;](https://image.slidesharecdn.com/c-250109035803-902aa064/85/c-pdf-programming-fundamentals-and-basics-if-IT-143-320.jpg)
![C++
132
return avg;
}
When the above code is compiled together and executed, it produces the
following result:
Average value is: 214.4
Return Pointer from Functions
As we have seen in last chapter how C++ allows to return an array from a
function, similar way C++ allows you to return a pointer from a function. To do
so, you would have to declare a function returning a pointer as in the following
example:
int * myFunction()
{
.
.
.
}
Second point to remember is that, it is not good idea to return the address of a
local variable to outside of the function, so you would have to define the local
variable as staticvariable.
Now, consider the following function, which will generate 10 random numbers
and return them using an array name which represents a pointer i.e., address of
first array element.
#include <iostream>
#include <ctime>
using namespace std;
// function to generate and retrun random numbers.
int * getRandom( )
{
static int r[10];](https://image.slidesharecdn.com/c-250109035803-902aa064/85/c-pdf-programming-fundamentals-and-basics-if-IT-144-320.jpg)
![C++
133
// set the seed
srand( (unsigned)time( NULL ) );
for (int i = 0; i < 10; ++i)
{
r[i] = rand();
cout << r[i] << endl;
}
return r;
}
// main function to call above defined function.
int main ()
{
// a pointer to an int.
int *p;
p = getRandom();
for ( int i = 0; i < 10; i++ )
{
cout << "*(p + " << i << ") : ";
cout << *(p + i) << endl;
}
return 0;
}
When the above code is compiled together and executed, it produces result
something as follows:
624723190
1468735695
807113585
976495677](https://image.slidesharecdn.com/c-250109035803-902aa064/85/c-pdf-programming-fundamentals-and-basics-if-IT-145-320.jpg)
![C++
138
int temp;
temp = x; /* save the value at address x */
x = y; /* put y into x */
y = temp; /* put x into y */
return;
}
When the above code is compiled and executed, it produces the following result:
Before swap, value of a :100
Before swap, value of b :200
After swap, value of a :200
After swap, value of b :100
Reference as Return Value
A C++ program can be made easier to read and maintain by using references
rather than pointers. A C++ function can return a reference in a similar way as it
returns a pointer.
When a function returns a reference, it returns an implicit pointer to its return
value. This way, a function can be used on the left side of an assignment
statement. For example, consider this simple program:
#include <iostream>
#include <ctime>
using namespace std;
double vals[] = {10.1, 12.6, 33.1, 24.1, 50.0};
double& setValues( int i )
{
return vals[i]; // return a reference to the ith element
}
// main function to call above defined function.
int main ()](https://image.slidesharecdn.com/c-250109035803-902aa064/85/c-pdf-programming-fundamentals-and-basics-if-IT-150-320.jpg)
![C++
139
{
cout << "Value before change" << endl;
for ( int i = 0; i < 5; i++ )
{
cout << "vals[" << i << "] = ";
cout << vals[i] << endl;
}
setValues(1) = 20.23; // change 2nd element
setValues(3) = 70.8; // change 4th element
cout << "Value after change" << endl;
for ( int i = 0; i < 5; i++ )
{
cout << "vals[" << i << "] = ";
cout << vals[i] << endl;
}
return 0;
}
When the above code is compiled together and executed, it produces the
following result:
Value before change
vals[0] = 10.1
vals[1] = 12.6
vals[2] = 33.1
vals[3] = 24.1
vals[4] = 50
Value after change
vals[0] = 10.1
vals[1] = 20.23
vals[2] = 33.1
vals[3] = 70.8](https://image.slidesharecdn.com/c-250109035803-902aa064/85/c-pdf-programming-fundamentals-and-basics-if-IT-151-320.jpg)
![C++
140
vals[4] = 50
When returning a reference, be careful that the object being referred to does not
go out of scope. So it is not legal to return a reference to local var. But you can
always return a reference on a static variable.
int& func() {
int q;
//! return q; // Compile time error
static int x;
return x; // Safe, x lives outside this scope
}](https://image.slidesharecdn.com/c-250109035803-902aa064/85/c-pdf-programming-fundamentals-and-basics-if-IT-152-320.jpg)
![C++
146
using namespace std;
int main( )
{
char str[] = "Hello C++";
cout << "Value of str is : " << str << endl;
}
When the above code is compiled and executed, it produces the following result:
Value of str is : Hello C++
The C++ compiler also determines the data type of variable to be output and
selects the appropriate stream insertion operator to display the value. The <<
operator is overloaded to output data items of built-in types integer, float,
double, strings and pointer values.
The insertion operator << may be used more than once in a single statement as
shown above and endl is used to add a new-line at the end of the line.
The Standard Input Stream (cin)
The predefined object cin is an instance of istream class. The cin object is said
to be attached to the standard input device, which usually is the keyboard.
The cin is used in conjunction with the stream extraction operator, which is
written as >> which are two greater than signs as shown in the following
example.
#include <iostream>
using namespace std;
int main( )
{
char name[50];
cout << "Please enter your name: ";
cin >> name;
cout << "Your name is: " << name << endl;](https://image.slidesharecdn.com/c-250109035803-902aa064/85/c-pdf-programming-fundamentals-and-basics-if-IT-158-320.jpg)
![C++
147
}
When the above code is compiled and executed, it will prompt you to enter a
name. You enter a value and then hit enter to see the following result:
Please enter your name: cplusplus
Your name is: cplusplus
The C++ compiler also determines the data type of the entered value and
selects the appropriate stream extraction operator to extract the value and store
it in the given variables.
The stream extraction operator >> may be used more than once in a single
statement. To request more than one datum you can use the following:
cin >> name >> age;
This will be equivalent to the following two statements:
cin >> name;
cin >> age;
The Standard Error Stream (cerr)
The predefined object cerr is an instance of ostream class. The cerr object is
said to be attached to the standard error device, which is also a display screen
but the object cerr is un-buffered and each stream insertion to cerr causes its
output to appear immediately.
The cerr is also used in conjunction with the stream insertion operator as shown
in the following example.
#include <iostream>
using namespace std;
int main( )
{
char str[] = "Unable to read....";
cerr << "Error message : " << str << endl;
}
When the above code is compiled and executed, it produces the following result:](https://image.slidesharecdn.com/c-250109035803-902aa064/85/c-pdf-programming-fundamentals-and-basics-if-IT-159-320.jpg)
![C++
148
Error message : Unable to read....
The Standard Log Stream (clog)
The predefined object clog is an instance of ostream class. The clog object is
said to be attached to the standard error device, which is also a display screen
but the object clog is buffered. This means that each insertion to clog could
cause its output to be held in a buffer until the buffer is filled or until the buffer
is flushed.
The clog is also used in conjunction with the stream insertion operator as shown
in the following example.
#include <iostream>
using namespace std;
int main( )
{
char str[] = "Unable to read....";
clog << "Error message : " << str << endl;
}
When the above code is compiled and executed, it produces the following result:
Error message : Unable to read....
You would not be able to see any difference in cout, cerr and clog with these
small examples, but while writing and executing big programs the difference
becomes obvious. So it is good practice to display error messages using cerr
stream and while displaying other log messages then clog should be used.](https://image.slidesharecdn.com/c-250109035803-902aa064/85/c-pdf-programming-fundamentals-and-basics-if-IT-160-320.jpg)
![C++
149
C/C++ arrays allow you to define variables that combine several data items of
the same kind, but structure is another user defined data type which allows you
to combine data items of different kinds.
Structures are used to represent a record, suppose you want to keep track of
your books in a library. You might want to track the following attributes about
each book:
Title
Author
Subject
Book ID
Defining a Structure
To define a structure, you must use the struct statement. The struct statement
defines a new data type, with more than one member, for your program. The
format of the struct statement is this:
struct [structure tag]
{
member definition;
member definition;
...
member definition;
} [one or more structure variables];
The structure tag is optional and each member definition is a normal variable
definition, such as int i; or float f; or any other valid variable definition. At the
end of the structure's definition, before the final semicolon, you can specify one
or more structure variables but it is optional. Here is the way you would declare
the Book structure:
struct Books
{
char title[50];
char author[50];
char subject[100];
22. DATA STRUCTURES](https://image.slidesharecdn.com/c-250109035803-902aa064/85/c-pdf-programming-fundamentals-and-basics-if-IT-161-320.jpg)
![C++
150
int book_id;
}book;
Accessing Structure Members
To access any member of a structure, we use the member access operator
(.). The member access operator is coded as a period between the structure
variable name and the structure member that we wish to access. You would
use struct keyword to define variables of structure type. Following is the
example to explain usage of structure:
#include <iostream>
#include <cstring>
using namespace std;
struct Books
{
char title[50];
char author[50];
char subject[100];
int book_id;
};
int main( )
{
struct Books Book1; // Declare Book1 of type Book
struct Books Book2; // Declare Book2 of type Book
// book 1 specification
strcpy( Book1.title, "Learn C++ Programming");
strcpy( Book1.author, "Chand Miyan");
strcpy( Book1.subject, "C++ Programming");
Book1.book_id = 6495407;
// book 2 specification
strcpy( Book2.title, "Telecom Billing");](https://image.slidesharecdn.com/c-250109035803-902aa064/85/c-pdf-programming-fundamentals-and-basics-if-IT-162-320.jpg)
![C++
152
using namespace std;
void printBook( struct Books book );
struct Books
{
char title[50];
char author[50];
char subject[100];
int book_id;
};
int main( )
{
struct Books Book1; // Declare Book1 of type Book
struct Books Book2; // Declare Book2 of type Book
// book 1 specification
strcpy( Book1.title, "Learn C++ Programming");
strcpy( Book1.author, "Chand Miyan");
strcpy( Book1.subject, "C++ Programming");
Book1.book_id = 6495407;
// book 2 specification
strcpy( Book2.title, "Telecom Billing");
strcpy( Book2.author, "Yakit Singha");
strcpy( Book2.subject, "Telecom");
Book2.book_id = 6495700;
// Print Book1 info
printBook( Book1 );
// Print Book2 info
printBook( Book2 );](https://image.slidesharecdn.com/c-250109035803-902aa064/85/c-pdf-programming-fundamentals-and-basics-if-IT-164-320.jpg)
![C++
154
#include <iostream>
#include <cstring>
using namespace std;
void printBook( struct Books *book );
struct Books
{
char title[50];
char author[50];
char subject[100];
int book_id;
};
int main( )
{
struct Books Book1; // Declare Book1 of type Book
struct Books Book2; // Declare Book2 of type Book
// Book 1 specification
strcpy( Book1.title, "Learn C++ Programming");
strcpy( Book1.author, "Chand Miyan");
strcpy( Book1.subject, "C++ Programming");
Book1.book_id = 6495407;
// Book 2 specification
strcpy( Book2.title, "Telecom Billing");
strcpy( Book2.author, "Yakit Singha");
strcpy( Book2.subject, "Telecom");
Book2.book_id = 6495700;
// Print Book1 info, passing address of structure
printBook( &Book1 );](https://image.slidesharecdn.com/c-250109035803-902aa064/85/c-pdf-programming-fundamentals-and-basics-if-IT-166-320.jpg)
![C++
155
// Print Book1 info, passing address of structure
printBook( &Book2 );
return 0;
}
// This function accept pointer to structure as parameter.
void printBook( struct Books *book )
{
cout << "Book title : " << book->title <<endl;
cout << "Book author : " << book->author <<endl;
cout << "Book subject : " << book->subject <<endl;
cout << "Book id : " << book->book_id <<endl;
}
When the above code is compiled and executed, it produces the following result:
Book title : Learn C++ Programming
Book author : Chand Miyan
Book subject : C++ Programming
Book id : 6495407
Book title : Telecom Billing
Book author : Yakit Singha
Book subject : Telecom
Book id : 6495700
The typedef Keyword
There is an easier way to define structs or you could "alias" types you create.
For example:
typedef struct
{
char title[50];
char author[50];
char subject[100];
int book_id;
}Books;](https://image.slidesharecdn.com/c-250109035803-902aa064/85/c-pdf-programming-fundamentals-and-basics-if-IT-167-320.jpg)
![C++
200
cout << "Volume of Box2 : " << volume <<endl;
// Add two object as follows:
Box3 = Box1 + Box2;
// volume of box 3
volume = Box3.getVolume();
cout << "Volume of Box3 : " << volume <<endl;
return 0;
}
When the above code is compiled and executed, it produces the following result:
Volume of Box1 : 210
Volume of Box2 : 1560
Volume of Box3 : 5400
Overloadable/Non-overloadable Operators
Following is the list of operators which can be overloaded:
+ - * / % ^
& | ~ ! , =
< > <= >= ++ --
<< >> == != && ||
+= -= /= %= ^= &=
|= *= <<= >>= [] ()
-> ->* new new [] delete delete []
Following is the list of operators, which cannot be overloaded:](https://image.slidesharecdn.com/c-250109035803-902aa064/85/c-pdf-programming-fundamentals-and-basics-if-IT-212-320.jpg)
![C++
201
:: .* . ?:
Operator Overloading Examples
Here are various operator overloading examples to help you in understanding
the concept.
S.N. Operators and Example
1 Unary operators overloading
2 Binary operators overloading
3 Relational operators overloading
4 Input/Output operators overloading
5 ++ and -- operators overloading
6 Assignment operators overloading
7 Function call () operator overloading
8 Subscripting [] operator overloading
9 Class member access operator -> overloading
Unary Operators Overloading
The unary operators operate on a single operand and following are the examples
of Unary operators:
The increment (++) and decrement (--) operators.
The unary minus (-) operator.
The logical not (!) operator.
The unary operators operate on the object for which they were called and
normally, this operator appears on the left side of the object, as in !obj, -obj,
and ++obj but sometime they can be used as postfix as well like obj++ or obj--.
Following example explain how minus (-) operator can be overloaded for prefix
as well as postfix usage.](https://image.slidesharecdn.com/c-250109035803-902aa064/85/c-pdf-programming-fundamentals-and-basics-if-IT-213-320.jpg)
![C++
217
cout << "F: " << feet << " I:" << inches << endl;
}
};
int main()
{
Distance D1(11, 10), D2;
cout << "First Distance : ";
D1.displayDistance();
D2 = D1(10, 10, 10); // invoke operator()
cout << "Second Distance :";
D2.displayDistance();
return 0;
}
When the above code is compiled and executed, it produces the following result:
First Distance : F: 11 I:10
Second Distance :F: 30 I:120
Subscripting [ ] Operator Overloading
The subscript operator [] is normally used to access array elements. This
operator can be overloaded to enhance the existing functionality of C++ arrays.
Following example explains how a subscript operator [] can be overloaded.
#include <iostream>
using namespace std;
const int SIZE = 10;
class safearay
{
private:](https://image.slidesharecdn.com/c-250109035803-902aa064/85/c-pdf-programming-fundamentals-and-basics-if-IT-229-320.jpg)
![C++
218
int arr[SIZE];
public:
safearay()
{
register int i;
for(i = 0; i < SIZE; i++)
{
arr[i] = i;
}
}
int &operator[](int i)
{
if( i > SIZE )
{
cout << "Index out of bounds" <<endl;
// return first element.
return arr[0];
}
return arr[i];
}
};
int main()
{
safearay A;
cout << "Value of A[2] : " << A[2] <<endl;
cout << "Value of A[5] : " << A[5]<<endl;
cout << "Value of A[12] : " << A[12]<<endl;
return 0;
}
When the above code is compiled and executed, it produces the following result:
Value of A[2] : 2](https://image.slidesharecdn.com/c-250109035803-902aa064/85/c-pdf-programming-fundamentals-and-basics-if-IT-230-320.jpg)
![C++
219
Value of A[5] : 5
Index out of bounds
Value of A[12] : 0
Class Member Access Operator - > Overloading
The class member access operator (->) can be overloaded but it is bit trickier. It
is defined to give a class type a "pointer-like" behavior. The operator -> must be
a member function. If used, its return type must be a pointer or an object of a
class to which you can apply.
The operator-> is used often in conjunction with the pointer-dereference
operator * to implement "smart pointers." These pointers are objects that
behave like normal pointers except they perform other tasks when you access
an object through them, such as automatic object deletion either when the
pointer is destroyed, or the pointer is used to point to another object.
The dereferencing operator-> can be defined as a unary postfix operator. That
is, given a class:
class Ptr{
//...
X * operator->();
};
Objects of class Ptr can be used to access members of class X in a very similar
manner to the way pointers are used. For example:
void f(Ptr p )
{
p->m = 10 ; // (p.operator->())->m = 10
}
The statement p->m is interpreted as (p.operator->())->m. Using the same
concept, following example explains how a class access operator -> can be
overloaded.
#include <iostream>
#include <vector>
using namespace std;
// Consider an actual class.
class Obj {](https://image.slidesharecdn.com/c-250109035803-902aa064/85/c-pdf-programming-fundamentals-and-basics-if-IT-231-320.jpg)
![C++
221
if(index >= oc.a.size()) return false;
if(oc.a[++index] == 0) return false;
return true;
}
bool operator++(int) // Postfix version
{
return operator++();
}
// overload operator->
Obj* operator->() const
{
if(!oc.a[index])
{
cout << "Zero value";
return (Obj*)0;
}
return oc.a[index];
}
};
int main() {
const int sz = 10;
Obj o[sz];
ObjContainer oc;
for(int i = 0; i < sz; i++)
{
oc.add(&o[i]);
}
SmartPointer sp(oc); // Create an iterator
do {
sp->f(); // smart pointer call
sp->g();
} while(sp++);
return 0;](https://image.slidesharecdn.com/c-250109035803-902aa064/85/c-pdf-programming-fundamentals-and-basics-if-IT-233-320.jpg)
![C++
240
You read information from a file into your program using the stream extraction
operator (>>) just as you use that operator to input information from the
keyboard. The only difference is that you use an ifstream or fstream object
instead of the cin object.
Read & Write Example
Following is the C++ program which opens a file in reading and writing mode.
After writing information entered by the user to a file named afile.dat, the
program reads information from the file and outputs it onto the screen:
#include <fstream>
#include <iostream>
using namespace std;
int main ()
{
char data[100];
// open a file in write mode.
ofstream outfile;
outfile.open("afile.dat");
cout << "Writing to the file" << endl;
cout << "Enter your name: ";
cin.getline(data, 100);
// write inputted data into the file.
outfile << data << endl;
cout << "Enter your age: ";
cin >> data;
cin.ignore();
// again write inputted data into the file.
outfile << data << endl;](https://image.slidesharecdn.com/c-250109035803-902aa064/85/c-pdf-programming-fundamentals-and-basics-if-IT-252-320.jpg)
![C++
247
std::bad_alloc This can be thrown by new.
std::bad_cast This can be thrown by dynamic_cast.
std::bad_exception This is useful device to handle unexpected exceptions
in a C++ program.
std::bad_typeid This can be thrown by typeid.
std::logic_error An exception that theoretically can be detected by
reading the code.
std::domain_error This is an exception thrown when a mathematically
invalid domain is used.
std::invalid_argument This is thrown due to invalid arguments.
std::length_error This is thrown when a too big std::string is created.
std::out_of_range This can be thrown by the ‘at’ method, for example a
std::vector and std::bitset<>::operator[]().
std::runtime_error An exception that theoretically cannot be detected by
reading the code.
std::overflow_error This is thrown if a mathematical overflow occurs.
std::range_error This is occurred when you try to store a value which is
out of range.
std::underflow_error This is thrown if a mathematical underflow occurs.
Define New Exceptions
You can define your own exceptions by inheriting and overriding exception class
functionality. Following is the example, which shows how you can use
std::exception class to implement your own exception in standard way:
#include <iostream>
#include <exception>
using namespace std;](https://image.slidesharecdn.com/c-250109035803-902aa064/85/c-pdf-programming-fundamentals-and-basics-if-IT-259-320.jpg)
![C++
251
Value of pvalue : 29495
Dynamic Memory Allocation for Arrays
Consider you want to allocate memory for an array of characters, i.e., string of
20 characters. Using the same syntax what we have used above we can allocate
memory dynamically as shown below.
char* pvalue = NULL; // Pointer initialized with null
pvalue = new char[20]; // Request memory for the variable
To remove the array that we have just created the statement would look like
this:
delete [] pvalue; // Delete array pointed to by pvalue
Following is the syntax of new operator for a multi-dimensional array as follows:
int ROW = 2;
int COL = 3;
double **pvalue = new double* [ROW]; // Allocate memory for rows
// Now allocate memory for columns
for(int i = 0; i < COL; i++) {
pvalue[i] = new double[COL];
}
The syntax to release the memory for multi-dimensional will be as follows:
for(int i = 0; i < COL; i++) {
delete[] pvalue[i];
}
delete [] pvalue;
Dynamic Memory Allocation for Objects
Objects are no different from simple data types. For example, consider the
following code where we are going to use an array of objects to clarify the
concept:
#include <iostream>
using namespace std;](https://image.slidesharecdn.com/c-250109035803-902aa064/85/c-pdf-programming-fundamentals-and-basics-if-IT-263-320.jpg)
![C++
252
class Box
{
public:
Box() {
cout << "Constructor called!" <<endl;
}
~Box() {
cout << "Destructor called!" <<endl;
}
};
int main( )
{
Box* myBoxArray = new Box[4];
delete [] myBoxArray; // Delete array
return 0;
}
If you were to allocate an array of four Box objects, the Simple constructor
would be called four times and similarly while deleting these objects, destructor
will also be called same number of times.
If we compile and run above code, this would produce the following result:
Constructor called!
Constructor called!
Constructor called!
Constructor called!
Destructor called!
Destructor called!
Destructor called!
Destructor called!](https://image.slidesharecdn.com/c-250109035803-902aa064/85/c-pdf-programming-fundamentals-and-basics-if-IT-264-320.jpg)
![C++
276
pthread_exit(NULL);
}
int main ()
{
pthread_t threads[NUM_THREADS];
int rc;
int i;
for( i=0; i < NUM_THREADS; i++ ){
cout << "main() : creating thread, " << i << endl;
rc = pthread_create(&threads[i], NULL,
PrintHello, (void *)i);
if (rc){
cout << "Error:unable to create thread," << rc << endl;
exit(-1);
}
}
pthread_exit(NULL);
}
Compile the following program using -lpthread library as follows:
$gcc test.cpp -lpthread
Now, execute your program which gives the following output:
main() : creating thread, 0
main() : creating thread, 1
main() : creating thread, 2
main() : creating thread, 3
main() : creating thread, 4
Hello World! Thread ID, 0
Hello World! Thread ID, 1
Hello World! Thread ID, 2
Hello World! Thread ID, 3
Hello World! Thread ID, 4](https://image.slidesharecdn.com/c-250109035803-902aa064/85/c-pdf-programming-fundamentals-and-basics-if-IT-288-320.jpg)
![C++
277
Passing Arguments to Threads
This example shows how to pass multiple arguments via a structure. You can
pass any data type in a thread callback because it points to void as explained in
the following example:
#include <iostream>
#include <cstdlib>
#include <pthread.h>
using namespace std;
#define NUM_THREADS 5
struct thread_data{
int thread_id;
char *message;
};
void *PrintHello(void *threadarg)
{
struct thread_data *my_data;
my_data = (struct thread_data *) threadarg;
cout << "Thread ID : " << my_data->thread_id ;
cout << " Message : " << my_data->message << endl;
pthread_exit(NULL);
}
int main ()
{
pthread_t threads[NUM_THREADS];
struct thread_data td[NUM_THREADS];
int rc;](https://image.slidesharecdn.com/c-250109035803-902aa064/85/c-pdf-programming-fundamentals-and-basics-if-IT-289-320.jpg)
![C++
278
int i;
for( i=0; i < NUM_THREADS; i++ ){
cout <<"main() : creating thread, " << i << endl;
td[i].thread_id = i;
td[i].message = "This is message";
rc = pthread_create(&threads[i], NULL,
PrintHello, (void *)&td[i]);
if (rc){
cout << "Error:unable to create thread," << rc << endl;
exit(-1);
}
}
pthread_exit(NULL);
}
When the above code is compiled and executed, it produces the following result:
main() : creating thread, 0
main() : creating thread, 1
main() : creating thread, 2
main() : creating thread, 3
main() : creating thread, 4
Thread ID : 3 Message : This is message
Thread ID : 2 Message : This is message
Thread ID : 0 Message : This is message
Thread ID : 1 Message : This is message
Thread ID : 4 Message : This is message
Joining and Detaching Threads
There are following two routines which we can use to join or detach threads:
pthread_join (threadid, status)
pthread_detach (threadid)](https://image.slidesharecdn.com/c-250109035803-902aa064/85/c-pdf-programming-fundamentals-and-basics-if-IT-290-320.jpg)
![C++
279
The pthread_join() subroutine blocks the calling thread until the specified
‘threadid’ thread terminates. When a thread is created, one of its attributes
defines whether it is joinable or detached. Only threads that are created as
joinable can be joined. If a thread is created as detached, it can never be joined.
This example demonstrates how to wait for thread completions by using the
Pthread join routine.
#include <iostream>
#include <cstdlib>
#include <pthread.h>
#include <unistd.h>
using namespace std;
#define NUM_THREADS 5
void *wait(void *t)
{
int i;
long tid;
tid = (long)t;
sleep(1);
cout << "Sleeping in thread " << endl;
cout << "Thread with id : " << tid << " ...exiting " << endl;
pthread_exit(NULL);
}
int main ()
{
int rc;
int i;
pthread_t threads[NUM_THREADS];
pthread_attr_t attr;
void *status;](https://image.slidesharecdn.com/c-250109035803-902aa064/85/c-pdf-programming-fundamentals-and-basics-if-IT-291-320.jpg)
![C++
280
// Initialize and set thread joinable
pthread_attr_init(&attr);
pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_JOINABLE);
for( i=0; i < NUM_THREADS; i++ ){
cout << "main() : creating thread, " << i << endl;
rc = pthread_create(&threads[i], NULL, wait, (void *)i );
if (rc){
cout << "Error:unable to create thread," << rc << endl;
exit(-1);
}
}
// free attribute and wait for the other threads
pthread_attr_destroy(&attr);
for( i=0; i < NUM_THREADS; i++ ){
rc = pthread_join(threads[i], &status);
if (rc){
cout << "Error:unable to join," << rc << endl;
exit(-1);
}
cout << "Main: completed thread id :" << i ;
cout << " exiting with status :" << status << endl;
}
cout << "Main: program exiting." << endl;
pthread_exit(NULL);
}
When the above code is compiled and executed, it produces the following result:
main() : creating thread, 0
main() : creating thread, 1
main() : creating thread, 2](https://image.slidesharecdn.com/c-250109035803-902aa064/85/c-pdf-programming-fundamentals-and-basics-if-IT-292-320.jpg)
![C++
287
Here is small CGI program to list out all the CGI variables.
#include <iostream>
#include <stdlib.h>
using namespace std;
const string ENV[ 24 ] = {
"COMSPEC", "DOCUMENT_ROOT", "GATEWAY_INTERFACE",
"HTTP_ACCEPT", "HTTP_ACCEPT_ENCODING",
"HTTP_ACCEPT_LANGUAGE", "HTTP_CONNECTION",
"HTTP_HOST", "HTTP_USER_AGENT", "PATH",
"QUERY_STRING", "REMOTE_ADDR", "REMOTE_PORT",
"REQUEST_METHOD", "REQUEST_URI", "SCRIPT_FILENAME",
"SCRIPT_NAME", "SERVER_ADDR", "SERVER_ADMIN",
"SERVER_NAME","SERVER_PORT","SERVER_PROTOCOL",
"SERVER_SIGNATURE","SERVER_SOFTWARE" };
int main ()
{
cout << "Content-type:text/htmlrnrn";
cout << "<html>n";
cout << "<head>n";
cout << "<title>CGI Environment Variables</title>n";
cout << "</head>n";
cout << "<body>n";
cout << "<table border = "0" cellspacing = "2">";
for ( int i = 0; i < 24; i++ )
{
cout << "<tr><td>" << ENV[ i ] << "</td><td>";
// attempt to retrieve value of environment variable
char *value = getenv( ENV[ i ].c_str() );](https://image.slidesharecdn.com/c-250109035803-902aa064/85/c-pdf-programming-fundamentals-and-basics-if-IT-299-320.jpg)
![C++
307
// create a vector to store int
vector<int> vec;
int i;
// display the original size of vec
cout << "vector size = " << vec.size() << endl;
// push 5 values into the vector
for(i = 0; i < 5; i++){
vec.push_back(i);
}
// display extended size of vec
cout << "extended vector size = " << vec.size() << endl;
// access 5 values from the vector
for(i = 0; i < 5; i++){
cout << "value of vec [" << i << "] = " << vec[i] << endl;
}
// use iterator to access the values
vector<int>::iterator v = vec.begin();
while( v != vec.end()) {
cout << "value of v = " << *v << endl;
v++;
}
return 0;
}
When the above code is compiled and executed, it produces the following result:
vector size = 0
extended vector size = 5
value of vec [0] = 0](https://image.slidesharecdn.com/c-250109035803-902aa064/85/c-pdf-programming-fundamentals-and-basics-if-IT-319-320.jpg)
![C++
308
value of vec [1] = 1
value of vec [2] = 2
value of vec [3] = 3
value of vec [4] = 4
value of v = 0
value of v = 1
value of v = 2
value of v = 3
value of v = 4
Here are following points to be noted related to various functions we used in the
above example:
The push_back( ) member function inserts value at the end of the vector,
expanding its size as needed.
The size( ) function displays the size of the vector.
The function begin( ) returns an iterator to the start of the vector.
The function end( ) returns an iterator to the end of the vector.](https://image.slidesharecdn.com/c-250109035803-902aa064/85/c-pdf-programming-fundamentals-and-basics-if-IT-320-320.jpg)
c++.pdf programming fundamentals and basics if IT
- 2. i About the Tutorial C++ is a middle-level programming language developed by Bjarne Stroustrup starting in 1979 at Bell Labs. C++ runs on a variety of platforms, such as Windows, Mac OS, and the various versions of UNIX. This tutorial adopts a simple and practical approach to describe the concepts of C++. Audience This tutorial has been prepared for the beginners to help them understand the basic to advanced concepts related to C++. Prerequisites Before you start practicing with various types of examples given in this tutorial,we are making an assumption that you are already aware of the basics of computer program and computer programming language. Copyright & Disclaimer Copyright 2014 by Tutorials Point (I) Pvt. Ltd. All the content and graphics published in this e-book are the property of Tutorials Point (I) Pvt. Ltd. The user of this e-book is prohibited to reuse, retain, copy, distribute or republish any contents or a part of contents of this e-book in any manner without written consent of the publisher. We strive to update the contents of our website and tutorials as timely and as precisely as possible, however, the contents may contain inaccuracies or errors. Tutorials Point (I) Pvt. Ltd. provides no guarantee regarding the accuracy, timeliness or completeness of our website or its contents including this tutorial. If you discover any errors on our website or in this tutorial, please notify us at contact@tutorialspoint.com Table of Contents About the Tutorial....................................................................................................................................i Audience..................................................................................................................................................i Prerequisites............................................................................................................................................i
- 3. ii Copyright & Disclaimer.............................................................................................................................i Table of Contents.....................................................................................................................................i 1. OVERVIEW...........................................................................................................................1 Object-Oriented Programming................................................................................................................1 Standard Libraries ...................................................................................................................................1 The ANSI Standard ..................................................................................................................................1 Learning C++............................................................................................................................................2 Use of C++ ...............................................................................................................................................2 2. ENVIORNMENT SETUP.........................................................................................................3 Try it Option Online.................................................................................................................................3 Local Environment Setup.........................................................................................................................3 Installing GNU C/C++ Compiler:...............................................................................................................4 3. BASIC SYNTAX......................................................................................................................6 C++ Program Structure:...........................................................................................................................6 Compile & Execute C++ Program: ............................................................................................................7 Semicolons & Blocks in C++.....................................................................................................................7 C++ Identifiers.........................................................................................................................................8 C++ Keywords..........................................................................................................................................8 Trigraphs.................................................................................................................................................9 Whitespace in C++.................................................................................................................................10 4. COMMENTS IN C++ ...........................................................................................................11 5. DATA TYPES.......................................................................................................................13 Primitive Built-in Types .........................................................................................................................13 typedef Declarations.............................................................................................................................15
- 4. iii Enumerated Types ................................................................................................................................16 6. VARIABLE TYPES ................................................................................................................17 Variable Definition in C++......................................................................................................................17 Variable Declaration in C++ ...................................................................................................................18 Lvalues and Rvalues ..............................................................................................................................20 7. VARIABLE SCOPE ...............................................................................................................21 Local Variables ......................................................................................................................................21 Global Variables....................................................................................................................................22 Initializing Local and Global Variables ...................................................................................................23 8. CONSTANTS/LITERALS .......................................................................................................24 Integer Literals ......................................................................................................................................24 Floating-point Literals ...........................................................................................................................24 Boolean Literals.....................................................................................................................................25 Character Literals ..................................................................................................................................25 String Literals ........................................................................................................................................26 Defining Constants................................................................................................................................27 9. MODIFIER TYPES................................................................................................................29 Type Qualifiers in C++............................................................................................................................30 10. STORAGE CLASSES.............................................................................................................31 The auto Storage Class ..........................................................................................................................31 The register Storage Class .....................................................................................................................31 The static Storage Class.........................................................................................................................31 The extern Storage Class .......................................................................................................................33 The mutable Storage Class ....................................................................................................................34 11. OPERATORS.......................................................................................................................35
- 5. iv Arithmetic Operators ............................................................................................................................35 Relational Operators.............................................................................................................................37 Logical Operators ..................................................................................................................................40 Bitwise Operators .................................................................................................................................41 Assignment Operators...........................................................................................................................44 Misc Operators......................................................................................................................................47 Operators Precedence in C++ ................................................................................................................48 12. LOOP TYPES.......................................................................................................................51 While Loop............................................................................................................................................52 for Loop.................................................................................................................................................54 do…while Loop......................................................................................................................................56 nested Loops.........................................................................................................................................58 Loop Control Statements.......................................................................................................................60 Break Statement ...................................................................................................................................61 continue Statement ..............................................................................................................................63 goto Statement .....................................................................................................................................65 The Infinite Loop ...................................................................................................................................67 13. DECISION-MAKING STATEMENTS ......................................................................................69 If Statement ..........................................................................................................................................70 if…else Statement .................................................................................................................................72 if...else if...else Statement.....................................................................................................................73 Switch Statement..................................................................................................................................75 Nested if Statement ..............................................................................................................................78 The ? : Operator ....................................................................................................................................81 14. FUNCTIONS .......................................................................................................................82 Defining a Function ...............................................................................................................................82
- 6. v Function Declarations ...........................................................................................................................83 Calling a Function..................................................................................................................................84 Function Arguments..............................................................................................................................85 Call by Value .........................................................................................................................................86 Call by Pointer.......................................................................................................................................87 Call by Reference...................................................................................................................................89 Default Values for Parameters ..............................................................................................................90 15. NUMBERS..........................................................................................................................93 Defining Numbers in C++.......................................................................................................................93 Math Operations in C++ ........................................................................................................................94 Random Numbers in C++.......................................................................................................................96 16. ARRAYS..............................................................................................................................98 Declaring Arrays....................................................................................................................................98 Initializing Arrays ..................................................................................................................................98 Accessing Array Elements......................................................................................................................99 Arrays in C++ .......................................................................................................................................100 Pointer to an Array..............................................................................................................................103 Passing Arrays to Functions.................................................................................................................105 Return Array from Functions...............................................................................................................107 17. STRINGS...........................................................................................................................111 The C-Style Character String................................................................................................................111 The String Class in C++.........................................................................................................................114 18. POINTERS ........................................................................................................................116 What are Pointers? .............................................................................................................................116 Using Pointers in C++...........................................................................................................................117 Pointers in C++ ....................................................................................................................................118
- 7. vi Null Pointers .......................................................................................................................................119 Pointer Arithmetic...............................................................................................................................120 Pointers vs Arrays ...............................................................................................................................124 Array of Pointers.................................................................................................................................126 Pointer to a Pointer.............................................................................................................................128 Passing Pointers to Functions..............................................................................................................130 Return Pointer from Functions............................................................................................................132 19. REFERENCES....................................................................................................................135 References vs Pointers ........................................................................................................................135 Creating References in C++..................................................................................................................135 References as Parameters...................................................................................................................137 Reference as Return Value ..................................................................................................................138 20. DATE AND TIME...............................................................................................................141 Current Date and Time........................................................................................................................142 Format Time using struct tm ...............................................................................................................143 21. BASIC INPUT/OUTPUT .....................................................................................................145 I/O Library Header Files.......................................................................................................................145 The Standard Output Stream (cout) ....................................................................................................145 The Standard Input Stream (cin)..........................................................................................................146 The Standard Error Stream (cerr) ........................................................................................................147 The Standard Log Stream (clog)...........................................................................................................148 22. DATA STRUCTURES..........................................................................................................149 Defining a Structure ............................................................................................................................149 Accessing Structure Members .............................................................................................................150
- 8. vii Structures as Function Arguments.......................................................................................................151 Pointers to Structures .........................................................................................................................153 The typedef Keyword..........................................................................................................................155 23. CLASSES AND OBJECTS ....................................................................................................157 C++ Class Definitions ...........................................................................................................................157 Define C++ Objects..............................................................................................................................157 Accessing the Data Members ..............................................................................................................158 Classes & Objects in Detail ..................................................................................................................159 Class Access Modifiers.........................................................................................................................163 The public Members............................................................................................................................164 The private Members..........................................................................................................................165 The protected Members......................................................................................................................167 Constructor & Destructor....................................................................................................................169 Parameterized Constructor .................................................................................................................170 The Class Destructor............................................................................................................................173 Copy Constructor ................................................................................................................................174 Friend Functions..................................................................................................................................179 Inline Functions...................................................................................................................................181 this Pointer .........................................................................................................................................182 Pointer to C++ Classes .........................................................................................................................184 Static Members of a Class ...................................................................................................................185 Static Function Members ....................................................................................................................187 24. INHERITANCE...................................................................................................................190 Base & Derived Classes........................................................................................................................190 Access Control and Inheritance ...........................................................................................................192
- 9. viii Type of Inheritance .............................................................................................................................192 Multiple Inheritance ...........................................................................................................................193 25. OVERLOADING (OPERATOR & FUNCTION).......................................................................196 Function Overloading in C++ ...............................................................................................................196 Operators Overloading in C++ .............................................................................................................197 Overloadable/Non-overloadable Operators........................................................................................200 Operator Overloading Examples .........................................................................................................201 Unary Operators Overloading .............................................................................................................201 Increment (++) and Decrement (- -) Operators ....................................................................................203 Binary Operators Overloading.............................................................................................................205 Relational Operators Overloading.......................................................................................................208 Input/Output Operators Overloading..................................................................................................210 ++ and - - Operators Overloading ........................................................................................................212 Assignment Operators Overloading ....................................................................................................214 Function Call () Operator Overloading.................................................................................................215 Subscripting [ ] Operator Overloading.................................................................................................217 Class Member Access Operator - > Overloading ..................................................................................219 26. POLYMORPHISM..............................................................................................................223 Virtual Function ..................................................................................................................................226 Pure Virtual Functions.........................................................................................................................226 27. DATA ABSTRACTION........................................................................................................227 Access Labels Enforce Abstraction.......................................................................................................228 Benefits of Data Abstraction ...............................................................................................................228 Data Abstraction Example...................................................................................................................228 Designing Strategy ..............................................................................................................................230 28. DATA ENCAPSULATION....................................................................................................231
- 10. ix Data Encapsulation Example ...............................................................................................................232 Designing Strategy ..............................................................................................................................233 29. INTERFACES.....................................................................................................................234 Abstract Class Example .......................................................................................................................234 Designing Strategy ..............................................................................................................................236 30. FILES AND STREAMS........................................................................................................238 Opening a File .....................................................................................................................................238 Closing a File .......................................................................................................................................239 Writing to a File...................................................................................................................................239 Reading from a File .............................................................................................................................239 Read & Write Example ........................................................................................................................240 File Position Pointers...........................................................................................................................242 31. EXCEPTION HANDLING......................................................................................................243 Throwing Exceptions...........................................................................................................................244 Catching Exceptions ............................................................................................................................244 C++ Standard Exceptions.....................................................................................................................246 Define New Exceptions .......................................................................................................................247 32. DYNAMIC MEMORY.........................................................................................................249 The new and delete Operators............................................................................................................249 Dynamic Memory Allocation for Arrays ..............................................................................................251 Dynamic Memory Allocation for Objects.............................................................................................251 33. NAMESPACES ..................................................................................................................253 Defining a Namespace.........................................................................................................................253 The using directive ..............................................................................................................................254
- 11. x Discontiguous Namespaces.................................................................................................................256 Nested Namespaces............................................................................................................................256 34. TEMPLATES .....................................................................................................................258 Function Template ..............................................................................................................................258 Class Template ....................................................................................................................................259 35. PREPROCESSOR...............................................................................................................263 The #define Preprocessor....................................................................................................................263 Function-Like Macros..........................................................................................................................264 Conditional Compilation .....................................................................................................................264 The # and # # Operators ......................................................................................................................266 Predefined C++ Macros .......................................................................................................................268 36. SIGNAL HANDLING ..........................................................................................................270 The signal() Function ...........................................................................................................................270 The raise() Function.............................................................................................................................272 37. MULTITHREADING...........................................................................................................274 Creating Threads.................................................................................................................................274 Terminating Threads ...........................................................................................................................275 Passing Arguments to Threads ............................................................................................................277 Joining and Detaching Threads............................................................................................................278 38. WEB PROGRAMMING......................................................................................................282 What is CGI?........................................................................................................................................282 Web Browsing.....................................................................................................................................282 CGI Architecture Diagram....................................................................................................................282 Web Server Configuration...................................................................................................................283 First CGI Program ................................................................................................................................284
- 12. xi My First CGI program ..........................................................................................................................284 HTTP Header .......................................................................................................................................285 CGI Environment Variables..................................................................................................................285 C++ CGI Library....................................................................................................................................289 GET and POST Methods.......................................................................................................................289 Passing Information Using GET Method ..............................................................................................289 Simple URL Example: Get Method.......................................................................................................290 Simple FORM Example: GET Method...................................................................................................291 Passing Information Using POST Method ............................................................................................292 Passing Checkbox Data to CGI Program...............................................................................................292 Passing Radio Button Data to CGI Program .........................................................................................294 Passing Text Area Data to CGI Program...............................................................................................296 Passing Dropdown Box Data to CGI Program.......................................................................................298 Using Cookies in CGI............................................................................................................................299 How It Works ......................................................................................................................................299 Setting up Cookies...............................................................................................................................300 Retrieving Cookies...............................................................................................................................301 File Upload Example............................................................................................................................303 39. STL TUTORIAL..................................................................................................................306 40. STANDARD LIBRARY.........................................................................................................309 The Standard Function Library ............................................................................................................309 The Object Oriented Class Library .......................................................................................................309
- 13. C++ 1 C++ is a statically typed, compiled, general-purpose, case-sensitive, free-form programming language that supports procedural, object-oriented, and generic programming. C++ is regarded as a middle-level language, as it comprises a combination of both high-level and low-level language features. C++ was developed by Bjarne Stroustrup starting in 1979 at Bell Labs in Murray Hill, New Jersey, as an enhancement to the C language and originally named C with Classes but later it was renamed C++ in 1983. C++ is a superset of C, and that virtually any legal C program is a legal C++ program. Note: A programming language is said to use static typing when type checking is performed during compile-time as opposed to run-time. Object-Oriented Programming C++ fully supports object-oriented programming, including the four pillars of object-oriented development: Encapsulation Data hiding Inheritance Polymorphism Standard Libraries Standard C++ consists of three important parts: The core language giving all the building blocks including variables, data types and literals, etc. The C++ Standard Library giving a rich set of functions manipulating files, strings, etc. The Standard Template Library (STL) giving a rich set of methods manipulating data structures, etc. The ANSI Standard The ANSI standard is an attempt to ensure that C++ is portable; that code you write for Microsoft's compiler will compile without errors, using a compiler on a Mac, UNIX, a Windows box, or an Alpha. 1. OVERVIEW
- 14. C++ 2 The ANSI standard has been stable for a while, and all the major C++ compiler manufacturers support the ANSI standard. Learning C++ The most important thing while learning C++ is to focus on concepts. The purpose of learning a programming language is to become a better programmer; that is, to become more effective at designing and implementing new systems and at maintaining old ones. C++ supports a variety of programming styles. You can write in the style of Fortran, C, Smalltalk, etc., in any language. Each style can achieve its aims effectively while maintaining runtime and space efficiency. Use of C++ C++ is used by hundreds of thousands of programmers in essentially every application domain. C++ is being highly used to write device drivers and other software that rely on direct manipulation of hardware under real-time constraints. C++ is widely used for teaching and research because it is clean enough for successful teaching of basic concepts. Anyone who has used either an Apple Macintosh or a PC running Windows has indirectly used C++ because the primary user interfaces of these systems are written in C++.
- 15. C++ 3 Try it Option Online You really do not need to set up your own environment to start learning C++ programming language. Reason is very simple, we have already set up C++ Programming environment online, so that you can compile and execute all the available examples online at the same time when you are doing your theory work. This gives you confidence in what you are reading and to check the result with different options. Feel free to modify any example and execute it online. Try the following example using our online compiler option available at http://www.compileonline.com/ #include <iostream> using namespace std; int main() { cout << "Hello World"; return 0; } For most of the examples given in this tutorial, you will find Try it option in our website code sections at the top right corner that will take you to the online compiler. So just make use of it and enjoy your learning. Local Environment Setup If you are still willing to set up your environment for C++, you need to have the following two softwares on your computer. Text Editor: This will be used to type your program. Examples of few editors include Windows Notepad, OS Edit command, Brief, Epsilon, EMACS, and vim or vi. Name and version of text editor can vary on different operating systems. For example, Notepad will be used on Windows and vim or vi can be used on windows as well as Linux, or UNIX. The files you create with your editor are called source files and for C++ they typically are named with the extension .cpp, .cp, or .c. A text editor should be in place to start your C++ programming. 2. ENVIORNMENT SETUP
- 16. C++ 4 C++ Compiler: This is an actual C++ compiler, which will be used to compile your source code into final executable program. Most C++ compilers don't care what extension you give to your source code, but if you don't specify otherwise, many will use .cpp by default. Most frequently used and free available compiler is GNU C/C++ compiler, otherwise you can have compilers either from HP or Solaris if you have the respective Operating Systems. Installing GNU C/C++ Compiler: UNIX/Linux Installation: If you are using Linux or UNIX then check whether GCC is installed on your system by entering the following command from the command line: $ g++ -v If you have installed GCC, then it should print a message such as the following: Using built-in specs. Target: i386-redhat-linux Configured with: ../configure --prefix=/usr ....... Thread model: posix gcc version 4.1.2 20080704 (Red Hat 4.1.2-46) If GCC is not installed, then you will have to install it yourself using the detailed instructions available at http://gcc.gnu.org/install/ . Mac OS X Installation: If you use Mac OS X, the easiest way to obtain GCC is to download the Xcode development environment from Apple's website and follow the simple installation instructions. Xcode is currently available at developer.apple.com/technologies/tools/. Windows Installation: To install GCC at Windows you need to install MinGW. To install MinGW, go to the MinGW homepage, www.mingw.org, and follow the link to the MinGW download page. Download the latest version of the MinGW installation program which should be named MinGW-<version>.exe. While installing MinGW, at a minimum, you must install gcc-core, gcc-g++, binutils, and the MinGW runtime, but you may wish to install more.
- 17. C++ 5 Add the bin subdirectory of your MinGW installation to your PATH environment variable so that you can specify these tools on the command line by their simple names. When the installation is complete, you will be able to run gcc, g++, ar, ranlib, dlltool, and several other GNU tools from the Windows command line.
- 18. C++ 6 When we consider a C++ program, it can be defined as a collection of objects that communicate via invoking each other's methods. Let us now briefly look into what a class, object, methods, and instant variables mean. Object - Objects have states and behaviors. Example: A dog has states - color, name, breed as well as behaviors - wagging, barking, and eating. An object is an instance of a class. Class - A class can be defined as a template/blueprint that describes the behaviors/states that object of its type support. Methods - A method is basically a behavior. A class can contain many methods. It is in methods where the logics are written, data is manipulated and all the actions are executed. Instant Variables - Each object has its unique set of instant variables. An object's state is created by the values assigned to these instant variables. C++ Program Structure: Let us look at a simple code that would print the words Hello World. #include <iostream> using namespace std; // main() is where program execution begins. int main() { cout << "Hello World"; // prints Hello World return 0; } Let us look at the various parts of the above program: 1. The C++ language defines several headers, which contain information that is either necessary or useful to your program. For this program, the header <iostream> is needed. 2. The line using namespace std; tells the compiler to use the std namespace. Namespaces are a relatively recent addition to C++. 3. BASIC SYNTAX
- 19. C++ 7 3. The next line ‘// main() is where program execution begins.’ is a single-line comment available in C++. Single-line comments begin with // and stop at the end of the line. 4. The line int main() is the main function where program execution begins. 5. The next line cout << "This is my first C++ program."; causes the message "This is my first C++ program" to be displayed on the screen. 6. The next line return 0; terminates main() function and causes it to return the value 0 to the calling process. Compile & Execute C++ Program: Let's look at how to save the file, compile and run the program. Please follow the steps given below: 1. Open a text editor and add the code as above. 2. Save the file as: hello.cpp 3. Open a command prompt and go to the directory where you saved the file. 4. Type 'g++ hello.cpp' and press enter to compile your code. If there are no errors in your code the command prompt will take you to the next line and would generate a.out executable file. 5. Now, type 'a.out' to run your program. 6. You will be able to see ' Hello World ' printed on the window. $ g++ hello.cpp $ ./a.out Hello World Make sure that g++ is in your path and that you are running it in the directory containing file hello.cpp. You can compile C/C++ programs using makefile. For more details, you can check our ‘Makefile Tutorial’. Semicolons & Blocks in C++ In C++, the semicolon is a statement terminator. That is, each individual statement must be ended with a semicolon. It indicates the end of one logical entity. For example, following are three different statements: x = y; y = y+1;
- 20. C++ 8 add(x, y); A block is a set of logically connected statements that are surrounded by opening and closing braces. For example: { cout << "Hello World"; // prints Hello World return 0; } C++ does not recognize the end of the line as a terminator. For this reason, it does not matter where you put a statement in a line. For example: x = y; y = y+1; add(x, y); is the same as x = y; y = y+1; add(x, y); C++ Identifiers A C++ identifier is a name used to identify a variable, function, class, module, or any other user-defined item. An identifier starts with a letter A to Z or a to z or an underscore (_) followed by zero or more letters, underscores, and digits (0 to 9). C++ does not allow punctuation characters such as @, $, and % within identifiers. C++ is a case-sensitive programming language. Thus, Manpower and manpower are two different identifiers in C++. Here are some examples of acceptable identifiers: mohd zara abc move_name a_123 myname50 _temp j a23b9 retVal C++ Keywords The following list shows the reserved words in C++. These reserved words may not be used as constant or variable or any other identifier names. asm else new this auto enum operator throw
- 21. C++ 9 bool explicit private true break export protected try case extern public typedef catch false register typeid char float reinterpret_cast typename class for return union const friend short unsigned const_cast goto signed using continue if sizeof virtual default inline static void delete int static_cast volatile do long struct wchar_t double mutable switch while dynamic_cast namespace template Trigraphs A few characters have an alternative representation, called a trigraph sequence. A trigraph is a three-character sequence that represents a single character and the sequence always starts with two question marks. Trigraphs are expanded anywhere they appear, including within string literals and character literals, in comments, and in preprocessor directives. Following are most frequently used trigraph sequences: Trigraph Replacement
- 22. C++ 10 ??= # ??/ ??' ^ ??( [ ??) ] ??! | ??< { ??> } ??- ~ All the compilers do not support trigraphs and they are not advised to be used because of their confusing nature. Whitespace in C++ A line containing only whitespace, possibly with a comment, is known as a blank line, and C++ compiler totally ignores it. Whitespace is the term used in C++ to describe blanks, tabs, newline characters and comments. Whitespace separates one part of a statement from another and enables the compiler to identify where one element in a statement, such as int, ends and the next element begins. Statement 1: int age; In the above statement there must be at least one whitespace character (usually a space) between int and age for the compiler to be able to distinguish them. Statement 2: fruit = apples + oranges; // Get the total fruit In the above statement 2, no whitespace characters are necessary between fruit and =, or between = and apples, although you are free to include some if you wish for readability purpose.
- 23. C++ 11 Program comments are explanatory statements that you can include in the C++ code. These comments help anyone reading the source code. All programming languages allow for some form of comments. C++ supports single-line and multi-line comments. All characters available inside any comment are ignored by C++ compiler. C++ comments start with /* and end with */. For example: /* This is a comment */ /* C++ comments can also * span multiple lines */ A comment can also start with //, extending to the end of the line. For example: #include <iostream> using namespace std; main() { cout << "Hello World"; // prints Hello World return 0; } When the above code is compiled, it will ignore // prints Hello World and final executable will produce the following result: Hello World Within a /* and */ comment, // characters have no special meaning. Within a // comment, /* and */ have no special meaning. Thus, you can "nest" one kind of comment within the other kind. For example: /* Comment out printing of Hello World: 4. COMMENTS IN C++
- 24. C++ 12 cout << "Hello World"; // prints Hello World */
- 25. C++ 13 While writing program in any language, you need to use various variables to store various information. Variables are nothing but reserved memory locations to store values. This means that when you create a variable you reserve some space in memory. You may like to store information of various data types like character, wide character, integer, floating point, double floating point, boolean etc. Based on the data type of a variable, the operating system allocates memory and decides what can be stored in the reserved memory. Primitive Built-in Types C++ offers the programmer a rich assortment of built-in as well as user defined data types. Following table lists down seven basic C++ data types: Type Keyword Boolean bool Character char Integer int Floating point float Double floating point double Valueless void Wide character wchar_t Several of the basic types can be modified using one or more of these type modifiers: signed unsigned short long 5. DATA TYPES
- 26. C++ 14 The following table shows the variable type, how much memory it takes to store the value in memory, and what is maximum and minimum value which can be stored in such type of variables. Type Typical Bit Width Typical Range char 1byte -127 to 127 or 0 to 255 unsigned char 1byte 0 to 255 signed char 1byte -127 to 127 int 4bytes -2147483648 to 2147483647 unsigned int 4bytes 0 to 4294967295 signed int 4bytes -2147483648 to 2147483647 short int 2bytes -32768 to 32767 unsigned short int Range 0 to 65,535 signed short int Range -32768 to 32767 long int 4bytes -2,147,483,647 to 2,147,483,647 signed long int 4bytes same as long int unsigned long int 4bytes 0 to 4,294,967,295 float 4bytes +/- 3.4e +/- 38 (~7 digits) double 8bytes +/- 1.7e +/- 308 (~15 digits) long double 8bytes +/- 1.7e +/- 308 (~15 digits) wchar_t 2 or 4 bytes 1 wide character The size of variables might be different from those shown in the above table, depending on the compiler and the computer you are using.
- 27. C++ 15 Following is the example, which will produce correct size of various data types on your computer. #include <iostream> using namespace std; int main() { cout << "Size of char : " << sizeof(char) << endl; cout << "Size of int : " << sizeof(int) << endl; cout << "Size of short int : " << sizeof(short int) << endl; cout << "Size of long int : " << sizeof(long int) << endl; cout << "Size of float : " << sizeof(float) << endl; cout << "Size of double : " << sizeof(double) << endl; cout << "Size of wchar_t : " << sizeof(wchar_t) << endl; return 0; } This example uses endl, which inserts a new-line character after every line and << operator is being used to pass multiple values out to the screen. We are also using sizeof() function to get size of various data types. When the above code is compiled and executed, it produces the following result which can vary from machine to machine: Size of char : 1 Size of int : 4 Size of short int : 2 Size of long int : 4 Size of float : 4 Size of double : 8 Size of wchar_t : 4 typedef Declarations You can create a new name for an existing type using typedef. Following is the simple syntax to define a new type using typedef: typedef type newname; For example, the following tells the compiler that feet is another name for int:
- 28. C++ 16 typedef int feet; Now, the following declaration is perfectly legal and creates an integer variable called distance: feet distance; Enumerated Types An enumerated type declares an optional type name and a set of zero or more identifiers that can be used as values of the type. Each enumerator is a constant whose type is the enumeration. Creating an enumeration requires the use of the keyword enum. The general form of an enumeration type is: enum enum-name { list of names } var-list; Here, the enum-name is the enumeration's type name. The list of names is comma separated. For example, the following code defines an enumeration of colors called colors and the variable c of type color. Finally, c is assigned the value "blue". enum color { red, green, blue } c; c = blue; By default, the value of the first name is 0, the second name has the value 1, and the third has the value 2, and so on. But you can give a name, a specific value by adding an initializer. For example, in the following enumeration, green will have the value 5. enum color { red, green=5, blue }; Here, blue will have a value of 6 because each name will be one greater than the one that precedes it.
- 29. C++ 17 A variable provides us with named storage that our programs can manipulate. Each variable in C++ has a specific type, which determines the size and layout of the variable's memory; the range of values that can be stored within that memory; and the set of operations that can be applied to the variable. The name of a variable can be composed of letters, digits, and the underscore character. It must begin with either a letter or an underscore. Upper and lowercase letters are distinct because C++ is case-sensitive: There are following basic types of variable in C++ as explained in last chapter: Type Description bool Stores either value true or false. char Typically a single octet (one byte). This is an integer type. int The most natural size of integer for the machine. float A single-precision floating point value. double A double-precision floating point value. void Represents the absence of type. wchar_t A wide character type. C++ also allows to define various other types of variables, which we will cover in subsequent chapters like Enumeration, Pointer, Array, Reference, Data structures, and Classes. Following section will cover how to define, declare and use various types of variables. Variable Definition in C++ A variable definition tells the compiler where and how much storage to create for the variable. A variable definition specifies a data type, and contains a list of one or more variables of that type as follows: 6. VARIABLE TYPES
- 30. C++ 18 type variable_list; Here, type must be a valid C++ data type including char, w_char, int, float, double, bool or any user-defined object, etc., and variable_list may consist of one or more identifier names separated by commas. Some valid declarations are shown here: int i, j, k; char c, ch; float f, salary; double d; The line int i, j, k; both declares and defines the variables i, j and k; which instructs the compiler to create variables named i, j and k of type int. Variables can be initialized (assigned an initial value) in their declaration. The initializer consists of an equal sign followed by a constant expression as follows: type variable_name = value; Some examples are: extern int d = 3, f = 5; // declaration of d and f. int d = 3, f = 5; // definition and initializing d and f. byte z = 22; // definition and initializes z. char x = 'x'; // the variable x has the value 'x'. For definition without an initializer: variables with static storage duration are implicitly initialized with NULL (all bytes have the value 0); the initial value of all other variables is undefined. Variable Declaration in C++ A variable declaration provides assurance to the compiler that there is one variable existing with the given type and name so that compiler proceed for further compilation without needing complete detail about the variable. A variable declaration has its meaning at the time of compilation only, compiler needs actual variable declaration at the time of linking of the program. A variable declaration is useful when you are using multiple files and you define your variable in one of the files which will be available at the time of linking of the program. You will use extern keyword to declare a variable at any place. Though you can declare a variable multiple times in your C++ program, but it can be defined only once in a file, a function or a block of code. Example:
- 31. C++ 19 Try the following example where a variable has been declared at the top, but it has been defined inside the main function: #include <iostream> using namespace std; // Variable declaration: extern int a, b; extern int c; extern float f; int main () { // Variable definition: int a, b; int c; float f; // actual initialization a = 10; b = 20; c = a + b; cout << c << endl ; f = 70.0/3.0; cout << f << endl ; return 0; } When the above code is compiled and executed, it produces the following result: 30 23.3333
- 32. C++ 20 Same concept applies on function declaration where you provide a function name at the time of its declaration and its actual definition can be given anywhere else. For example: // function declaration int func(); int main() { // function call int i = func(); } // function definition int func() { return 0; } Lvalues and Rvalues There are two kinds of expressions in C++: lvalue : Expressions that refer to a memory location is called "lvalue" expression. An lvalue may appear as either the left-hand or right-hand side of an assignment. rvalue : The term rvalue refers to a data value that is stored at some address in memory. An rvalue is an expression that cannot have a value assigned to it which means an rvalue may appear on the right- but not left-hand side of an assignment. Variables are lvalues and so may appear on the left-hand side of an assignment. Numeric literals are rvalues and so may not be assigned and cannot appear on the left-hand side. Following is a valid statement: int g = 20; But the following is not a valid statement and would generate compile-time error: 10 = 20;
- 33. C++ 21 A scope is a region of the program and broadly speaking there are three places, where variables can be declared: Inside a function or a block which is called local variables, In the definition of function parameters which is called formal parameters. Outside of all functions which is called global variables. We will learn what a function is, and it's parameter in subsequent chapters. Here let us explain what local and global variables are. Local Variables Variables that are declared inside a function or block are local variables. They can be used only by statements that are inside that function or block of code. Local variables are not known to functions outside their own. Following is the example using local variables: #include <iostream> using namespace std; int main () { // Local variable declaration: int a, b; int c; // actual initialization a = 10; b = 20; c = a + b; cout << c; return 0; } 7. VARIABLE SCOPE
- 34. C++ 22 Global Variables Global variables are defined outside of all the functions, usually on top of the program. The global variables will hold their value throughout the life-time of your program. A global variable can be accessed by any function. That is, a global variable is available for use throughout your entire program after its declaration. Following is the example using global and local variables: #include <iostream> using namespace std; // Global variable declaration: int g; int main () { // Local variable declaration: int a, b; // actual initialization a = 10; b = 20; g = a + b; cout << g; return 0; } A program can have same name for local and global variables but value of local variable inside a function will take preference. For example: #include <iostream> using namespace std; // Global variable declaration: int g = 20;
- 35. C++ 23 int main () { // Local variable declaration: int g = 10; cout << g; return 0; } When the above code is compiled and executed, it produces the following result: 10 Initializing Local and Global Variables When a local variable is defined, it is not initialized by the system, you must initialize it yourself. Global variables are initialized automatically by the system when you define them as follows: Data Type Initializer int 0 char '0' float 0 double 0 pointer NULL It is a good programming practice to initialize variables properly, otherwise sometimes program would produce unexpected result.
- 36. C++ 24 Constants refer to fixed values that the program may not alter and they are called literals. Constants can be of any of the basic data types and can be divided into Integer Numerals, Floating-Point Numerals, Characters, Strings and Boolean Values. Again, constants are treated just like regular variables except that their values cannot be modified after their definition. Integer Literals An integer literal can be a decimal, octal, or hexadecimal constant. A prefix specifies the base or radix: 0x or 0X for hexadecimal, 0 for octal, and nothing for decimal. An integer literal can also have a suffix that is a combination of U and L, for unsigned and long, respectively. The suffix can be uppercase or lowercase and can be in any order. Here are some examples of integer literals: 212 // Legal 215u // Legal 0xFeeL // Legal 078 // Illegal: 8 is not an octal digit 032UU // Illegal: cannot repeat a suffix Following are other examples of various types of Integer literals: 85 // decimal 0213 // octal 0x4b // hexadecimal 30 // int 30u // unsigned int 30l // long 30ul // unsigned long Floating-point Literals A floating-point literal has an integer part, a decimal point, a fractional part, and an exponent part. You can represent floating point literals either in decimal form or exponential form. 8. CONSTANTS/LITERALS
- 37. C++ 25 While representing using decimal form, you must include the decimal point, the exponent, or both and while representing using exponential form, you must include the integer part, the fractional part, or both. The signed exponent is introduced by e or E. Here are some examples of floating-point literals: 3.14159 // Legal 314159E-5L // Legal 510E // Illegal: incomplete exponent 210f // Illegal: no decimal or exponent .e55 // Illegal: missing integer or fraction Boolean Literals There are two Boolean literals and they are part of standard C++ keywords: A value of true representing true. A value of false representing false. You should not consider the value of true equal to 1 and value of false equal to 0. Character Literals Character literals are enclosed in single quotes. If the literal begins with L (uppercase only), it is a wide character literal (e.g., L'x') and should be stored in wchar_t type of variable. Otherwise, it is a narrow character literal (e.g., 'x') and can be stored in a simple variable of char type. A character literal can be a plain character (e.g., 'x'), an escape sequence (e.g., 't'), or a universal character (e.g., 'u02C0'). There are certain characters in C++ when they are preceded by a backslash they will have special meaning and they are used to represent like newline (n) or tab (t). Here, you have a list of some of such escape sequence codes: Escape sequence Meaning character ' ' character " " character ? ? character
- 38. C++ 26 a Alert or bell b Backspace f Form feed n Newline r Carriage return t Horizontal tab v Vertical tab ooo Octal number of one to three digits xhh . . . Hexadecimal number of one or more digits Following is the example to show a few escape sequence characters: #include <iostream> using namespace std; int main() { cout << "HellotWorldnn"; return 0; } When the above code is compiled and executed, it produces the following result: Hello World String Literals String literals are enclosed in double quotes. A string contains characters that are similar to character literals: plain characters, escape sequences, and universal characters.
- 39. C++ 27 You can break a long line into multiple lines using string literals and separate them using whitespaces. Here are some examples of string literals. All the three forms are identical strings. "hello, dear" "hello, dear" "hello, " "d" "ear" Defining Constants There are two simple ways in C++ to define constants: Using #define preprocessor. Using const keyword. The #define Preprocessor Following is the form to use #define preprocessor to define a constant: #define identifier value Following example explains it in detail: #include <iostream> using namespace std; #define LENGTH 10 #define WIDTH 5 #define NEWLINE 'n' int main() { int area; area = LENGTH * WIDTH;
- 40. C++ 28 cout << area; cout << NEWLINE; return 0; } When the above code is compiled and executed, it produces the following result: 50 The const Keyword You can use const prefix to declare constants with a specific type as follows: const type variable = value; Following example explains it in detail: #include <iostream> using namespace std; int main() { const int LENGTH = 10; const int WIDTH = 5; const char NEWLINE = 'n'; int area; area = LENGTH * WIDTH; cout << area; cout << NEWLINE; return 0; } When the above code is compiled and executed, it produces the following result: 50 Note that it is a good programming practice to define constants in CAPITALS.
- 41. C++ 29 C++ allows the char, int, and double data types to have modifiers preceding them. A modifier is used to alter the meaning of the base type so that it more precisely fits the needs of various situations. The data type modifiers are listed here: signed unsigned long short The modifiers signed, unsigned, long, and short can be applied to integer base types. In addition, signed and unsigned can be applied to char, and long can be applied to double. The modifiers signed and unsigned can also be used as prefix to long or short modifiers. For example, unsigned long int. C++ allows a shorthand notation for declaring unsigned, short, or long integers. You can simply use the word unsigned, short, or long, without int. It automatically implies int. For example, the following two statements both declare unsigned integer variables. unsigned x; unsigned int y; To understand the difference between the way signed and unsigned integer modifiers are interpreted by C++, you should run the following short program: #include <iostream> using namespace std; /* This program shows the difference between * signed and unsigned integers. */ int main() { short int i; // a signed short integer short unsigned int j; // an unsigned short integer 9. MODIFIER TYPES
- 42. C++ 30 j = 50000; i = j; cout << i << " " << j; return 0; } When this program is run, following is the output: -15536 50000 The above result is because the bit pattern that represents 50,000 as a short unsigned integer is interpreted as -15,536 by a short. Type Qualifiers in C++ The type qualifiers provide additional information about the variables they precede. Qualifier Meaning const Objects of type const cannot be changed by your program during execution volatile The modifier volatile tells the compiler that a variable's value may be changed in ways not explicitly specified by the program. restrict A pointer qualified by restrict is initially the only means by which the object it points to can be accessed. Only C99 adds a new type qualifier called restrict.
- 43. C++ 31 A storage class defines the scope (visibility) and life-time of variables and/or functions within a C++ Program. These specifiers precede the type that they modify. There are following storage classes, which can be used in a C++ Program auto register static extern mutable The auto Storage Class The auto storage class is the default storage class for all local variables. { int mount; auto int month; } The example above defines two variables with the same storage class, auto can only be used within functions, i.e., local variables. The register Storage Class The register storage class is used to define local variables that should be stored in a register instead of RAM. This means that the variable has a maximum size equal to the register size (usually one word) and can't have the unary '&' operator applied to it (as it does not have a memory location). { register int miles; } The register should only be used for variables that require quick access such as counters. It should also be noted that defining 'register' does not mean that the variable will be stored in a register. It means that it MIGHT be stored in a register depending on hardware and implementation restrictions. The static Storage Class 10. STORAGE CLASSES
- 44. C++ 32 The static storage class instructs the compiler to keep a local variable in existence during the life-time of the program instead of creating and destroying it each time it comes into and goes out of scope. Therefore, making local variables static allows them to maintain their values between function calls. The static modifier may also be applied to global variables. When this is done, it causes that variable's scope to be restricted to the file in which it is declared. In C++, when static is used on a class data member, it causes only one copy of that member to be shared by all objects of its class. #include <iostream> // Function declaration void func(void); static int count = 10; /* Global variable */ main() { while(count--) { func(); } return 0; } // Function definition void func( void ) { static int i = 5; // local static variable i++; std::cout << "i is " << i ; std::cout << " and count is " << count << std::endl; } When the above code is compiled and executed, it produces the following result: i is 6 and count is 9 i is 7 and count is 8
- 45. C++ 33 i is 8 and count is 7 i is 9 and count is 6 i is 10 and count is 5 i is 11 and count is 4 i is 12 and count is 3 i is 13 and count is 2 i is 14 and count is 1 i is 15 and count is 0 The extern Storage Class The extern storage class is used to give a reference of a global variable that is visible to ALL the program files. When you use 'extern' the variable cannot be initialized as all it does is point the variable name at a storage location that has been previously defined. When you have multiple files and you define a global variable or function, which will be used in other files also, then extern will be used in another file to give reference of defined variable or function. Just for understanding extern is used to declare a global variable or function in another file. The extern modifier is most commonly used when there are two or more files sharing the same global variables or functions as explained below. First File: main.cpp #include <iostream> int count ; extern void write_extern(); main() { count = 5; write_extern(); } Second File: support.cpp #include <iostream>
- 46. C++ 34 extern int count; void write_extern(void) { std::cout << "Count is " << count << std::endl; } Here, extern keyword is being used to declare count in another file. Now compile these two files as follows: $g++ main.cpp support.cpp -o write This will produce write executable program, try to execute write and check the result as follows: $./write 5 The mutable Storage Class The mutable specifier applies only to class objects, which are discussed later in this tutorial. It allows a member of an object to override const member function. That is, a mutable member can be modified by a const member function.
- 47. C++ 35 An operator is a symbol that tells the compiler to perform specific mathematical or logical manipulations. C++ is rich in built-in operators and provide the following types of operators: Arithmetic Operators Relational Operators Logical Operators Bitwise Operators Assignment Operators Misc Operators This chapter will examine the arithmetic, relational, logical, bitwise, assignment and other operators one by one. Arithmetic Operators There are following arithmetic operators supported by C++ language: Assume variable A holds 10 and variable B holds 20, then: Operator Description Example + Adds two operands A + B will give 30 - Subtracts second operand from the first A - B will give -10 * Multiplies both operands A * B will give 200 / Divides numerator by de- numerator B / A will give 2 % Modulus Operator and remainder of after an integer division B % A will give 0 11. OPERATORS
- 48. C++ 36 ++ Increment operator, increases integer value by one A++ will give 11 -- Decrement operator, decreases integer value by one A-- will give 9 Try the following example to understand all the arithmetic operators available in C++. Copy and paste the following C++ program in test.cpp file and compile and run this program. #include <iostream> using namespace std; main() { int a = 21; int b = 10; int c ; c = a + b; cout << "Line 1 - Value of c is :" << c << endl ; c = a - b; cout << "Line 2 - Value of c is :" << c << endl ; c = a * b; cout << "Line 3 - Value of c is :" << c << endl ; c = a / b; cout << "Line 4 - Value of c is :" << c << endl ; c = a % b; cout << "Line 5 - Value of c is :" << c << endl ; c = a++; cout << "Line 6 - Value of c is :" << c << endl ; c = a--; cout << "Line 7 - Value of c is :" << c << endl ; return 0;
- 49. C++ 37 } When the above code is compiled and executed, it produces the following result: Line 1 - Value of c is :31 Line 2 - Value of c is :11 Line 3 - Value of c is :210 Line 4 - Value of c is :2 Line 5 - Value of c is :1 Line 6 - Value of c is :21 Line 7 - Value of c is :22 Relational Operators There are following relational operators supported by C++ language Assume variable A holds 10 and variable B holds 20, then: Operator Description Example == Checks if the values of two operands are equal or not, if yes then condition becomes true. (A == B) is not true. != Checks if the values of two operands are equal or not, if values are not equal then condition becomes true. (A != B) is true. > Checks if the value of left operand is greater than the value of right operand, if yes then condition becomes true. (A > B) is not true. < Checks if the value of left operand is less than the value of right operand, if yes then condition becomes true. (A < B) is true. >= Checks if the value of left (A >= B) is not true.
- 50. C++ 38 operand is greater than or equal to the value of right operand, if yes then condition becomes true. <= Checks if the value of left operand is less than or equal to the value of right operand, if yes then condition becomes true. (A <= B) is true. Try the following example to understand all the relational operators available in C++. Copy and paste the following C++ program in test.cpp file and compile and run this program. #include <iostream> using namespace std; main() { int a = 21; int b = 10; int c ; if( a == b ) { cout << "Line 1 - a is equal to b" << endl ; } else { cout << "Line 1 - a is not equal to b" << endl ; } if ( a < b ) { cout << "Line 2 - a is less than b" << endl ;
- 51. C++ 39 } else { cout << "Line 2 - a is not less than b" << endl ; } if ( a > b ) { cout << "Line 3 - a is greater than b" << endl ; } else { cout << "Line 3 - a is not greater than b" << endl ; } /* Let's change the values of a and b */ a = 5; b = 20; if ( a <= b ) { cout << "Line 4 - a is either less than or equal to b" << endl ; } if ( b >= a ) { cout << "Line 5 - b is either greater than or equal to b" << endl ; } return 0; } When the above code is compiled and executed, it produces the following result: Line 1 - a is not equal to b Line 2 - a is not less than b Line 3 - a is greater than b Line 4 - a is either less than or equal to b
- 52. C++ 40 Line 5 - b is either greater than or equal to b Logical Operators There are following logical operators supported by C++ language. Assume variable A holds 1 and variable B holds 0, then: Operator Description Example && Called Logical AND operator. If both the operands are non-zero, then condition becomes true. (A && B) is false. || Called Logical OR Operator. If any of the two operands is non- zero, then condition becomes true. (A || B) is true. ! Called Logical NOT Operator. Use to reverses the logical state of its operand. If a condition is true, then Logical NOT operator will make false. !(A && B) is true. Try the following example to understand all the logical operators available in C++. Copy and paste the following C++ program in test.cpp file and compile and run this program. #include <iostream> using namespace std; main() { int a = 5; int b = 20; int c ;
- 53. C++ 41 if ( a && b ) { cout << "Line 1 - Condition is true"<< endl ; } if ( a || b ) { cout << "Line 2 - Condition is true"<< endl ; } /* Let's change the values of a and b */ a = 0; b = 10; if ( a && b ) { cout << "Line 3 - Condition is true"<< endl ; } else { cout << "Line 4 - Condition is not true"<< endl ; } if ( !(a && b) ) { cout << "Line 5 - Condition is true"<< endl ; } return 0; } When the above code is compiled and executed, it produces the following result: Line 1 - Condition is true Line 2 - Condition is true Line 4 - Condition is not true Line 5 - Condition is true Bitwise Operators
- 54. C++ 42 Bitwise operator works on bits and perform bit-by-bit operation. The truth tables for &, |, and ^ are as follows: p q p & q p | q p ^ q 0 0 0 0 0 0 1 0 1 1 1 1 1 1 0 1 0 0 1 1 Assume if A = 60; and B = 13; now in binary format they will be as follows: A = 0011 1100 B = 0000 1101 ----------------- A&B = 0000 1100 A|B = 0011 1101 A^B = 0011 0001 ~A = 1100 0011 The Bitwise operators supported by C++ language are listed in the following table. Assume variable A holds 60 and variable B holds 13, then: Operator Description Example & Binary AND Operator copies a bit to the result if it exists in both operands. (A & B) will give 12 which is 0000 1100 | Binary OR Operator copies a bit if it exists in either operand. (A | B) will give 61 which is 0011 1101 ^ Binary XOR Operator copies the bit if it is set in one operand but not both. (A ^ B) will give 49 which is 0011 0001
- 55. C++ 43 ~ Binary Ones Complement Operator is unary and has the effect of 'flipping' bits. (~A ) will give -61 which is 1100 0011 in 2's complement form due to a signed binary number. << Binary Left Shift Operator. The left operands value is moved left by the number of bits specified by the right operand. A << 2 will give 240 which is 1111 0000 >> Binary Right Shift Operator. The left operands value is moved right by the number of bits specified by the right operand. A >> 2 will give 15 which is 0000 1111 Try the following example to understand all the bitwise operators available in C++. Copy and paste the following C++ program in test.cpp file and compile and run this program. #include <iostream> using namespace std; main() { unsigned int a = 60; // 60 = 0011 1100 unsigned int b = 13; // 13 = 0000 1101 int c = 0; c = a & b; // 12 = 0000 1100 cout << "Line 1 - Value of c is : " << c << endl ; c = a | b; // 61 = 0011 1101 cout << "Line 2 - Value of c is: " << c << endl ; c = a ^ b; // 49 = 0011 0001 cout << "Line 3 - Value of c is: " << c << endl ;
- 56. C++ 44 c = ~a; // -61 = 1100 0011 cout << "Line 4 - Value of c is: " << c << endl ; c = a << 2; // 240 = 1111 0000 cout << "Line 5 - Value of c is: " << c << endl ; c = a >> 2; // 15 = 0000 1111 cout << "Line 6 - Value of c is: " << c << endl ; return 0; } When the above code is compiled and executed, it produces the following result: Line 1 - Value of c is : 12 Line 2 - Value of c is: 61 Line 3 - Value of c is: 49 Line 4 - Value of c is: -61 Line 5 - Value of c is: 240 Line 6 - Value of c is: 15 Assignment Operators There are following assignment operators supported by C++ language: Operator Description Example = Simple assignment operator, Assigns values from right side operands to left side operand. C = A + B will assign value of A + B into C += Add AND assignment operator, It adds right operand to the left operand and assign the result to left operand. C += A is equivalent to C = C + A
- 57. C++ 45 -= Subtract AND assignment operator, It subtracts right operand from the left operand and assign the result to left operand. C -= A is equivalent to C = C - A *= Multiply AND assignment operator, It multiplies right operand with the left operand and assign the result to left operand. C *= A is equivalent to C = C * A /= Divide AND assignment operator, It divides left operand with the right operand and assign the result to left operand. C /= A is equivalent to C = C / A %= Modulus AND assignment operator, It takes modulus using two operands and assign the result to left operand. C %= A is equivalent to C = C % A <<= Left shift AND assignment operator. C <<= 2 is same as C = C << 2 >>= Right shift AND assignment operator. C >>= 2 is same as C = C >> 2 &= Bitwise AND assignment operator. C &= 2 is same as C = C & 2 ^= Bitwise exclusive OR and assignment operator. C ^= 2 is same as C = C ^ 2 |= Bitwise inclusive OR and assignment operator. C |= 2 is same as C = C | 2 Try the following example to understand all the assignment operators available in C++.
- 58. C++ 46 Copy and paste the following C++ program in test.cpp file and compile and run this program. #include <iostream> using namespace std; main() { int a = 21; int c ; c = a; cout << "Line 1 - = Operator, Value of c = : " <<c<< endl ; c += a; cout << "Line 2 - += Operator, Value of c = : " <<c<< endl ; c -= a; cout << "Line 3 - -= Operator, Value of c = : " <<c<< endl ; c *= a; cout << "Line 4 - *= Operator, Value of c = : " <<c<< endl ; c /= a; cout << "Line 5 - /= Operator, Value of c = : " <<c<< endl ; c = 200; c %= a; cout << "Line 6 - %= Operator, Value of c = : " <<c<< endl ; c <<= 2; cout << "Line 7 - <<= Operator, Value of c = : " <<c<< endl ; c >>= 2; cout << "Line 8 - >>= Operator, Value of c = : " <<c<< endl ;
- 59. C++ 47 c &= 2; cout << "Line 9 - &= Operator, Value of c = : " <<c<< endl ; c ^= 2; cout << "Line 10 - ^= Operator, Value of c = : " <<c<< endl ; c |= 2; cout << "Line 11 - |= Operator, Value of c = : " <<c<< endl ; return 0; } When the above code is compiled and executed, it produces the following result: Line 1 - = Operator, Value of c = : 21 Line 2 - += Operator, Value of c = : 42 Line 3 - -= Operator, Value of c = : 21 Line 4 - *= Operator, Value of c = : 441 Line 5 - /= Operator, Value of c = : 21 Line 6 - %= Operator, Value of c = : 11 Line 7 - <<= Operator, Value of c = : 44 Line 8 - >>= Operator, Value of c = : 11 Line 9 - &= Operator, Value of c = : 2 Line 10 - ^= Operator, Value of c = : 0 Line 11 - |= Operator, Value of c = : 2 Misc Operators The following table lists some other operators that C++ supports. Operator Description sizeof sizeof operator returns the size of a variable. For example, sizeof(a), where ‘a’ is integer, and will return 4.
- 60. C++ 48 Condition ? X : Y Conditional operator (?). If Condition is true then it returns value of X otherwise returns value of Y. , Comma operator causes a sequence of operations to be performed. The value of the entire comma expression is the value of the last expression of the comma-separated list. . (dot) and -> (arrow) Member operators are used to reference individual members of classes, structures, and unions. Cast Casting operators convert one data type to another. For example, int(2.2000) would return 2. & Pointer operator ‘&’ returns the address of a variable. For example &a; will give actual address of the variable. * Pointer operator * is pointer to a variable. For example *var; will pointer to a variable var. Operators Precedence in C++ Operator precedence determines the grouping of terms in an expression. This affects how an expression is evaluated. Certain operators have higher precedence than others; for example, the multiplication operator has higher precedence than the addition operator: For example x = 7 + 3 * 2; here, x is assigned 13, not 20 because operator * has higher precedence than +, so it first gets multiplied with 3*2 and then adds into 7. Here, operators with the highest precedence appear at the top of the table, those with the lowest appear at the bottom. Within an expression, higher precedence operators will be evaluated first. Category Operator Associativity Postfix () [] -> . ++ - - Left to right Unary + - ! ~ ++ - - (type)* & sizeof Right to left Multiplicative * / % Left to right
- 61. C++ 49 Additive + - Left to right Shift << >> Left to right Relational < <= > >= Left to right Equality == != Left to right Bitwise AND & Left to right Bitwise XOR ^ Left to right Bitwise OR | Left to right Logical AND && Left to right Logical OR || Left to right Conditional ?: Right to left Assignment = += -= *= /= %=>>= <<= &= ^= |= Right to left Comma , Left to right Try the following example to understand operator’s precedence concept available in C++. Copy and paste the following C++ program in test.cpp file and compile and run this program. Check the simple difference with and without parenthesis. This will produce different results because (), /, * and + have different precedence. Higher precedence operators will be evaluated first: #include <iostream> using namespace std; main() { int a = 20;
- 62. C++ 50 int b = 10; int c = 15; int d = 5; int e; e = (a + b) * c / d; // ( 30 * 15 ) / 5 cout << "Value of (a + b) * c / d is :" << e << endl ; e = ((a + b) * c) / d; // (30 * 15 ) / 5 cout << "Value of ((a + b) * c) / d is :" << e << endl ; e = (a + b) * (c / d); // (30) * (15/5) cout << "Value of (a + b) * (c / d) is :" << e << endl ; e = a + (b * c) / d; // 20 + (150/5) cout << "Value of a + (b * c) / d is :" << e << endl ; return 0; } When the above code is compiled and executed, it produces the following result: Value of (a + b) * c / d is :90 Value of ((a + b) * c) / d is :90 Value of (a + b) * (c / d) is :90 Value of a + (b * c) / d is :50
- 63. C++ 51 There may be a situation, when you need to execute a block of code several number of times. In general, statements are executed sequentially: The first statement in a function is executed first, followed by the second, and so on. Programming languages provide various control structures that allow for more complicated execution paths. A loop statement allows us to execute a statement or group of statements multiple times and following is the general from of a loop statement in most of the programming languages: C++ programming language provides the following type of loops to handle looping requirements. Loop Type Description while loop Repeats a statement or group of statements while a given condition is true. It tests the condition before executing the loop body. for loop Execute a sequence of statements multiple times and abbreviates the code that manages the loop variable. 12. LOOP TYPES
- 64. C++ 52 do...while loop Like a ‘while’ statement, except that it tests the condition at the end of the loop body. nested loops You can use one or more loop inside any another ‘while’, ‘for’ or ‘do..while’ loop. While Loop A while loop statement repeatedly executes a target statement as long as a given condition is true. Syntax The syntax of a while loop in C++ is: while(condition) { statement(s); } Here, statement(s) may be a single statement or a block of statements. The condition may be any expression, and true is any non-zero value. The loop iterates while the condition is true. When the condition becomes false, program control passes to the line immediately following the loop. Flow Diagram
- 65. C++ 53 Here, key point of the while loop is that the loop might not ever run. When the condition is tested and the result is false, the loop body will be skipped and the first statement after the while loop will be executed. Example #include <iostream> using namespace std; int main () { // Local variable declaration: int a = 10; // while loop execution while( a < 20 ) { cout << "value of a: " << a << endl; a++; }
- 66. C++ 54 return 0; } When the above code is compiled and executed, it produces the following result: value of a: 10 value of a: 11 value of a: 12 value of a: 13 value of a: 14 value of a: 15 value of a: 16 value of a: 17 value of a: 18 value of a: 19 for Loop A for loop is a repetition control structure that allows you to efficiently write a loop that needs to execute a specific number of times. Syntax The syntax of a for loop in C++ is: for ( init; condition; increment ) { statement(s); } Here is the flow of control in a for loop: 1. The init step is executed first, and only once. This step allows you to declare and initialize any loop control variables. You are not required to put a statement here, as long as a semicolon appears. 2. Next, the condition is evaluated. If it is true, the body of the loop is executed. If it is false, the body of the loop does not execute and flow of control jumps to the next statement just after the for loop. 3. After the body of the for loop executes, the flow of control jumps back up to the increment statement. This statement allows you to update any
- 67. C++ 55 loop control variables. This statement can be left blank, as long as a semicolon appears after the condition. 4. The condition is now evaluated again. If it is true, the loop executes and the process repeats itself (body of loop, then increment step, and then again condition). After the condition becomes false, the for loop terminates. Flow Diagram Example #include <iostream> using namespace std; int main () { // for loop execution for( int a = 10; a < 20; a = a + 1 )
- 68. C++ 56 { cout << "value of a: " << a << endl; } return 0; } When the above code is compiled and executed, it produces the following result: value of a: 10 value of a: 11 value of a: 12 value of a: 13 value of a: 14 value of a: 15 value of a: 16 value of a: 17 value of a: 18 value of a: 19 do…while Loop Unlike for and while loops, which test the loop condition at the top of the loop, the do...while loop checks its condition at the bottom of the loop. A do...while loop is similar to a while loop, except that a do...while loop is guaranteed to execute at least one time. Syntax The syntax of a do...while loop in C++ is: do { statement(s); }while( condition ); Notice that the conditional expression appears at the end of the loop, so the statement(s) in the loop execute once before the condition is tested.
- 69. C++ 57 If the condition is true, the flow of control jumps back up to do, and the statement(s) in the loop execute again. This process repeats until the given condition becomes false. Flow Diagram Example #include <iostream> using namespace std; int main () { // Local variable declaration: int a = 10; // do loop execution do { cout << "value of a: " << a << endl; a = a + 1; }while( a < 20 ); return 0;
- 70. C++ 58 } When the above code is compiled and executed, it produces the following result: value of a: 10 value of a: 11 value of a: 12 value of a: 13 value of a: 14 value of a: 15 value of a: 16 value of a: 17 value of a: 18 value of a: 19 nested Loops A loop can be nested inside of another loop. C++ allows at least 256 levels of nesting. Syntax The syntax for a nested for loop statement in C++ is as follows: for ( init; condition; increment ) { for ( init; condition; increment ) { statement(s); } statement(s); // you can put more statements. } The syntax for a nested while loop statement in C++ is as follows: while(condition) { while(condition) { statement(s);
- 71. C++ 59 } statement(s); // you can put more statements. } The syntax for a nested do...while loop statement in C++ is as follows: do { statement(s); // you can put more statements. do { statement(s); }while( condition ); }while( condition ); Example The following program uses a nested for loop to find the prime numbers from 2 to 100: #include <iostream> using namespace std; int main () { int i, j; for(i=2; i<100; i++) { for(j=2; j <= (i/j); j++) if(!(i%j)) break; // if factor found, not prime if(j > (i/j)) cout << i << " is primen"; } return 0; } This would produce the following result:
- 72. C++ 60 2 is prime 3 is prime 5 is prime 7 is prime 11 is prime 13 is prime 17 is prime 19 is prime 23 is prime 29 is prime 31 is prime 37 is prime 41 is prime 43 is prime 47 is prime 53 is prime 59 is prime 61 is prime 67 is prime 71 is prime 73 is prime 79 is prime 83 is prime 89 is prime 97 is prime Loop Control Statements Loop control statements change execution from its normal sequence. When execution leaves a scope, all automatic objects that were created in that scope are destroyed. C++ supports the following control statements. Control Statement Description break statement Terminates the loop or switch statement and transfers execution to the statement immediately
- 73. C++ 61 following the loop or switch. continue statement Causes the loop to skip the remainder of its body and immediately retest its condition prior to reiterating. goto statement Transfers control to the labeled statement. Though it is not advised to use goto statement in your program. Break Statement The break statement has the following two usages in C++: When the break statement is encountered inside a loop, the loop is immediately terminated and program control resumes at the next statement following the loop. It can be used to terminate a case in the switch statement (covered in the next chapter). If you are using nested loops (i.e., one loop inside another loop), the break statement will stop the execution of the innermost loop and start executing the next line of code after the block. Syntax The syntax of a break statement in C++ is: break; Flow Diagram
- 74. C++ 62 Example #include <iostream> using namespace std; int main () { // Local variable declaration: int a = 10; // do loop execution do { cout << "value of a: " << a << endl; a = a + 1; if( a > 15) { // terminate the loop break; } }while( a < 20 );
- 75. C++ 63 return 0; } When the above code is compiled and executed, it produces the following result: value of a: prettyprint notranslate10 value of a: 11 value of a: 12 value of a: 13 value of a: 14 value of a: 15 continue Statement The continue statement works somewhat like the break statement. Instead of forcing termination, however, continue forces the next iteration of the loop to take place, skipping any code in between. For the for loop, continue causes the conditional test and increment portions of the loop to execute. For the while and do...while loops, program control passes to the conditional tests. Syntax The syntax of a continue statement in C++ is: continue; Flow Diagram
- 76. C++ 64 Example #include <iostream> using namespace std; int main () { // Local variable declaration: int a = 10; // do loop execution do { if( a == 15) { // skip the iteration. a = a + 1; continue; } cout << "value of a: " << a << endl; a = a + 1; }while( a < 20 );
- 77. C++ 65 return 0; } When the above code is compiled and executed, it produces the following result: value of a: 10 value of a: 11 value of a: 12 value of a: 13 value of a: 14 value of a: 16 value of a: 17 value of a: 18 value of a: 19 goto Statement A goto statement provides an unconditional jump from the goto to a labeled statement in the same function. NOTE: Use of goto statement is highly discouraged because it makes difficult to trace the control flow of a program, making the program hard to understand and hard to modify. Any program that uses a goto can be rewritten so that it doesn't need the goto. Syntax The syntax of a goto statement in C++ is: goto label; .. . label: statement; Where label is an identifier that identifies a labeled statement. A labeled statement is any statement that is preceded by an identifier followed by a colon (:). Flow Diagram
- 78. C++ 66 Example #include <iostream> using namespace std; int main () { // Local variable declaration: int a = 10; // do loop execution LOOP:do { if( a == 15) { // skip the iteration. a = a + 1; goto LOOP; } cout << "value of a: " << a << endl; a = a + 1; }while( a < 20 );
- 79. C++ 67 return 0; } When the above code is compiled and executed, it produces the following result: value of a: 10 value of a: 11 value of a: 12 value of a: 13 value of a: 14 value of a: 16 value of a: 17 value of a: 18 value of a: 19 One good use of goto is to exit from a deeply nested routine. For example, consider the following code fragment: for(...) { for(...) { while(...) { if(...) goto stop; . . . } } } stop: cout << "Error in program.n"; Eliminating the goto would force a number of additional tests to be performed. A simplebreak statement would not work here, because it would only cause the program to exit from the innermost loop. The Infinite Loop A loop becomes infinite loop if a condition never becomes false. The for loop is traditionally used for this purpose. Since none of the three expressions that form
- 80. C++ 68 the ‘for’ loop are required, you can make an endless loop by leaving the conditional expression empty. #include <iostream> using namespace std; int main () { for( ; ; ) { printf("This loop will run forever.n"); } return 0; } When the conditional expression is absent, it is assumed to be true. You may have an initialization and increment expression, but C++ programmers more commonly use the ‘for (;;)’ construct to signify an infinite loop. NOTE: You can terminate an infinite loop by pressing Ctrl + C keys.
- 81. C++ 69 Decision making structures require that the programmer specify one or more conditions to be evaluated or tested by the program, along with a statement or statements to be executed if the condition is determined to be true, and optionally, other statements to be executed if the condition is determined to be false. Following is the general from of a typical decision making structure found in most of the programming languages: C++ programming language provides following types of decision making statements. Statement Description if statement An ‘if’ statement consists of a boolean expression followed by one or more statements. if...else statement An ‘if’ statement can be followed by an optional ‘else’ statement, which executes when the boolean expression is false. switch statement A ‘switch’ statement allows a variable to be tested 13. DECISION-MAKING STATEMENTS
- 82. C++ 70 for equality against a list of values. nested if statements You can use one ‘if’ or ‘else if’ statement inside another ‘if’ or ‘else if’ statement(s). nested switch statements You can use one ‘switch’ statement inside another ‘switch’ statement(s). If Statement An if statement consists of a boolean expression followed by one or more statements. Syntax The syntax of an if statement in C++ is: if(boolean_expression) { // statement(s) will execute if the boolean expression is true } If the boolean expression evaluates to true, then the block of code inside the if statement will be executed. If boolean expression evaluates to false, then the first set of code after the end of the if statement (after the closing curly brace) will be executed. Flow Diagram
- 83. C++ 71 Example #include <iostream> using namespace std; int main () { // local variable declaration: int a = 10; // check the boolean condition if( a < 20 ) { // if condition is true then print the following cout << "a is less than 20;" << endl; } cout << "value of a is : " << a << endl; return 0; }
- 84. C++ 72 When the above code is compiled and executed, it produces the following result: a is less than 20; value of a is : 10 if…else Statement An if statement can be followed by an optional else statement, which executes when the boolean expression is false. Syntax The syntax of an if...else statement in C++ is: if(boolean_expression) { // statement(s) will execute if the boolean expression is true } else { // statement(s) will execute if the boolean expression is false } If the boolean expression evaluates to true, then the if block of code will be executed, otherwise else block of code will be executed. Flow Diagram
- 85. C++ 73 Example #include <iostream> using namespace std; int main () { // local variable declaration: int a = 100; // check the boolean condition if( a < 20 ) { // if condition is true then print the following cout << "a is less than 20;" << endl; } else { // if condition is false then print the following cout << "a is not less than 20;" << endl; } cout << "value of a is : " << a << endl; return 0; } When the above code is compiled and executed, it produces the following result: a is not less than 20; value of a is : 100 if...else if...else Statement An if statement can be followed by an optional else if...else statement, which is very usefull to test various conditions using single if...else if statement. When using if , else if , else statements there are few points to keep in mind. An if can have zero or one else's and it must come after any else if's.
- 86. C++ 74 An if can have zero to many else if's and they must come before the else. Once an else if succeeds, none of he remaining else if's or else's will be tested. Syntax The syntax of an if...else if...else statement in C++ is: if(boolean_expression 1) { // Executes when the boolean expression 1 is true } else if( boolean_expression 2) { // Executes when the boolean expression 2 is true } else if( boolean_expression 3) { // Executes when the boolean expression 3 is true } else { // executes when the none of the above condition is true. } Example #include <iostream> using namespace std; int main () { // local variable declaration: int a = 100; // check the boolean condition if( a == 10 )
- 87. C++ 75 { // if condition is true then print the following cout << "Value of a is 10" << endl; } else if( a == 20 ) { // if else if condition is true cout << "Value of a is 20" << endl; } else if( a == 30 ) { // if else if condition is true cout << "Value of a is 30" << endl; } else { // if none of the conditions is true cout << "Value of a is not matching" << endl; } cout << "Exact value of a is : " << a << endl; return 0; } When the above code is compiled and executed, it produces the following result: Value of a is not matching Exact value of a is : 100 Switch Statement A switch statement allows a variable to be tested for equality against a list of values. Each value is called a case, and the variable being switched on is checked for each case. Syntax The syntax for a switch statement in C++ is as follows:
- 88. C++ 76 switch(expression){ case constant-expression : statement(s); break; //optional case constant-expression : statement(s); break; //optional // you can have any number of case statements. default : //Optional statement(s); } The following rules apply to a switch statement: The expression used in a switch statement must have an integral or enumerated type, or be of a class type in which the class has a single conversion function to an integral or enumerated type. You can have any number of case statements within a switch. Each case is followed by the value to be compared to and a colon. The constant-expression for a case must be the same data type as the variable in the switch, and it must be a constant or a literal. When the variable being switched on is equal to a case, the statements following that case will execute until a break statement is reached. When a break statement is reached, the switch terminates, and the flow of control jumps to the next line following the switch statement. Not every case needs to contain a break. If no break appears, the flow of control will fall through to subsequent cases until a break is reached. A switch statement can have an optional default case, which must appear at the end of the switch. The default case can be used for performing a task when none of the cases is true. No break is needed in the default case. Flow Diagram
- 89. C++ 77 Example #include <iostream> using namespace std; int main () { // local variable declaration: char grade = 'D'; switch(grade) { case 'A' : cout << "Excellent!" << endl; break; case 'B' : case 'C' : cout << "Well done" << endl; break;
- 90. C++ 78 case 'D' : cout << "You passed" << endl; break; case 'F' : cout << "Better try again" << endl; break; default : cout << "Invalid grade" << endl; } cout << "Your grade is " << grade << endl; return 0; } This would produce the following result: You passed Your grade is D Nested if Statement It is always legal to nest if-else statements, which means you can use one if or else if statement inside another if or else if statement(s). Syntax The syntax for a nested if statement is as follows: if( boolean_expression 1) { // Executes when the boolean expression 1 is true if(boolean_expression 2) { // Executes when the boolean expression 2 is true } } You can nest else if...else in the similar way as you have nested if statement.
- 91. C++ 79 Example #include <iostream> using namespace std; int main () { // local variable declaration: int a = 100; int b = 200; // check the boolean condition if( a == 100 ) { // if condition is true then check the following if( b == 200 ) { // if condition is true then print the following cout << "Value of a is 100 and b is 200" << endl; } } cout << "Exact value of a is : " << a << endl; cout << "Exact value of b is : " << b << endl; return 0; } When the above code is compiled and executed, it produces the following result: Value of a is 100 and b is 200 Exact value of a is : 100 Exact value of b is : 200 Nested switch Statements It is possible to have a switch as part of the statement sequence of an outer switch. Even if the case constants of the inner and outer switch contain common values, no conflicts will arise.
- 92. C++ 80 C++ specifies that at least 256 levels of nesting be allowed for switch statements. Syntax The syntax for a nested switch statement is as follows: switch(ch1) { case 'A': cout << "This A is part of outer switch"; switch(ch2) { case 'A': cout << "This A is part of inner switch"; break; case 'B': // ... } break; case 'B': // ... } Example #include <iostream> using namespace std; int main () { // local variable declaration: int a = 100; int b = 200; switch(a) { case 100: cout << "This is part of outer switch" << endl; switch(b) { case 200: cout << "This is part of inner switch" << endl;
- 93. C++ 81 } } cout << "Exact value of a is : " << a << endl; cout << "Exact value of b is : " << b << endl; return 0; } This would produce the following result: This is part of outer switch This is part of inner switch Exact value of a is : 100 Exact value of b is : 200 The ? : Operator We have covered conditional operator “? :” in previous chapter which can be used to replace if...else statements. It has the following general form: Exp1 ? Exp2 : Exp3; Exp1, Exp2, and Exp3 are expressions. Notice the use and placement of the colon. The value of a ‘?’ expression is determined like this: Exp1 is evaluated. If it is true, then Exp2 is evaluated and becomes the value of the entire ‘?’ expression. If Exp1 is false, then Exp3 is evaluated and its value becomes the value of the expression.
- 94. C++ 82 A function is a group of statements that together perform a task. Every C++ program has at least one function, which is main(), and all the most trivial programs can define additional functions. You can divide up your code into separate functions. How you divide up your code among different functions is up to you, but logically the division usually is such that each function performs a specific task. A function declaration tells the compiler about a function's name, return type, and parameters. A function definition provides the actual body of the function. The C++ standard library provides numerous built-in functions that your program can call. For example, function strcat() to concatenate two strings, function memcpy() to copy one memory location to another location, and many more functions. A function is known with various names like a method or a sub-routine or a procedure etc. Defining a Function The general form of a C++ function definition is as follows: return_type function_name( parameter list ) { body of the function } A C++ function definition consists of a function header and a function body. Here are all the parts of a function: Return Type: A function may return a value. The return_type is the data type of the value the function returns. Some functions perform the desired operations without returning a value. In this case, the return_type is the keyword void. Function Name: This is the actual name of the function. The function name and the parameter list together constitute the function signature. Parameters: A parameter is like a placeholder. When a function is invoked, you pass a value to the parameter. This value is referred to as actual parameter or argument. The parameter list refers to the type, order, and number of the parameters of a function. Parameters are optional; that is, a function may contain no parameters. 14. FUNCTIONS
- 95. C++ 83 Function Body: The function body contains a collection of statements that define what the function does. Example: Following is the source code for a function called max(). This function takes two parameters num1 and num2 and returns the maximum between the two: // function returning the max between two numbers int max(int num1, int num2) { // local variable declaration int result; if (num1 > num2) result = num1; else result = num2; return result; } Function Declarations A function declaration tells the compiler about a function name and how to call the function. The actual body of the function can be defined separately. A function declaration has the following parts: return_type function_name( parameter list ); For the above defined function max(), following is the function declaration: int max(int num1, int num2); Parameter names are not important in function declaration only their type is required, so following is also valid declaration: int max(int, int); Function declaration is required when you define a function in one source file and you call that function in another file. In such case, you should declare the function at the top of the file calling the function.
- 96. C++ 84 Calling a Function While creating a C++ function, you give a definition of what the function has to do. To use a function, you will have to call or invoke that function. When a program calls a function, program control is transferred to the called function. A called function performs defined task and when it’s return statement is executed or when its function-ending closing brace is reached, it returns program control back to the main program. To call a function, you simply need to pass the required parameters along with function name, and if function returns a value, then you can store returned value. For example: #include <iostream> using namespace std; // function declaration int max(int num1, int num2); int main () { // local variable declaration: int a = 100; int b = 200; int ret; // calling a function to get max value. ret = max(a, b); cout << "Max value is : " << ret << endl; return 0; } // function returning the max between two numbers int max(int num1, int num2) { // local variable declaration
- 97. C++ 85 int result; if (num1 > num2) result = num1; else result = num2; return result; } I kept max() function along with main() function and compiled the source code. While running final executable, it would produce the following result: Max value is : 200 Function Arguments If a function is to use arguments, it must declare variables that accept the values of the arguments. These variables are called the formal parameters of the function. The formal parameters behave like other local variables inside the function and are created upon entry into the function and destroyed upon exit. While calling a function, there are two ways that arguments can be passed to a function: Call Type Description Call by value This method copies the actual value of an argument into the formal parameter of the function. In this case, changes made to the parameter inside the function have no effect on the argument. Call by pointer This method copies the address of an argument into the formal parameter. Inside the function, the address is used to access the actual argument used in the call. This means that changes made to the parameter affect the argument. Call by reference This method copies the reference of an argument into the formal parameter. Inside the function, the reference is used to access the actual argument used
- 98. C++ 86 in the call. This means that changes made to the parameter affect the argument. Call by Value The call by value method of passing arguments to a function copies the actual value of an argument into the formal parameter of the function. In this case, changes made to the parameter inside the function have no effect on the argument. By default, C++ uses call by value to pass arguments. In general, this means that code within a function cannot alter the arguments used to call the function. Consider the function swap() definition as follows. // function definition to swap the values. void swap(int x, int y) { int temp; temp = x; /* save the value of x */ x = y; /* put y into x */ y = temp; /* put x into y */ return; } Now, let us call the function swap() by passing actual values as in the following example: #include <iostream> using namespace std; // function declaration void swap(int x, int y); int main () { // local variable declaration: int a = 100;
- 99. C++ 87 int b = 200; cout << "Before swap, value of a :" << a << endl; cout << "Before swap, value of b :" << b << endl; // calling a function to swap the values. swap(a, b); cout << "After swap, value of a :" << a << endl; cout << "After swap, value of b :" << b << endl; return 0; } When the above code is put together in a file, compiled and executed, it produces the following result: Before swap, value of a :100 Before swap, value of b :200 After swap, value of a :100 After swap, value of b :200 Which shows that there is no change in the values though they had been changed inside the function. Call by Pointer The call by pointer method of passing arguments to a function copies the address of an argument into the formal parameter. Inside the function, the address is used to access the actual argument used in the call. This means that changes made to the parameter affect the passed argument. To pass the value by pointer, argument pointers are passed to the functions just like any other value. So accordingly you need to declare the function parameters as pointer types as in the following function swap(), which exchanges the values of the two integer variables pointed to by its arguments. // function definition to swap the values. void swap(int *x, int *y) { int temp;
- 100. C++ 88 temp = *x; /* save the value at address x */ *x = *y; /* put y into x */ *y = temp; /* put x into y */ return; } To check the more detail about C++ pointers, kindly check C++ Pointers chapter. For now, let us call the function swap() by passing values by pointer as in the following example: #include <iostream> using namespace std; // function declaration void swap(int *x, int *y); int main () { // local variable declaration: int a = 100; int b = 200; cout << "Before swap, value of a :" << a << endl; cout << "Before swap, value of b :" << b << endl; /* calling a function to swap the values. * &a indicates pointer to a ie. address of variable a and * &b indicates pointer to b ie. address of variable b. */ swap(&a, &b); cout << "After swap, value of a :" << a << endl; cout << "After swap, value of b :" << b << endl;
- 101. C++ 89 return 0; } When the above code is put together in a file, compiled and executed, it produces the following result: Before swap, value of a :100 Before swap, value of b :200 After swap, value of a :200 After swap, value of b :100 Call by Reference The call by reference method of passing arguments to a function copies the reference of an argument into the formal parameter. Inside the function, the reference is used to access the actual argument used in the call. This means that changes made to the parameter affect the passed argument. To pass the value by reference, argument reference is passed to the functions just like any other value. So accordingly you need to declare the function parameters as reference types as in the following function swap(), which exchanges the values of the two integer variables pointed to by its arguments. // function definition to swap the values. void swap(int &x, int &y) { int temp; temp = x; /* save the value at address x */ x = y; /* put y into x */ y = temp; /* put x into y */ return; } For now, let us call the function swap() by passing values by reference as in the following example: #include <iostream> using namespace std; // function declaration void swap(int &x, int &y);
- 102. C++ 90 int main () { // local variable declaration: int a = 100; int b = 200; cout << "Before swap, value of a :" << a << endl; cout << "Before swap, value of b :" << b << endl; /* calling a function to swap the values using variable reference.*/ swap(a, b); cout << "After swap, value of a :" << a << endl; cout << "After swap, value of b :" << b << endl; return 0; } When the above code is put together in a file, compiled and executed, it produces the following result: Before swap, value of a :100 Before swap, value of b :200 After swap, value of a :200 After swap, value of b :100 By default, C++ uses call by value to pass arguments. In general, this means that code within a function cannot alter the arguments used to call the function and above mentioned example while calling max() function used the same method. Default Values for Parameters When you define a function, you can specify a default value for each of the last parameters. This value will be used if the corresponding argument is left blank when calling to the function.
- 103. C++ 91 This is done by using the assignment operator and assigning values for the arguments in the function definition. If a value for that parameter is not passed when the function is called, the default given value is used, but if a value is specified, this default value is ignored and the passed value is used instead. Consider the following example: #include <iostream> using namespace std; int sum(int a, int b=20) { int result; result = a + b; return (result); } int main () { // local variable declaration: int a = 100; int b = 200; int result; // calling a function to add the values. result = sum(a, b); cout << "Total value is :" << result << endl; // calling a function again as follows. result = sum(a); cout << "Total value is :" << result << endl; return 0; } When the above code is compiled and executed, it produces the following result:
- 104. C++ 92 Total value is :300 Total value is :120
- 105. C++ 93 Normally, when we work with Numbers, we use primitive data types such as int, short, long, float and double, etc. The number data types, their possible values and number ranges have been explained while discussing C++ Data Types. Defining Numbers in C++ You have already defined numbers in various examples given in previous chapters. Here is another consolidated example to define various types of numbers in C++: #include <iostream> using namespace std; int main () { // number definition: short s; int i; long l; float f; double d; // number assignments; s = 10; i = 1000; l = 1000000; f = 230.47; d = 30949.374; // number printing; cout << "short s :" << s << endl; cout << "int i :" << i << endl; cout << "long l :" << l << endl; cout << "float f :" << f << endl; 15. NUMBERS
- 106. C++ 94 cout << "double d :" << d << endl; return 0; } When the above code is compiled and executed, it produces the following result: short s :10 int i :1000 long l :1000000 float f :230.47 double d :30949.4 Math Operations in C++ In addition to the various functions you can create, C++ also includes some useful functions you can use. These functions are available in standard C and C++ libraries and called built-in functions. These are functions that can be included in your program and then use. C++ has a rich set of mathematical operations, which can be performed on various numbers. Following table lists down some useful built-in mathematical functions available in C++. To utilize these functions you need to include the math header file <cmath>. S.N. Function & Purpose 1 double cos(double); This function takes an angle (as a double) and returns the cosine. 2 double sin(double); This function takes an angle (as a double) and returns the sine. 3 double tan(double); This function takes an angle (as a double) and returns the tangent. 4 double log(double); This function takes a number and returns the natural log of that number.
- 107. C++ 95 5 double pow(double, double); The first is a number you wish to raise and the second is the power you wish to raise it t 6 double hypot(double, double); If you pass this function the length of two sides of a right triangle, it will return you the length of the hypotenuse. 7 double sqrt(double); You pass this function a number and it gives you the square root. 8 int abs(int); This function returns the absolute value of an integer that is passed to it. 9 double fabs(double); This function returns the absolute value of any decimal number passed to it. 10 double floor(double); Finds the integer which is less than or equal to the argument passed to it. Following is a simple example to show few of the mathematical operations: #include <iostream> #include <cmath> using namespace std; int main () { // number definition: short s = 10; int i = -1000; long l = 100000; float f = 230.47;
- 108. C++ 96 double d = 200.374; // mathematical operations; cout << "sin(d) :" << sin(d) << endl; cout << "abs(i) :" << abs(i) << endl; cout << "floor(d) :" << floor(d) << endl; cout << "sqrt(f) :" << sqrt(f) << endl; cout << "pow( d, 2) :" << pow(d, 2) << endl; return 0; } When the above code is compiled and executed, it produces the following result: sign(d) :-0.634939 abs(i) :1000 floor(d) :200 sqrt(f) :15.1812 pow( d, 2 ) :40149.7 Random Numbers in C++ There are many cases where you will wish to generate a random number. There are actually two functions you will need to know about random number generation. The first is rand(), this function will only return a pseudo random number. The way to fix this is to first call the srand() function. Following is a simple example to generate few random numbers. This example makes use of time() function to get the number of seconds on your system time, to randomly seed the rand() function: #include <iostream> #include <ctime> #include <cstdlib> using namespace std; int main () { int i,j;
- 109. C++ 97 // set the seed srand( (unsigned)time( NULL ) ); /* generate 10 random numbers. */ for( i = 0; i < 10; i++ ) { // generate actual random number j= rand(); cout <<" Random Number : " << j << endl; } return 0; } When the above code is compiled and executed, it produces the following result: Random Number : 1748144778 Random Number : 630873888 Random Number : 2134540646 Random Number : 219404170 Random Number : 902129458 Random Number : 920445370 Random Number : 1319072661 Random Number : 257938873 Random Number : 1256201101 Random Number : 580322989
- 110. C++ 98 C++ provides a data structure, the array, which stores a fixed-size sequential collection of elements of the same type. An array is used to store a collection of data, but it is often more useful to think of an array as a collection of variables of the same type. Instead of declaring individual variables, such as number0, number1, ..., and number99, you declare one array variable such as numbers and use numbers[0], numbers[1], and ..., numbers[99] to represent individual variables. A specific element in an array is accessed by an index. All arrays consist of contiguous memory locations. The lowest address corresponds to the first element and the highest address to the last element. Declaring Arrays To declare an array in C++, the programmer specifies the type of the elements and the number of elements required by an array as follows: type arrayName [ arraySize ]; This is called a single-dimension array. The arraySize must be an integer constant greater than zero and type can be any valid C++ data type. For example, to declare a 10-element array called balance of type double, use this statement: double balance[10]; Initializing Arrays You can initialize C++ array elements either one by one or using a single statement as follows: double balance[5] = {1000.0, 2.0, 3.4, 17.0, 50.0}; The number of values between braces { } cannot be larger than the number of elements that we declare for the array between square brackets [ ]. Following is an example to assign a single element of the array: If you omit the size of the array, an array just big enough to hold the initialization is created. Therefore, if you write: double balance[] = {1000.0, 2.0, 3.4, 17.0, 50.0}; You will create exactly the same array as you did in the previous example. 16. ARRAYS
- 111. C++ 99 balance[4] = 50.0; The above statement assigns element number 5th in the array a value of 50.0. Array with 4th index will be 5th, i.e., last element because all arrays have 0 as the index of their first element which is also called base index. Following is the pictorial representation of the same array we discussed above: Accessing Array Elements An element is accessed by indexing the array name. This is done by placing the index of the element within square brackets after the name of the array. For example: double salary = balance[9]; The above statement will take 10th element from the array and assign the value to salary variable. Following is an example, which will use all the above- mentioned three concepts viz. declaration, assignment and accessing arrays: #include <iostream> using namespace std; #include <iomanip> using std::setw; int main () { int n[ 10 ]; // n is an array of 10 integers // initialize elements of array n to 0 for ( int i = 0; i < 10; i++ ) { n[ i ] = i + 100; // set element at location i to i + 100 } cout << "Element" << setw( 13 ) << "Value" << endl; // output each array element's value
- 112. C++ 100 for ( int j = 0; j < 10; j++ ) { cout << setw( 7 )<< j << setw( 13 ) << n[ j ] << endl; } return 0; } This program makes use of setw() function to format the output. When the above code is compiled and executed, it produces the following result: Element Value 0 100 1 101 2 102 3 103 4 104 5 105 6 106 7 107 8 108 9 109 Arrays in C++ Arrays are important to C++ and should need lots of more detail. There are following few important concepts, which should be clear to a C++ programmer: Concept Description Multi-dimensional arrays C++ supports multidimensional arrays. The simplest form of the multidimensional array is the two-dimensional array. Pointer to an array You can generate a pointer to the first element of an array by simply specifying the array name, without any index. Passing arrays to functions You can pass to the function a pointer to an
- 113. C++ 101 array by specifying the array's name without an index. Return array from functions C++ allows a function to return an array. Multi-dimensional Arrays C++ allows multidimensional arrays. Here is the general form of a multidimensional array declaration: type name[size1][size2]...[sizeN]; For example, the following declaration creates a three dimensional 5 . 10 . 4 integer array: int threedim[5][10][4]; Two-Dimensional Arrays The simplest form of the multidimensional array is the two-dimensional array. A two-dimensional array is, in essence, a list of one-dimensional arrays. To declare a two-dimensional integer array of size x,y, you would write something as follows: type arrayName [ x ][ y ]; Where type can be any valid C++ data type and arrayName will be a valid C++ identifier. A two-dimensional array can be think as a table, which will have x number of rows and y number of columns. A 2-dimensional array a, which contains three rows and four columns can be shown as below: Thus, every element in array a is identified by an element name of the form a[ i ][ j ], where a is the name of the array, and i and j are the subscripts that uniquely identify each element in a. Initializing Two-Dimensional Arrays
- 114. C++ 102 Multidimensioned arrays may be initialized by specifying bracketed values for each row. Following is an array with 3 rows and each row have 4 columns. int a[3][4] = { {0, 1, 2, 3} , /* initializers for row indexed by 0 */ {4, 5, 6, 7} , /* initializers for row indexed by 1 */ {8, 9, 10, 11} /* initializers for row indexed by 2 */ }; The nested braces, which indicate the intended row, are optional. The following initialization is equivalent to previous example: int a[3][4] = {0,1,2,3,4,5,6,7,8,9,10,11}; Accessing Two-Dimensional Array Elements An element in 2-dimensional array is accessed by using the subscripts, i.e., row index and column index of the array. For example: int val = a[2][3]; The above statement will take 4th element from the 3rd row of the array. You can verify it in the above digram. #include <iostream> using namespace std; int main () { // an array with 5 rows and 2 columns. int a[5][2] = { {0,0}, {1,2}, {2,4}, {3,6},{4,8}}; // output each array element's value for ( int i = 0; i < 5; i++ ) for ( int j = 0; j < 2; j++ ) { cout << "a[" << i << "][" << j << "]: "; cout << a[i][j]<< endl; }
- 115. C++ 103 return 0; } When the above code is compiled and executed, it produces the following result: a[0][0]: 0 a[0][1]: 0 a[1][0]: 1 a[1][1]: 2 a[2][0]: 2 a[2][1]: 4 a[3][0]: 3 a[3][1]: 6 a[4][0]: 4 a[4][1]: 8 As explained above, you can have arrays with any number of dimensions, although it is likely that most of the arrays you create will be of one or two dimensions. Pointer to an Array It is most likely that you would not understand this chapter until you go through the chapter related C++ Pointers. So assuming you have bit understanding on pointers in C++, let us start: An array name is a constant pointer to the first element of the array. Therefore, in the declaration: double balance[50]; balance is a pointer to &balance[0], which is the address of the first element of the array balance. Thus, the following program fragment assigns p the address of the first element ofbalance: double *p; double balance[10]; p = balance; It is legal to use array names as constant pointers, and vice versa. Therefore, *(balance + 4) is a legitimate way of accessing the data at balance[4].
- 116. C++ 104 Once you store the address of first element in p, you can access array elements using *p, *(p+1), *(p+2) and so on. Below is the example to show all the concepts discussed above: #include <iostream> using namespace std; int main () { // an array with 5 elements. double balance[5] = {1000.0, 2.0, 3.4, 17.0, 50.0}; double *p; p = balance; // output each array element's value cout << "Array values using pointer " << endl; for ( int i = 0; i < 5; i++ ) { cout << "*(p + " << i << ") : "; cout << *(p + i) << endl; } cout << "Array values using balance as address " << endl; for ( int i = 0; i < 5; i++ ) { cout << "*(balance + " << i << ") : "; cout << *(balance + i) << endl; } return 0; } When the above code is compiled and executed, it produces the following result: Array values using pointer *(p + 0) : 1000
- 117. C++ 105 *(p + 1) : 2 *(p + 2) : 3.4 *(p + 3) : 17 *(p + 4) : 50 Array values using balance as address *(balance + 0) : 1000 *(balance + 1) : 2 *(balance + 2) : 3.4 *(balance + 3) : 17 *(balance + 4) : 50 In the above example, p is a pointer to double which means it can store address of a variable of double type. Once we have address in p, then *p will give us value available at the address stored in p, as we have shown in the above example. Passing Arrays to Functions C++ does not allow to pass an entire array as an argument to a function. However, You can pass a pointer to an array by specifying the array's name without an index. If you want to pass a single-dimension array as an argument in a function, you would have to declare function formal parameter in one of following three ways and all three declaration methods produce similar results because each tells the compiler that an integer pointer is going to be received. Way-1 Formal parameters as a pointer as follows: void myFunction(int *param) { . . . } Way-2 Formal parameters as a sized array as follows: void myFunction(int param[10])
- 118. C++ 106 { . . . } Way-3 Formal parameters as an unsized array as follows: void myFunction(int param[]) { . . . } Now, consider the following function, which will take an array as an argument along with another argument and based on the passed arguments, it will return average of the numbers passed through the array as follows: double getAverage(int arr[], int size) { int i, sum = 0; double avg; for (i = 0; i < size; ++i) { sum += arr[i]; } avg = double(sum) / size; return avg; } Now, let us call the above function as follows: #include <iostream>
- 119. C++ 107 using namespace std; // function declaration: double getAverage(int arr[], int size); int main () { // an int array with 5 elements. int balance[5] = {1000, 2, 3, 17, 50}; double avg; // pass pointer to the array as an argument. avg = getAverage( balance, 5 ) ; // output the returned value cout << "Average value is: " << avg << endl; return 0; } When the above code is compiled together and executed, it produces the following result: Average value is: 214.4 As you can see, the length of the array doesn't matter as far as the function is concerned because C++ performs no bounds checking for the formal parameters. Return Array from Functions C++ does not allow to return an entire array as an argument to a function. However, you can return a pointer to an array by specifying the array's name without an index. If you want to return a single-dimension array from a function, you would have to declare a function returning a pointer as in the following example: int * myFunction() { .
- 120. C++ 108 . . } Second point to remember is that C++ does not advocate to return the address of a local variable to outside of the function so you would have to define the local variable as staticvariable. Now, consider the following function, which will generate 10 random numbers and return them using an array and call this function as follows: #include <iostream> #include <ctime> using namespace std; // function to generate and retrun random numbers. int * getRandom( ) { static int r[10]; // set the seed srand( (unsigned)time( NULL ) ); for (int i = 0; i < 10; ++i) { r[i] = rand(); cout << r[i] << endl; } return r; } // main function to call above defined function. int main () { // a pointer to an int. int *p;
- 121. C++ 109 p = getRandom(); for ( int i = 0; i < 10; i++ ) { cout << "*(p + " << i << ") : "; cout << *(p + i) << endl; } return 0; } When the above code is compiled together and executed, it produces result something as follows: 624723190 1468735695 807113585 976495677 613357504 1377296355 1530315259 1778906708 1820354158 667126415 *(p + 0) : 624723190 *(p + 1) : 1468735695 *(p + 2) : 807113585 *(p + 3) : 976495677 *(p + 4) : 613357504 *(p + 5) : 1377296355 *(p + 6) : 1530315259 *(p + 7) : 1778906708 *(p + 8) : 1820354158 *(p + 9) : 667126415
- 122. C++ 110
- 123. C++ 111 C++ provides following two types of string representations: The C-style character string. The string class type introduced with Standard C++. The C-Style Character String The C-style character string originated within the C language and continues to be supported within C++. This string is actually a one-dimensional array of characters which is terminated by a null character '0'. Thus a null-terminated string contains the characters that comprise the string followed by a null. The following declaration and initialization create a string consisting of the word "Hello". To hold the null character at the end of the array, the size of the character array containing the string is one more than the number of characters in the word "Hello." char greeting[6] = {'H', 'e', 'l', 'l', 'o', '0'}; If you follow the rule of array initialization, then you can write the above statement as follows: char greeting[] = "Hello"; Following is the memory presentation of above defined string in C/C++: Actually, you do not place the null character at the end of a string constant. The C++ compiler automatically places the '0' at the end of the string when it initializes the array. Let us try to print above-mentioned string: #include <iostream> using namespace std; 17. STRINGS
- 124. C++ 112 int main () { char greeting[6] = {'H', 'e', 'l', 'l', 'o', '0'}; cout << "Greeting message: "; cout << greeting << endl; return 0; } When the above code is compiled and executed, it produces the following result: Greeting message: Hello C++ supports a wide range of functions that manipulate null-terminated strings: S.N. Function & Purpose 1 strcpy(s1, s2); Copies string s2 into string s1. 2 strcat(s1, s2); Concatenates string s2 onto the end of string s1. 3 strlen(s1); Returns the length of string s1. 4 strcmp(s1, s2); Returns 0 if s1 and s2 are the same; less than 0 if s1<s2; greater than 0 if s1>s2. 5 strchr(s1, ch); Returns a pointer to the first occurrence of character ch in string s1. 6 strstr(s1, s2);
- 125. C++ 113 Returns a pointer to the first occurrence of string s2 in string s1. Following example makes use of few of the above-mentioned functions: #include <iostream> #include <cstring> using namespace std; int main () { char str1[10] = "Hello"; char str2[10] = "World"; char str3[10]; int len ; // copy str1 into str3 strcpy( str3, str1); cout << "strcpy( str3, str1) : " << str3 << endl; // concatenates str1 and str2 strcat( str1, str2); cout << "strcat( str1, str2): " << str1 << endl; // total lenghth of str1 after concatenation len = strlen(str1); cout << "strlen(str1) : " << len << endl; return 0; } When the above code is compiled and executed, it produces result something as follows: strcpy( str3, str1) : Hello
- 126. C++ 114 strcat( str1, str2): HelloWorld strlen(str1) : 10 The String Class in C++ The standard C++ library provides a string class type that supports all the operations mentioned above, additionally much more functionality. Let us check the following example: #include <iostream> #include <string> using namespace std; int main () { string str1 = "Hello"; string str2 = "World"; string str3; int len ; // copy str1 into str3 str3 = str1; cout << "str3 : " << str3 << endl; // concatenates str1 and str2 str3 = str1 + str2; cout << "str1 + str2 : " << str3 << endl; // total length of str3 after concatenation len = str3.size(); cout << "str3.size() : " << len << endl; return 0; } When the above code is compiled and executed, it produces result something as follows:
- 127. C++ 115 str3 : Hello str1 + str2 : HelloWorld str3.size() : 10
- 128. C++ 116 C++ pointers are easy and fun to learn. Some C++ tasks are performed more easily with pointers, and other C++ tasks, such as dynamic memory allocation, cannot be performed without them. As you know every variable is a memory location and every memory location has its address defined which can be accessed using ampersand (&) operator which denotes an address in memory. Consider the following which will print the address of the variables defined: #include <iostream> using namespace std; int main () { int var1; char var2[10]; cout << "Address of var1 variable: "; cout << &var1 << endl; cout << "Address of var2 variable: "; cout << &var2 << endl; return 0; } When the above code is compiled and executed, it produces the following result: Address of var1 variable: 0xbfebd5c0 Address of var2 variable: 0xbfebd5b6 What are Pointers? A pointer is a variable whose value is the address of another variable. Like any variable or constant, you must declare a pointer before you can work with it. The general form of a pointer variable declaration is: 18. POINTERS
- 129. C++ 117 type *var-name; Here, type is the pointer's base type; it must be a valid C++ type and var- name is the name of the pointer variable. The asterisk you used to declare a pointer is the same asterisk that you use for multiplication. However, in this statement the asterisk is being used to designate a variable as a pointer. Following are the valid pointer declaration: int *ip; // pointer to an integer double *dp; // pointer to a double float *fp; // pointer to a float char *ch // pointer to character The actual data type of the value of all pointers, whether integer, float, character, or otherwise, is the same, a long hexadecimal number that represents a memory address. The only difference between pointers of different data types is the data type of the variable or constant that the pointer points to. Using Pointers in C++ There are few important operations, which we will do with the pointers very frequently. (a) We define a pointer variable. (b) Assign the address of a variable to a pointer. (c) Finally access the value at the address available in the pointer variable. This is done by using unary operator * that returns the value of the variable located at the address specified by its operand. Following example makes use of these operations: #include <iostream> using namespace std; int main () { int var = 20; // actual variable declaration. int *ip; // pointer variable ip = &var; // store address of var in pointer variable cout << "Value of var variable: "; cout << var << endl;
- 130. C++ 118 // print the address stored in ip pointer variable cout << "Address stored in ip variable: "; cout << ip << endl; // access the value at the address available in pointer cout << "Value of *ip variable: "; cout << *ip << endl; return 0; } When the above code is compiled and executed, it produces result something as follows: Value of var variable: 20 Address stored in ip variable: 0xbfc601ac Value of *ip variable: 20 Pointers in C++ Pointers have many but easy concepts and they are very important to C++ programming. There are following few important pointer concepts which should be clear to a C++ programmer: Concept Description C++ Null Pointers C++ supports null pointer, which is a constant with a value of zero defined in several standard libraries. C++ pointer arithmetic There are four arithmetic operators that can be used on pointers: ++, --, +, - C++ pointers vs arrays There is a close relationship between pointers and arrays. C++ array of pointers You can define arrays to hold a number of pointers. C++ pointer to pointer C++ allows you to have pointer on a pointer
- 131. C++ 119 and so on. Passing pointers to functions Passing an argument by reference or by address both enable the passed argument to be changed in the calling function by the called function. Return pointer from functions C++ allows a function to return a pointer to local variable, static variable and dynamically allocated memory as well. Null Pointers It is always a good practice to assign the pointer NULL to a pointer variable in case you do not have exact address to be assigned. This is done at the time of variable declaration. A pointer that is assigned NULL is called a null pointer. The NULL pointer is a constant with a value of zero defined in several standard libraries, including iostream. Consider the following program: #include <iostream> using namespace std; int main () { int *ptr = NULL; cout << "The value of ptr is " << ptr ; return 0; } When the above code is compiled and executed, it produces the following result: The value of ptr is 0 On most of the operating systems, programs are not permitted to access memory at address 0 because that memory is reserved by the operating system. However, the memory address 0 has special significance; it signals that the pointer is not intended to point to an accessible memory location. But by
- 132. C++ 120 convention, if a pointer contains the null (zero) value, it is assumed to point to nothing. To check for a null pointer you can use an if statement as follows: if(ptr) // succeeds if p is not null if(!ptr) // succeeds if p is null Thus, if all unused pointers are given the null value and you avoid the use of a null pointer, you can avoid the accidental misuse of an uninitialized pointer. Many times, uninitialized variables hold some junk values and it becomes difficult to debug the program. Pointer Arithmetic As you understood pointer is an address which is a numeric value; therefore, you can perform arithmetic operations on a pointer just as you can a numeric value. There are four arithmetic operators that can be used on pointers: ++, --, +, and - To understand pointer arithmetic, let us consider that ptr is an integer pointer which points to the address 1000. Assuming 32-bit integers, let us perform the following arithmatic operation on the pointer: ptr++ the ptr will point to the location 1004 because each time ptr is incremented, it will point to the next integer. This operation will move the pointer to next memory location without impacting actual value at the memory location. If ptr points to a character whose address is 1000, then above operation will point to the location 1001 because next character will be available at 1001. Incrementing a Pointer We prefer using a pointer in our program instead of an array because the variable pointer can be incremented, unlike the array name which cannot be incremented because it is a constant pointer. The following program increments the variable pointer to access each succeeding element of the array: #include <iostream> using namespace std; const int MAX = 3; int main () { int var[MAX] = {10, 100, 200};
- 133. C++ 121 int *ptr; // let us have array address in pointer. ptr = var; for (int i = 0; i < MAX; i++) { cout << "Address of var[" << i << "] = "; cout << ptr << endl; cout << "Value of var[" << i << "] = "; cout << *ptr << endl; // point to the next location ptr++; } return 0; } When the above code is compiled and executed, it produces result something as follows: Address of var[0] = 0xbfa088b0 Value of var[0] = 10 Address of var[1] = 0xbfa088b4 Value of var[1] = 100 Address of var[2] = 0xbfa088b8 Value of var[2] = 200 Decrementing a Pointer The same considerations apply to decrementing a pointer, which decreases its value by the number of bytes of its data type as shown below: #include <iostream> using namespace std; const int MAX = 3;
- 134. C++ 122 int main () { int var[MAX] = {10, 100, 200}; int *ptr; // let us have address of the last element in pointer. ptr = &var[MAX-1]; for (int i = MAX; i > 0; i--) { cout << "Address of var[" << i << "] = "; cout << ptr << endl; cout << "Value of var[" << i << "] = "; cout << *ptr << endl; // point to the previous location ptr--; } return 0; } When the above code is compiled and executed, it produces result something as follows: Address of var[3] = 0xbfdb70f8 Value of var[3] = 200 Address of var[2] = 0xbfdb70f4 Value of var[2] = 100 Address of var[1] = 0xbfdb70f0 Value of var[1] = 10 Pointer Comparisons Pointers may be compared by using relational operators, such as ==, <, and >. If p1 and p2 point to variables that are related to each other, such as elements of the same array, then p1 and p2 can be meaningfully compared.
- 135. C++ 123 The following program modifies the previous example one by incrementing the variable pointer so long as the address to which it points is either less than or equal to the address of the last element of the array, which is &var[MAX - 1]: #include <iostream> using namespace std; const int MAX = 3; int main () { int var[MAX] = {10, 100, 200}; int *ptr; // let us have address of the first element in pointer. ptr = var; int i = 0; while ( ptr <= &var[MAX - 1] ) { cout << "Address of var[" << i << "] = "; cout << ptr << endl; cout << "Value of var[" << i << "] = "; cout << *ptr << endl; // point to the previous location ptr++; i++; } return 0; } When the above code is compiled and executed, it produces result something as follows: Address of var[0] = 0xbfce42d0 Value of var[0] = 10
- 136. C++ 124 Address of var[1] = 0xbfce42d4 Value of var[1] = 100 Address of var[2] = 0xbfce42d8 Value of var[2] = 200 Pointers vs Arrays Pointers and arrays are strongly related. In fact, pointers and arrays are interchangeable in many cases. For example, a pointer that points to the beginning of an array can access that array by using either pointer arithmetic or array-style indexing. Consider the following program: #include <iostream> using namespace std; const int MAX = 3; int main () { int var[MAX] = {10, 100, 200}; int *ptr; // let us have array address in pointer. ptr = var; for (int i = 0; i < MAX; i++) { cout << "Address of var[" << i << "] = "; cout << ptr << endl; cout << "Value of var[" << i << "] = "; cout << *ptr << endl; // point to the next location ptr++; } return 0; }
- 137. C++ 125 When the above code is compiled and executed, it produces result something as follows: Address of var[0] = 0xbfa088b0 Value of var[0] = 10 Address of var[1] = 0xbfa088b4 Value of var[1] = 100 Address of var[2] = 0xbfa088b8 Value of var[2] = 200 However, pointers and arrays are not completely interchangeable. For example, consider the following program: #include <iostream> using namespace std; const int MAX = 3; int main () { int var[MAX] = {10, 100, 200}; for (int i = 0; i < MAX; i++) { *var = i; // This is a correct syntax var++; // This is incorrect. } return 0; } It is perfectly acceptable to apply the pointer operator * to var but it is illegal to modify var value. The reason for this is that var is a constant that points to the beginning of an array and can not be used as l-value. Because an array name generates a pointer constant, it can still be used in pointer-style expressions, as long as it is not modified. For example, the following is a valid statement that assigns var[2] the value 500: *(var + 2) = 500;
- 138. C++ 126 Above statement is valid and will compile successfully because var is not changed. Array of Pointers Before we understand the concept of array of pointers, let us consider the following example, which makes use of an array of 3 integers: #include <iostream> using namespace std; const int MAX = 3; int main () { int var[MAX] = {10, 100, 200}; for (int i = 0; i < MAX; i++) { cout << "Value of var[" << i << "] = "; cout << var[i] << endl; } return 0; } When the above code is compiled and executed, it produces the following result: Value of var[0] = 10 Value of var[1] = 100 Value of var[2] = 200 There may be a situation, when we want to maintain an array, which can store pointers to an int or char or any other data type available. Following is the declaration of an array of pointers to an integer: int *ptr[MAX]; This declares ptr as an array of MAX integer pointers. Thus, each element in ptr, now holds a pointer to an int value. Following example makes use of three integers which will be stored in an array of pointers as follows: #include <iostream>
- 139. C++ 127 using namespace std; const int MAX = 3; int main () { int var[MAX] = {10, 100, 200}; int *ptr[MAX]; for (int i = 0; i < MAX; i++) { ptr[i] = &var[i]; // assign the address of integer. } for (int i = 0; i < MAX; i++) { cout << "Value of var[" << i << "] = "; cout << *ptr[i] << endl; } return 0; } When the above code is compiled and executed, it produces the following result: Value of var[0] = 10 Value of var[1] = 100 Value of var[2] = 200 You can also use an array of pointers to character to store a list of strings as follows: #include <iostream> using namespace std; const int MAX = 4; int main () {
- 140. C++ 128 char *names[MAX] = { "Zara Ali", "Hina Ali", "Nuha Ali", "Sara Ali", }; for (int i = 0; i < MAX; i++) { cout << "Value of names[" << i << "] = "; cout << names[i] << endl; } return 0; } When the above code is compiled and executed, it produces the following result: Value of names[0] = Zara Ali Value of names[1] = Hina Ali Value of names[2] = Nuha Ali Value of names[3] = Sara Ali Pointer to a Pointer A pointer to a pointer is a form of multiple indirection or a chain of pointers. Normally, a pointer contains the address of a variable. When we define a pointer to a pointer, the first pointer contains the address of the second pointer, which points to the location that contains the actual value as shown below. A variable that is a pointer to a pointer must be declared as such. This is done by placing an additional asterisk in front of its name. For example, following is the declaration to declare a pointer to a pointer of type int: int **var;
- 141. C++ 129 When a target value is indirectly pointed to by a pointer to a pointer, accessing that value requires that the asterisk operator be applied twice, as is shown below in the example: #include <iostream> using namespace std; int main () { int var; int *ptr; int **pptr; var = 3000; // take the address of var ptr = &var; // take the address of ptr using address of operator & pptr = &ptr; // take the value using pptr cout << "Value of var :" << var << endl; cout << "Value available at *ptr :" << *ptr << endl; cout << "Value available at **pptr :" << **pptr << endl; return 0; } When the above code is compiled and executed, it produces the following result: Value of var :3000 Value available at *ptr :3000 Value available at **pptr :3000
- 142. C++ 130 Passing Pointers to Functions C++ allows you to pass a pointer to a function. To do so, simply declare the function parameter as a pointer type. Following a simple example where we pass an unsigned long pointer to a function and change the value inside the function which reflects back in the calling function: #include <iostream> #include <ctime> using namespace std; void getSeconds(unsigned long *par); int main () { unsigned long sec; getSeconds( &sec ); // print the actual value cout << "Number of seconds :" << sec << endl; return 0; } void getSeconds(unsigned long *par) { // get the current number of seconds *par = time( NULL ); return; } When the above code is compiled and executed, it produces the following result: Number of seconds :1294450468
- 143. C++ 131 The function which can accept a pointer, can also accept an array as shown in the following example: #include <iostream> using namespace std; // function declaration: double getAverage(int *arr, int size); int main () { // an int array with 5 elements. int balance[5] = {1000, 2, 3, 17, 50}; double avg; // pass pointer to the array as an argument. avg = getAverage( balance, 5 ) ; // output the returned value cout << "Average value is: " << avg << endl; return 0; } double getAverage(int *arr, int size) { int i, sum = 0; double avg; for (i = 0; i < size; ++i) { sum += arr[i]; } avg = double(sum) / size;
- 144. C++ 132 return avg; } When the above code is compiled together and executed, it produces the following result: Average value is: 214.4 Return Pointer from Functions As we have seen in last chapter how C++ allows to return an array from a function, similar way C++ allows you to return a pointer from a function. To do so, you would have to declare a function returning a pointer as in the following example: int * myFunction() { . . . } Second point to remember is that, it is not good idea to return the address of a local variable to outside of the function, so you would have to define the local variable as staticvariable. Now, consider the following function, which will generate 10 random numbers and return them using an array name which represents a pointer i.e., address of first array element. #include <iostream> #include <ctime> using namespace std; // function to generate and retrun random numbers. int * getRandom( ) { static int r[10];
- 145. C++ 133 // set the seed srand( (unsigned)time( NULL ) ); for (int i = 0; i < 10; ++i) { r[i] = rand(); cout << r[i] << endl; } return r; } // main function to call above defined function. int main () { // a pointer to an int. int *p; p = getRandom(); for ( int i = 0; i < 10; i++ ) { cout << "*(p + " << i << ") : "; cout << *(p + i) << endl; } return 0; } When the above code is compiled together and executed, it produces result something as follows: 624723190 1468735695 807113585 976495677
- 146. C++ 134 613357504 1377296355 1530315259 1778906708 1820354158 667126415 *(p + 0) : 624723190 *(p + 1) : 1468735695 *(p + 2) : 807113585 *(p + 3) : 976495677 *(p + 4) : 613357504 *(p + 5) : 1377296355 *(p + 6) : 1530315259 *(p + 7) : 1778906708 *(p + 8) : 1820354158 *(p + 9) : 667126415
- 147. C++ 135 A reference variable is an alias, that is, another name for an already existing variable. Once a reference is initialized with a variable, either the variable name or the reference name may be used to refer to the variable. References vs Pointers References are often confused with pointers but three major differences between references and pointers are: You cannot have NULL references. You must always be able to assume that a reference is connected to a legitimate piece of storage. Once a reference is initialized to an object, it cannot be changed to refer to another object. Pointers can be pointed to another object at any time. A reference must be initialized when it is created. Pointers can be initialized at any time. Creating References in C++ Think of a variable name as a label attached to the variable's location in memory. You can then think of a reference as a second label attached to that memory location. Therefore, you can access the contents of the variable through either the original variable name or the reference. For example, suppose we have the following example: int i = 17; We can declare reference variables for i as follows. int& r = i; Read the & in these declarations as reference. Thus, read the first declaration as "r is an integer reference initialized to i" and read the second declaration as "s is a double reference initialized to d." Following example makes use of references on int and double: #include <iostream> using namespace std; int main () { 19. REFERENCES
- 148. C++ 136 // declare simple variables int i; double d; // declare reference variables int& r = i; double& s = d; i = 5; cout << "Value of i : " << i << endl; cout << "Value of i reference : " << r << endl; d = 11.7; cout << "Value of d : " << d << endl; cout << "Value of d reference : " << s << endl; return 0; } When the above code is compiled together and executed, it produces the following result: Value of i : 5 Value of i reference : 5 Value of d : 11.7 Value of d reference : 11.7 References are usually used for function argument lists and function return values. So following are two important subjects related to C++ references which should be clear to a C++ programmer: Concept Description References as parameters C++ supports passing references as function parameter more safely than parameters. Reference as return value You can return reference from a C++ function
- 149. C++ 137 like any other data type. References as Parameters We have discussed how we implement call by reference concept using pointers. Here is another example of call by reference which makes use of C++ reference: #include <iostream> using namespace std; // function declaration void swap(int& x, int& y); int main () { // local variable declaration: int a = 100; int b = 200; cout << "Before swap, value of a :" << a << endl; cout << "Before swap, value of b :" << b << endl; /* calling a function to swap the values.*/ swap(a, b); cout << "After swap, value of a :" << a << endl; cout << "After swap, value of b :" << b << endl; return 0; } // function definition to swap the values. void swap(int& x, int& y) {
- 150. C++ 138 int temp; temp = x; /* save the value at address x */ x = y; /* put y into x */ y = temp; /* put x into y */ return; } When the above code is compiled and executed, it produces the following result: Before swap, value of a :100 Before swap, value of b :200 After swap, value of a :200 After swap, value of b :100 Reference as Return Value A C++ program can be made easier to read and maintain by using references rather than pointers. A C++ function can return a reference in a similar way as it returns a pointer. When a function returns a reference, it returns an implicit pointer to its return value. This way, a function can be used on the left side of an assignment statement. For example, consider this simple program: #include <iostream> #include <ctime> using namespace std; double vals[] = {10.1, 12.6, 33.1, 24.1, 50.0}; double& setValues( int i ) { return vals[i]; // return a reference to the ith element } // main function to call above defined function. int main ()
- 151. C++ 139 { cout << "Value before change" << endl; for ( int i = 0; i < 5; i++ ) { cout << "vals[" << i << "] = "; cout << vals[i] << endl; } setValues(1) = 20.23; // change 2nd element setValues(3) = 70.8; // change 4th element cout << "Value after change" << endl; for ( int i = 0; i < 5; i++ ) { cout << "vals[" << i << "] = "; cout << vals[i] << endl; } return 0; } When the above code is compiled together and executed, it produces the following result: Value before change vals[0] = 10.1 vals[1] = 12.6 vals[2] = 33.1 vals[3] = 24.1 vals[4] = 50 Value after change vals[0] = 10.1 vals[1] = 20.23 vals[2] = 33.1 vals[3] = 70.8
- 152. C++ 140 vals[4] = 50 When returning a reference, be careful that the object being referred to does not go out of scope. So it is not legal to return a reference to local var. But you can always return a reference on a static variable. int& func() { int q; //! return q; // Compile time error static int x; return x; // Safe, x lives outside this scope }
- 153. C++ 141 The C++ standard library does not provide a proper date type. C++ inherits the structs and functions for date and time manipulation from C. To access date and time related functions and structures, you would need to include <ctime> header file in your C++ program. There are four time-related types: clock_t, time_t, size_t, and tm. The types - clock_t, size_t and time_t are capable of representing the system time and date as some sort of integer. The structure type tm holds the date and time in the form of a C structure having the following elements: struct tm { int tm_sec; // seconds of minutes from 0 to 61 int tm_min; // minutes of hour from 0 to 59 int tm_hour; // hours of day from 0 to 24 int tm_mday; // day of month from 1 to 31 int tm_mon; // month of year from 0 to 11 int tm_year; // year since 1900 int tm_wday; // days since sunday int tm_yday; // days since January 1st int tm_isdst; // hours of daylight savings time } Following are the important functions, which we use while working with date and time in C or C++. All these functions are part of standard C and C++ library and you can check their detail using reference to C++ standard library given below. SN Function & Purpose 1 time_t time(time_t *time); This returns the current calendar time of the system in number of seconds elapsed since January 1, 1970. If the system has no time, .1 is returned. 2 char *ctime(const time_t *time); This returns a pointer to a string of the form day month year 20. DATE AND TIME
- 154. C++ 142 hours:minutes:seconds yearn0. 3 struct tm *localtime(const time_t *time); This returns a pointer to the tm structure representing local time. 4 clock_t clock(void); This returns a value that approximates the amount of time the calling program has been running. A value of .1 is returned if the time is not available. 5 char * asctime ( const struct tm * time ); This returns a pointer to a string that contains the information stored in the structure pointed to by time converted into the form: day month date hours:minutes:seconds yearn0 6 struct tm *gmtime(const time_t *time); This returns a pointer to the time in the form of a tm structure. The time is represented in Coordinated Universal Time (UTC), which is essentially Greenwich Mean Time (GMT). 7 time_t mktime(struct tm *time); This returns the calendar-time equivalent of the time found in the structure pointed to by time. 8 double difftime ( time_t time2, time_t time1 ); This function calculates the difference in seconds between time1 and time2. 9 size_t strftime(); This function can be used to format date and time in a specific format. Current Date and Time Suppose you want to retrieve the current system date and time, either as a local time or as a Coordinated Universal Time (UTC). Following is the example to achieve the same: #include <iostream> #include <ctime>
- 155. C++ 143 using namespace std; int main( ) { // current date/time based on current system time_t now = time(0); // convert now to string form char* dt = ctime(&now); cout << "The local date and time is: " << dt << endl; // convert now to tm struct for UTC tm *gmtm = gmtime(&now); dt = asctime(gmtm); cout << "The UTC date and time is:"<< dt << endl; } When the above code is compiled and executed, it produces the following result: The local date and time is: Sat Jan 8 20:07:41 2011 The UTC date and time is:Sun Jan 9 03:07:41 2011 Format Time using struct tm The tm structure is very important while working with date and time in either C or C++. This structure holds the date and time in the form of a C structure as mentioned above. Most of the time related functions makes use of tm structure. Following is an example which makes use of various date and time related functions and tm structure: While using structure in this chapter, I'm making an assumption that you have basic understanding on C structure and how to access structure members using arrow -> operator. #include <iostream> #include <ctime>
- 156. C++ 144 using namespace std; int main( ) { // current date/time based on current system time_t now = time(0); cout << "Number of sec since January 1,1970:" << now << endl; tm *ltm = localtime(&now); // print various components of tm structure. cout << "Year: "<< 1900 + ltm->tm_year << endl; cout << "Month: "<< 1 + ltm->tm_mon<< endl; cout << "Day: "<< ltm->tm_mday << endl; cout << "Time: "<< 1 + ltm->tm_hour << ":"; cout << 1 + ltm->tm_min << ":"; cout << 1 + ltm->tm_sec << endl; } When the above code is compiled and executed, it produces the following result: Number of sec since January 1, 1970:1294548238 Year: 2011 Month: 1 Day: 8 Time: 22: 44:59
- 157. C++ 145 The C++ standard libraries provide an extensive set of input/output capabilities which we will see in subsequent chapters. This chapter will discuss very basic and most common I/O operations required for C++ programming. C++ I/O occurs in streams, which are sequences of bytes. If bytes flow from a device like a keyboard, a disk drive, or a network connection etc. to main memory, this is called input operation and if bytes flow from main memory to a device like a display screen, a printer, a disk drive, or a network connection, etc., this is called output operation. I/O Library Header Files There are following header files important to C++ programs: Header File Function and Description <iostream> This file defines the cin, cout, cerr and clog objects, which correspond to the standard input stream, the standard output stream, the un-buffered standard error stream and the buffered standard error stream, respectively. <iomanip> This file declares services useful for performing formatted I/O with so-called parameterized stream manipulators, such as setw and setprecision. <fstream> This file declares services for user-controlled file processing. We will discuss about it in detail in File and Stream related chapter. The Standard Output Stream (cout) The predefined object cout is an instance of ostream class. The cout object is said to be "connected to" the standard output device, which usually is the display screen. The cout is used in conjunction with the stream insertion operator, which is written as << which are two less than signs as shown in the following example. #include <iostream> 21. BASIC INPUT/OUTPUT
- 158. C++ 146 using namespace std; int main( ) { char str[] = "Hello C++"; cout << "Value of str is : " << str << endl; } When the above code is compiled and executed, it produces the following result: Value of str is : Hello C++ The C++ compiler also determines the data type of variable to be output and selects the appropriate stream insertion operator to display the value. The << operator is overloaded to output data items of built-in types integer, float, double, strings and pointer values. The insertion operator << may be used more than once in a single statement as shown above and endl is used to add a new-line at the end of the line. The Standard Input Stream (cin) The predefined object cin is an instance of istream class. The cin object is said to be attached to the standard input device, which usually is the keyboard. The cin is used in conjunction with the stream extraction operator, which is written as >> which are two greater than signs as shown in the following example. #include <iostream> using namespace std; int main( ) { char name[50]; cout << "Please enter your name: "; cin >> name; cout << "Your name is: " << name << endl;
- 159. C++ 147 } When the above code is compiled and executed, it will prompt you to enter a name. You enter a value and then hit enter to see the following result: Please enter your name: cplusplus Your name is: cplusplus The C++ compiler also determines the data type of the entered value and selects the appropriate stream extraction operator to extract the value and store it in the given variables. The stream extraction operator >> may be used more than once in a single statement. To request more than one datum you can use the following: cin >> name >> age; This will be equivalent to the following two statements: cin >> name; cin >> age; The Standard Error Stream (cerr) The predefined object cerr is an instance of ostream class. The cerr object is said to be attached to the standard error device, which is also a display screen but the object cerr is un-buffered and each stream insertion to cerr causes its output to appear immediately. The cerr is also used in conjunction with the stream insertion operator as shown in the following example. #include <iostream> using namespace std; int main( ) { char str[] = "Unable to read...."; cerr << "Error message : " << str << endl; } When the above code is compiled and executed, it produces the following result:
- 160. C++ 148 Error message : Unable to read.... The Standard Log Stream (clog) The predefined object clog is an instance of ostream class. The clog object is said to be attached to the standard error device, which is also a display screen but the object clog is buffered. This means that each insertion to clog could cause its output to be held in a buffer until the buffer is filled or until the buffer is flushed. The clog is also used in conjunction with the stream insertion operator as shown in the following example. #include <iostream> using namespace std; int main( ) { char str[] = "Unable to read...."; clog << "Error message : " << str << endl; } When the above code is compiled and executed, it produces the following result: Error message : Unable to read.... You would not be able to see any difference in cout, cerr and clog with these small examples, but while writing and executing big programs the difference becomes obvious. So it is good practice to display error messages using cerr stream and while displaying other log messages then clog should be used.
- 161. C++ 149 C/C++ arrays allow you to define variables that combine several data items of the same kind, but structure is another user defined data type which allows you to combine data items of different kinds. Structures are used to represent a record, suppose you want to keep track of your books in a library. You might want to track the following attributes about each book: Title Author Subject Book ID Defining a Structure To define a structure, you must use the struct statement. The struct statement defines a new data type, with more than one member, for your program. The format of the struct statement is this: struct [structure tag] { member definition; member definition; ... member definition; } [one or more structure variables]; The structure tag is optional and each member definition is a normal variable definition, such as int i; or float f; or any other valid variable definition. At the end of the structure's definition, before the final semicolon, you can specify one or more structure variables but it is optional. Here is the way you would declare the Book structure: struct Books { char title[50]; char author[50]; char subject[100]; 22. DATA STRUCTURES
- 162. C++ 150 int book_id; }book; Accessing Structure Members To access any member of a structure, we use the member access operator (.). The member access operator is coded as a period between the structure variable name and the structure member that we wish to access. You would use struct keyword to define variables of structure type. Following is the example to explain usage of structure: #include <iostream> #include <cstring> using namespace std; struct Books { char title[50]; char author[50]; char subject[100]; int book_id; }; int main( ) { struct Books Book1; // Declare Book1 of type Book struct Books Book2; // Declare Book2 of type Book // book 1 specification strcpy( Book1.title, "Learn C++ Programming"); strcpy( Book1.author, "Chand Miyan"); strcpy( Book1.subject, "C++ Programming"); Book1.book_id = 6495407; // book 2 specification strcpy( Book2.title, "Telecom Billing");
- 163. C++ 151 strcpy( Book2.author, "Yakit Singha"); strcpy( Book2.subject, "Telecom"); Book2.book_id = 6495700; // Print Book1 info cout << "Book 1 title : " << Book1.title <<endl; cout << "Book 1 author : " << Book1.author <<endl; cout << "Book 1 subject : " << Book1.subject <<endl; cout << "Book 1 id : " << Book1.book_id <<endl; // Print Book2 info cout << "Book 2 title : " << Book2.title <<endl; cout << "Book 2 author : " << Book2.author <<endl; cout << "Book 2 subject : " << Book2.subject <<endl; cout << "Book 2 id : " << Book2.book_id <<endl; return 0; } When the above code is compiled and executed, it produces the following result: Book 1 title : Learn C++ Programming Book 1 author : Chand Miyan Book 1 subject : C++ Programming Book 1 id : 6495407 Book 2 title : Telecom Billing Book 2 author : Yakit Singha Book 2 subject : Telecom Book 2 id : 6495700 Structures as Function Arguments You can pass a structure as a function argument in very similar way as you pass any other variable or pointer. You would access structure variables in the similar way as you have accessed in the above example: #include <iostream> #include <cstring>
- 164. C++ 152 using namespace std; void printBook( struct Books book ); struct Books { char title[50]; char author[50]; char subject[100]; int book_id; }; int main( ) { struct Books Book1; // Declare Book1 of type Book struct Books Book2; // Declare Book2 of type Book // book 1 specification strcpy( Book1.title, "Learn C++ Programming"); strcpy( Book1.author, "Chand Miyan"); strcpy( Book1.subject, "C++ Programming"); Book1.book_id = 6495407; // book 2 specification strcpy( Book2.title, "Telecom Billing"); strcpy( Book2.author, "Yakit Singha"); strcpy( Book2.subject, "Telecom"); Book2.book_id = 6495700; // Print Book1 info printBook( Book1 ); // Print Book2 info printBook( Book2 );
- 165. C++ 153 return 0; } void printBook( struct Books book ) { cout << "Book title : " << book.title <<endl; cout << "Book author : " << book.author <<endl; cout << "Book subject : " << book.subject <<endl; cout << "Book id : " << book.book_id <<endl; } When the above code is compiled and executed, it produces the following result: Book title : Learn C++ Programming Book author : Chand Miyan Book subject : C++ Programming Book id : 6495407 Book title : Telecom Billing Book author : Yakit Singha Book subject : Telecom Book id : 6495700 Pointers to Structures You can define pointers to structures in very similar way as you define pointer to any other variable as follows: struct Books *struct_pointer; Now, you can store the address of a structure variable in the above defined pointer variable. To find the address of a structure variable, place the ‘&’ operator before the structure's name as follows: struct_pointer = &Book1; To access the members of a structure using a pointer to that structure, you must use the -> operator as follows: struct_pointer->title; Let us re-write above example using structure pointer, hope this will be easy for you to understand the concept:
- 166. C++ 154 #include <iostream> #include <cstring> using namespace std; void printBook( struct Books *book ); struct Books { char title[50]; char author[50]; char subject[100]; int book_id; }; int main( ) { struct Books Book1; // Declare Book1 of type Book struct Books Book2; // Declare Book2 of type Book // Book 1 specification strcpy( Book1.title, "Learn C++ Programming"); strcpy( Book1.author, "Chand Miyan"); strcpy( Book1.subject, "C++ Programming"); Book1.book_id = 6495407; // Book 2 specification strcpy( Book2.title, "Telecom Billing"); strcpy( Book2.author, "Yakit Singha"); strcpy( Book2.subject, "Telecom"); Book2.book_id = 6495700; // Print Book1 info, passing address of structure printBook( &Book1 );
- 167. C++ 155 // Print Book1 info, passing address of structure printBook( &Book2 ); return 0; } // This function accept pointer to structure as parameter. void printBook( struct Books *book ) { cout << "Book title : " << book->title <<endl; cout << "Book author : " << book->author <<endl; cout << "Book subject : " << book->subject <<endl; cout << "Book id : " << book->book_id <<endl; } When the above code is compiled and executed, it produces the following result: Book title : Learn C++ Programming Book author : Chand Miyan Book subject : C++ Programming Book id : 6495407 Book title : Telecom Billing Book author : Yakit Singha Book subject : Telecom Book id : 6495700 The typedef Keyword There is an easier way to define structs or you could "alias" types you create. For example: typedef struct { char title[50]; char author[50]; char subject[100]; int book_id; }Books;
- 168. C++ 156 Now, you can use Books directly to define variables of Books type without using struct keyword. Following is the example: Books Book1, Book2; You can use typedef keyword for non-structs as well as follows: typedef long int *pint32; pint32 x, y, z; x, y and z are all pointers to long ints.
- 169. C++ 157 The main purpose of C++ programming is to add object orientation to the C programming language and classes are the central feature of C++ that supports object-oriented programming and are often called user-defined types. A class is used to specify the form of an object and it combines data representation and methods for manipulating that data into one neat package. The data and functions within a class are called members of the class. C++ Class Definitions When you define a class, you define a blueprint for a data type. This doesn't actually define any data, but it does define what the class name means, that is, what an object of the class will consist of and what operations can be performed on such an object. A class definition starts with the keyword class followed by the class name; and the class body, enclosed by a pair of curly braces. A class definition must be followed either by a semicolon or a list of declarations. For example, we define the Box data type using the keyword class as follows: class Box { public: double length; // Length of a box double breadth; // Breadth of a box double height; // Height of a box }; The keyword public determines the access attributes of the members of the class that follows it. A public member can be accessed from outside the class anywhere within the scope of the class object. You can also specify the members of a class as private or protected which we will discuss in a sub-section. Define C++ Objects A class provides the blueprints for objects, so basically an object is created from a class. We declare objects of a class with exactly the same sort of declaration that we declare variables of basic types. Following statements declare two objects of class Box: Box Box1; // Declare Box1 of type Box Box Box2; // Declare Box2 of type Box 23. CLASSES AND OBJECTS
- 170. C++ 158 Both of the objects Box1 and Box2 will have their own copy of data members. Accessing the Data Members The public data members of objects of a class can be accessed using the direct member access operator (.). Let us try the following example to make the things clear: #include <iostream> using namespace std; class Box { public: double length; // Length of a box double breadth; // Breadth of a box double height; // Height of a box }; int main( ) { Box Box1; // Declare Box1 of type Box Box Box2; // Declare Box2 of type Box double volume = 0.0; // Store the volume of a box here // box 1 specification Box1.height = 5.0; Box1.length = 6.0; Box1.breadth = 7.0; // box 2 specification Box2.height = 10.0; Box2.length = 12.0; Box2.breadth = 13.0; // volume of box 1 volume = Box1.height * Box1.length * Box1.breadth;
- 171. C++ 159 cout << "Volume of Box1 : " << volume <<endl; // volume of box 2 volume = Box2.height * Box2.length * Box2.breadth; cout << "Volume of Box2 : " << volume <<endl; return 0; } When the above code is compiled and executed, it produces the following result: Volume of Box1 : 210 Volume of Box2 : 1560 It is important to note that private and protected members cannot be accessed directly using direct member access operator (.). We will learn how private and protected members can be accessed. Classes & Objects in Detail So far, you have got very basic idea about C++ Classes and Objects. There are further interesting concepts related to C++ Classes and Objects which we will discuss in various sub-sections listed below: Concept Description Class member functions A member function of a class is a function that has its definition or its prototype within the class definition like any other variable. Class access modifiers A class member can be defined as public, private or protected. By default members would be assumed as private. Constructor & destructor A class constructor is a special function in a class that is called when a new object of the class is created. A destructor is also a special function which is called when created object is deleted. C++ copy constructor The copy constructor is a constructor which creates an object by initializing it with an object of the same class, which has been
- 172. C++ 160 created previously. C++ friend functions A friend function is permitted full access to private and protected members of a class. C++ inline functions With an inline function, the compiler tries to expand the code in the body of the function in place of a call to the function. The this pointer in C++ Every object has a special pointer this which points to the object itself. Pointer to C++ classes A pointer to a class is done exactly the same way a pointer to a structure is. In fact a class is really just a structure with functions in it. Static members of a class Both data members and function members of a class can be declared as static. Class member functions A member function of a class is a function that has its definition or its prototype within the class definition like any other variable. It operates on any object of the class of which it is a member, and has access to all the members of a class for that object. Let us take previously defined class to access the members of the class using a member function instead of directly accessing them: class Box { public: double length; // Length of a box double breadth; // Breadth of a box double height; // Height of a box double getVolume(void);// Returns box volume }; Member functions can be defined within the class definition or separately using scope resolution operator, ::. Defining a member function within the
- 173. C++ 161 class definition declares the function inline, even if you do not use the inline specifier. So either you can defineVolume() function as below: class Box { public: double length; // Length of a box double breadth; // Breadth of a box double height; // Height of a box double getVolume(void) { return length * breadth * height; } }; If you like, you can define the same function outside the class using the scope resolution operator (::) as follows: double Box::getVolume(void) { return length * breadth * height; } Here, only important point is that you would have to use class name just before :: operator. A member function will be called using a dot operator (.) on a object where it will manipulate data related to that object only as follows: Box myBox; // Create an object myBox.getVolume(); // Call member function for the object Let us put above concepts to set and get the value of different class members in a class: #include <iostream> using namespace std; class Box
- 174. C++ 162 { public: double length; // Length of a box double breadth; // Breadth of a box double height; // Height of a box // Member functions declaration double getVolume(void); void setLength( double len ); void setBreadth( double bre ); void setHeight( double hei ); }; // Member functions definitions double Box::getVolume(void) { return length * breadth * height; } void Box::setLength( double len ) { length = len; } void Box::setBreadth( double bre ) { breadth = bre; } void Box::setHeight( double hei ) { height = hei; }
- 175. C++ 163 // Main function for the program int main( ) { Box Box1; // Declare Box1 of type Box Box Box2; // Declare Box2 of type Box double volume = 0.0; // Store the volume of a box here // box 1 specification Box1.setLength(6.0); Box1.setBreadth(7.0); Box1.setHeight(5.0); // box 2 specification Box2.setLength(12.0); Box2.setBreadth(13.0); Box2.setHeight(10.0); // volume of box 1 volume = Box1.getVolume(); cout << "Volume of Box1 : " << volume <<endl; // volume of box 2 volume = Box2.getVolume(); cout << "Volume of Box2 : " << volume <<endl; return 0; } When the above code is compiled and executed, it produces the following result: Volume of Box1 : 210 Volume of Box2 : 1560 Class Access Modifiers Data hiding is one of the important features of Object Oriented Programming which allows preventing the functions of a program to access directly the
- 176. C++ 164 internal representation of a class type. The access restriction to the class members is specified by the labeled public, private, and protected sections within the class body. The keywords public, private, and protected are called access specifiers. A class can have multiple public, protected, or private labeled sections. Each section remains in effect until either another section label or the closing right brace of the class body is seen. The default access for members and classes is private. class Base { public: // public members go here protected: // protected members go here private: // private members go here }; The public Members A public member is accessible from anywhere outside the class but within a program. You can set and get the value of public variables without any member function as shown in the following example: #include <iostream> using namespace std; class Line { public: double length;
- 177. C++ 165 void setLength( double len ); double getLength( void ); }; // Member functions definitions double Line::getLength(void) { return length ; } void Line::setLength( double len ) { length = len; } // Main function for the program int main( ) { Line line; // set line length line.setLength(6.0); cout << "Length of line : " << line.getLength() <<endl; // set line length without member function line.length = 10.0; // OK: because length is public cout << "Length of line : " << line.length <<endl; return 0; } When the above code is compiled and executed, it produces the following result: Length of line : 6 Length of line : 10 The private Members
- 178. C++ 166 A private member variable or function cannot be accessed, or even viewed from outside the class. Only the class and friend functions can access private members. By default all the members of a class would be private, for example in the following classwidth is a private member, which means until you label a member, it will be assumed a private member: class Box { double width; public: double length; void setWidth( double wid ); double getWidth( void ); }; Practically, we define data in private section and related functions in public section so that they can be called from outside of the class as shown in the following program. #include <iostream> using namespace std; class Box { public: double length; void setWidth( double wid ); double getWidth( void ); private: double width; }; // Member functions definitions double Box::getWidth(void)
- 179. C++ 167 { return width ; } void Box::setWidth( double wid ) { width = wid; } // Main function for the program int main( ) { Box box; // set box length without member function box.length = 10.0; // OK: because length is public cout << "Length of box : " << box.length <<endl; // set box width without member function // box.width = 10.0; // Error: because width is private box.setWidth(10.0); // Use member function to set it. cout << "Width of box : " << box.getWidth() <<endl; return 0; } When the above code is compiled and executed, it produces the following result: Length of box : 10 Width of box : 10 The protected Members A protected member variable or function is very similar to a private member but it provided one additional benefit that they can be accessed in child classes which are called derived classes.
- 180. C++ 168 You will learn derived classes and inheritance in next chapter. For now you can check following example where I have derived one child class SmallBox from a parent class Box. Following example is similar to above example and here width member will be accessible by any member function of its derived class SmallBox. #include <iostream> using namespace std; class Box { protected: double width; }; class SmallBox:Box // SmallBox is the derived class. { public: void setSmallWidth( double wid ); double getSmallWidth( void ); }; // Member functions of child class double SmallBox::getSmallWidth(void) { return width ; } void SmallBox::setSmallWidth( double wid ) { width = wid; } // Main function for the program int main( ) {
- 181. C++ 169 SmallBox box; // set box width using member function box.setSmallWidth(5.0); cout << "Width of box : "<< box.getSmallWidth() << endl; return 0; } When the above code is compiled and executed, it produces the following result: Width of box : 5 Constructor & Destructor A class constructor is a special member function of a class that is executed whenever we create new objects of that class. A constructor will have exact same name as the class and it does not have any return type at all, not even void. Constructors can be very useful for setting initial values for certain member variables. Following example explains the concept of constructor: #include <iostream> using namespace std; class Line { public: void setLength( double len ); double getLength( void ); Line(); // This is the constructor private: double length; }; // Member functions definitions including constructor
- 182. C++ 170 Line::Line(void) { cout << "Object is being created" << endl; } void Line::setLength( double len ) { length = len; } double Line::getLength( void ) { return length; } // Main function for the program int main( ) { Line line; // set line length line.setLength(6.0); cout << "Length of line : " << line.getLength() <<endl; return 0; } When the above code is compiled and executed, it produces the following result: Object is being created Length of line : 6 Parameterized Constructor A default constructor does not have any parameter, but if you need, a constructor can have parameters. This helps you to assign initial value to an object at the time of its creation as shown in the following example:
- 183. C++ 171 #include <iostream> using namespace std; class Line { public: void setLength( double len ); double getLength( void ); Line(double len); // This is the constructor private: double length; }; // Member functions definitions including constructor Line::Line( double len) { cout << "Object is being created, length = " << len << endl; length = len; } void Line::setLength( double len ) { length = len; } double Line::getLength( void ) { return length; } // Main function for the program int main( ) {
- 184. C++ 172 Line line(10.0); // get initially set length. cout << "Length of line : " << line.getLength() <<endl; // set line length again line.setLength(6.0); cout << "Length of line : " << line.getLength() <<endl; return 0; } When the above code is compiled and executed, it produces the following result: Object is being created, length = 10 Length of line : 10 Length of line : 6 Using Initialization Lists to Initialize Fields In case of parameterized constructor, you can use following syntax to initialize the fields: Line::Line( double len): length(len) { cout << "Object is being created, length = " << len << endl; } Above syntax is equal to the following syntax: Line::Line( double len) { cout << "Object is being created, length = " << len << endl; length = len; } If for a class C, you have multiple fields X, Y, Z, etc., to be initialized, then use can use same syntax and separate the fields by comma as follows: C::C( double a, double b, double c): X(a), Y(b), Z(c) {
- 185. C++ 173 .... } The Class Destructor A destructor is a special member function of a class that is executed whenever an object of it's class goes out of scope or whenever the delete expression is applied to a pointer to the object of that class. A destructor will have exact same name as the class prefixed with a tilde (~) and it can neither return a value nor can it take any parameters. Destructor can be very useful for releasing resources before coming out of the program like closing files, releasing memories etc. Following example explains the concept of destructor: #include <iostream> using namespace std; class Line { public: void setLength( double len ); double getLength( void ); Line(); // This is the constructor declaration ~Line(); // This is the destructor: declaration private: double length; }; // Member functions definitions including constructor Line::Line(void) { cout << "Object is being created" << endl; } Line::~Line(void) {
- 186. C++ 174 cout << "Object is being deleted" << endl; } void Line::setLength( double len ) { length = len; } double Line::getLength( void ) { return length; } // Main function for the program int main( ) { Line line; // set line length line.setLength(6.0); cout << "Length of line : " << line.getLength() <<endl; return 0; } When the above code is compiled and executed, it produces the following result: Object is being created Length of line : 6 Object is being deleted Copy Constructor
- 187. C++ 175 The copy constructor is a constructor which creates an object by initializing it with an object of the same class, which has been created previously. The copy constructor is used to: Initialize one object from another of the same type. Copy an object to pass it as an argument to a function. Copy an object to return it from a function. If a copy constructor is not defined in a class, the compiler itself defines one.If the class has pointer variables and has some dynamic memory allocations, then it is a must to have a copy constructor. The most common form of copy constructor is shown here: classname (const classname &obj) { // body of constructor } Here, obj is a reference to an object that is being used to initialize another object. #include <iostream> using namespace std; class Line { public: int getLength( void ); Line( int len ); // simple constructor Line( const Line &obj); // copy constructor ~Line(); // destructor private: int *ptr; }; // Member functions definitions including constructor Line::Line(int len) {
- 188. C++ 176 cout << "Normal constructor allocating ptr" << endl; // allocate memory for the pointer; ptr = new int; *ptr = len; } Line::Line(const Line &obj) { cout << "Copy constructor allocating ptr." << endl; ptr = new int; *ptr = *obj.ptr; // copy the value } Line::~Line(void) { cout << "Freeing memory!" << endl; delete ptr; } int Line::getLength( void ) { return *ptr; } void display(Line obj) { cout << "Length of line : " << obj.getLength() <<endl; } // Main function for the program int main( ) { Line line(10); display(line);
- 189. C++ 177 return 0; } When the above code is compiled and executed, it produces the following result: Normal constructor allocating ptr Copy constructor allocating ptr. Length of line : 10 Freeing memory! Freeing memory! Let us see the same example but with a small change to create another object using existing object of the same type: #include <iostream> using namespace std; class Line { public: int getLength( void ); Line( int len ); // simple constructor Line( const Line &obj); // copy constructor ~Line(); // destructor private: int *ptr; }; // Member functions definitions including constructor Line::Line(int len) { cout << "Normal constructor allocating ptr" << endl; // allocate memory for the pointer; ptr = new int;
- 190. C++ 178 *ptr = len; } Line::Line(const Line &obj) { cout << "Copy constructor allocating ptr." << endl; ptr = new int; *ptr = *obj.ptr; // copy the value } Line::~Line(void) { cout << "Freeing memory!" << endl; delete ptr; } int Line::getLength( void ) { return *ptr; } void display(Line obj) { cout << "Length of line : " << obj.getLength() <<endl; } // Main function for the program int main( ) { Line line1(10); Line line2 = line1; // This also calls copy constructor display(line1); display(line2);
- 191. C++ 179 return 0; } When the above code is compiled and executed, it produces the following result: Normal constructor allocating ptr Copy constructor allocating ptr. Copy constructor allocating ptr. Length of line : 10 Freeing memory! Copy constructor allocating ptr. Length of line : 10 Freeing memory! Freeing memory! Freeing memory! Friend Functions A friend function of a class is defined outside that class' scope but it has the right to access all private and protected members of the class. Even though the prototypes for friend functions appear in the class definition, friends are not member functions. A friend can be a function, function template, or member function, or a class or class template, in which case the entire class and all of its members are friends. To declare a function as a friend of a class, precede the function prototype in the class definition with keyword friend as follows: class Box { double width; public: double length; friend void printWidth( Box box ); void setWidth( double wid ); }; To declare all member functions of class ClassTwo as friends of class ClassOne, place a following declaration in the definition of class ClassOne:
- 192. C++ 180 friend class ClassTwo; Consider the following program: #include <iostream> using namespace std; class Box { double width; public: friend void printWidth( Box box ); void setWidth( double wid ); }; // Member function definition void Box::setWidth( double wid ) { width = wid; } // Note: printWidth() is not a member function of any class. void printWidth( Box box ) { /* Because printWidth() is a friend of Box, it can directly access any member of this class */ cout << "Width of box : " << box.width <<endl; } // Main function for the program int main( ) { Box box;
- 193. C++ 181 // set box width without member function box.setWidth(10.0); // Use friend function to print the wdith. printWidth( box ); return 0; } When the above code is compiled and executed, it produces the following result: Width of box : 10 Inline Functions C++ inline function is powerful concept that is commonly used with classes. If a function is inline, the compiler places a copy of the code of that function at each point where the function is called at compile time. Any change to an inline function could require all clients of the function to be recompiled because compiler would need to replace all the code once again otherwise it will continue with old functionality. To inline a function, place the keyword inline before the function name and define the function before any calls are made to the function. The compiler can ignore the inline qualifier in case defined function is more than a line. A function definition in a class definition is an inline function definition, even without the use of the inline specifier. Following is an example, which makes use of inline function to return max of two numbers: #include <iostream> using namespace std; inline int Max(int x, int y) { return (x > y)? x : y; } // Main function for the program
- 194. C++ 182 int main( ) { cout << "Max (20,10): " << Max(20,10) << endl; cout << "Max (0,200): " << Max(0,200) << endl; cout << "Max (100,1010): " << Max(100,1010) << endl; return 0; } When the above code is compiled and executed, it produces the following result: Max (20,10): 20 Max (0,200): 200 Max (100,1010): 1010 this Pointer Every object in C++ has access to its own address through an important pointer called thispointer. The this pointer is an implicit parameter to all member functions. Therefore, inside a member function, this may be used to refer to the invoking object. Friend functions do not have a this pointer, because friends are not members of a class. Only member functions have a this pointer. Let us try the following example to understand the concept of this pointer: #include <iostream> using namespace std; class Box { public: // Constructor definition Box(double l=2.0, double b=2.0, double h=2.0) { cout <<"Constructor called." << endl; length = l; breadth = b;
- 195. C++ 183 height = h; } double Volume() { return length * breadth * height; } int compare(Box box) { return this->Volume() > box.Volume(); } private: double length; // Length of a box double breadth; // Breadth of a box double height; // Height of a box }; int main(void) { Box Box1(3.3, 1.2, 1.5); // Declare box1 Box Box2(8.5, 6.0, 2.0); // Declare box2 if(Box1.compare(Box2)) { cout << "Box2 is smaller than Box1" <<endl; } else { cout << "Box2 is equal to or larger than Box1" <<endl; } return 0; } When the above code is compiled and executed, it produces the following result: Constructor called.
- 196. C++ 184 Constructor called. Box2 is equal to or larger than Box1 Pointer to C++ Classes A pointer to a C++ class is done exactly the same way as a pointer to a structure and to access members of a pointer to a class you use the member access operator -> operator, just as you do with pointers to structures. Also as with all pointers, you must initialize the pointer before using it. Let us try the following example to understand the concept of pointer to a class: #include <iostream> using namespace std; class Box { public: // Constructor definition Box(double l=2.0, double b=2.0, double h=2.0) { cout <<"Constructor called." << endl; length = l; breadth = b; height = h; } double Volume() { return length * breadth * height; } private: double length; // Length of a box double breadth; // Breadth of a box double height; // Height of a box };
- 197. C++ 185 int main(void) { Box Box1(3.3, 1.2, 1.5); // Declare box1 Box Box2(8.5, 6.0, 2.0); // Declare box2 Box *ptrBox; // Declare pointer to a class. // Save the address of first object ptrBox = &Box1; // Now try to access a member using member access operator cout << "Volume of Box1: " << ptrBox->Volume() << endl; // Save the address of first object ptrBox = &Box2; // Now try to access a member using member access operator cout << "Volume of Box2: " << ptrBox->Volume() << endl; return 0; } When the above code is compiled and executed, it produces the following result: Constructor called. Constructor called. Volume of Box1: 5.94 Volume of Box2: 102 Static Members of a Class We can define class members static using static keyword. When we declare a member of a class as static it means no matter how many objects of the class are created, there is only one copy of the static member. A static member is shared by all objects of the class. All static data is initialized to zero when the first object is created, if no other initialization is present. We can't put it in the class definition but it can be initialized outside the class as done in the following example by redeclaring the static variable, using the scope resolution operator :: to identify which class it belongs to.
- 198. C++ 186 Let us try the following example to understand the concept of static data members: #include <iostream> using namespace std; class Box { public: static int objectCount; // Constructor definition Box(double l=2.0, double b=2.0, double h=2.0) { cout <<"Constructor called." << endl; length = l; breadth = b; height = h; // Increase every time object is created objectCount++; } double Volume() { return length * breadth * height; } private: double length; // Length of a box double breadth; // Breadth of a box double height; // Height of a box }; // Initialize static member of class Box int Box::objectCount = 0; int main(void)
- 199. C++ 187 { Box Box1(3.3, 1.2, 1.5); // Declare box1 Box Box2(8.5, 6.0, 2.0); // Declare box2 // Print total number of objects. cout << "Total objects: " << Box::objectCount << endl; return 0; } When the above code is compiled and executed, it produces the following result: Constructor called. Constructor called. Total objects: 2 Static Function Members By declaring a function member as static, you make it independent of any particular object of the class. A static member function can be called even if no objects of the class exist and the static functions are accessed using only the class name and the scope resolution operator ::. A static member function can only access static data member, other static member functions and any other functions from outside the class. Static member functions have a class scope and they do not have access to the this pointer of the class. You could use a static member function to determine whether some objects of the class have been created or not. Let us try the following example to understand the concept of static function members: #include <iostream> using namespace std; class Box { public: static int objectCount; // Constructor definition
- 200. C++ 188 Box(double l=2.0, double b=2.0, double h=2.0) { cout <<"Constructor called." << endl; length = l; breadth = b; height = h; // Increase every time object is created objectCount++; } double Volume() { return length * breadth * height; } static int getCount() { return objectCount; } private: double length; // Length of a box double breadth; // Breadth of a box double height; // Height of a box }; // Initialize static member of class Box int Box::objectCount = 0; int main(void) { // Print total number of objects before creating object. cout << "Inital Stage Count: " << Box::getCount() << endl; Box Box1(3.3, 1.2, 1.5); // Declare box1 Box Box2(8.5, 6.0, 2.0); // Declare box2
- 201. C++ 189 // Print total number of objects after creating object. cout << "Final Stage Count: " << Box::getCount() << endl; return 0; } When the above code is compiled and executed, it produces the following result: Inital Stage Count: 0 Constructor called. Constructor called. Final Stage Count: 2
- 202. C++ 190 One of the most important concepts in object-oriented programming is that of inheritance. Inheritance allows us to define a class in terms of another class, which makes it easier to create and maintain an application. This also provides an opportunity to reuse the code functionality and fast implementation time. When creating a class, instead of writing completely new data members and member functions, the programmer can designate that the new class should inherit the members of an existing class. This existing class is called the base class, and the new class is referred to as the derived class. The idea of inheritance implements the is a relationship. For example, mammal IS-A animal, dog IS-A mammal hence dog IS-A animal as well and so on. Base & Derived Classes A class can be derived from more than one classes, which means it can inherit data and functions from multiple base classes. To define a derived class, we use a class derivation list to specify the base class(es). A class derivation list names one or more base classes and has the form: class derived-class: access-specifier base-class Where access-specifier is one of public, protected, or private, and base-class is the name of a previously defined class. If the access-specifier is not used, then it is private by default. Consider a base class Shape and its derived class Rectangle as follows: #include <iostream> using namespace std; // Base class class Shape { public: void setWidth(int w) { width = w; } 24. INHERITANCE
- 203. C++ 191 void setHeight(int h) { height = h; } protected: int width; int height; }; // Derived class class Rectangle: public Shape { public: int getArea() { return (width * height); } }; int main(void) { Rectangle Rect; Rect.setWidth(5); Rect.setHeight(7); // Print the area of the object. cout << "Total area: " << Rect.getArea() << endl; return 0; } When the above code is compiled and executed, it produces the following result:
- 204. C++ 192 Total area: 35 Access Control and Inheritance A derived class can access all the non-private members of its base class. Thus base-class members that should not be accessible to the member functions of derived classes should be declared private in the base class. We can summarize the different access types according to - who can access them, in the following way: Access public protected private Same class yes yes yes Derived classes yes yes no Outside classes yes no no A derived class inherits all base class methods with the following exceptions: Constructors, destructors and copy constructors of the base class. Overloaded operators of the base class. The friend functions of the base class. Type of Inheritance When deriving a class from a base class, the base class may be inherited through public, protected or private inheritance. The type of inheritance is specified by the access-specifier as explained above. We hardly use protected or private inheritance, but public inheritance is commonly used. While using different type of inheritance, following rules are applied: Public Inheritance: When deriving a class from a public base class, public members of the base class become public members of the derived class and protected members of the base class become protected members of the derived class. A base class's private members are never accessible directly from a derived class, but can be accessed through calls to the public and protected members of the base class. Protected Inheritance: When deriving from a protected base class, public and protected members of the base class become protected members of the derived class.
- 205. C++ 193 Private Inheritance: When deriving from a private base class, public and protected members of the base class become private members of the derived class. Multiple Inheritance A C++ class can inherit members from more than one class and here is the extended syntax: class derived-class: access baseA, access baseB.... Where access is one of public, protected, or private and would be given for every base class and they will be separated by comma as shown above. Let us try the following example: #include <iostream> using namespace std; // Base class Shape class Shape { public: void setWidth(int w) { width = w; } void setHeight(int h) { height = h; } protected: int width; int height; }; // Base class PaintCost class PaintCost {
- 206. C++ 194 public: int getCost(int area) { return area * 70; } }; // Derived class class Rectangle: public Shape, public PaintCost { public: int getArea() { return (width * height); } }; int main(void) { Rectangle Rect; int area; Rect.setWidth(5); Rect.setHeight(7); area = Rect.getArea(); // Print the area of the object. cout << "Total area: " << Rect.getArea() << endl; // Print the total cost of painting cout << "Total paint cost: $" << Rect.getCost(area) << endl; return 0;
- 207. C++ 195 } When the above code is compiled and executed, it produces the following result: Total area: 35 Total paint cost: $2450
- 208. C++ 196 C++ allows you to specify more than one definition for a function name or an operator in the same scope, which is called function overloading and operator overloading respectively. An overloaded declaration is a declaration that is declared with the same name as a previously declared declaration in the same scope, except that both declarations have different arguments and obviously different definition (implementation). When you call an overloaded function or operator, the compiler determines the most appropriate definition to use, by comparing the argument types you have used to call the function or operator with the parameter types specified in the definitions. The process of selecting the most appropriate overloaded function or operator is called overload resolution. Function Overloading in C++ You can have multiple definitions for the same function name in the same scope. The definition of the function must differ from each other by the types and/or the number of arguments in the argument list. You cannot overload function declarations that differ only by return type. Following is the example where same function print() is being used to print different data types: #include <iostream> using namespace std; class printData { public: void print(int i) { cout << "Printing int: " << i << endl; } void print(double f) { cout << "Printing float: " << f << endl; } 25. OVERLOADING (OPERATOR & FUNCTION)
- 209. C++ 197 void print(char* c) { cout << "Printing character: " << c << endl; } }; int main(void) { printData pd; // Call print to print integer pd.print(5); // Call print to print float pd.print(500.263); // Call print to print character pd.print("Hello C++"); return 0; } When the above code is compiled and executed, it produces the following result: Printing int: 5 Printing float: 500.263 Printing character: Hello C++ Operators Overloading in C++ You can redefine or overload most of the built-in operators available in C++. Thus, a programmer can use operators with user-defined types as well. Overloaded operators are functions with special names the keyword operator followed by the symbol for the operator being defined. Like any other function, an overloaded operator has a return type and a parameter list. Box operator+(const Box&); Declares the addition operator that can be used to add two Box objects and returns final Box object. Most overloaded operators may be defined as ordinary non-member functions or as class member functions. In case we define above function as non-member function of a class then we would have to pass two arguments for each operand as follows:
- 210. C++ 198 Box operator+(const Box&, const Box&); Following is the example to show the concept of operator over loading using a member function. Here an object is passed as an argument whose properties will be accessed using this object, the object which will call this operator can be accessed using this operator as explained below: #include <iostream> using namespace std; class Box { public: double getVolume(void) { return length * breadth * height; } void setLength( double len ) { length = len; } void setBreadth( double bre ) { breadth = bre; } void setHeight( double hei ) { height = hei; } // Overload + operator to add two Box objects. Box operator+(const Box& b) { Box box;
- 211. C++ 199 box.length = this->length + b.length; box.breadth = this->breadth + b.breadth; box.height = this->height + b.height; return box; } private: double length; // Length of a box double breadth; // Breadth of a box double height; // Height of a box }; // Main function for the program int main( ) { Box Box1; // Declare Box1 of type Box Box Box2; // Declare Box2 of type Box Box Box3; // Declare Box3 of type Box double volume = 0.0; // Store the volume of a box here // box 1 specification Box1.setLength(6.0); Box1.setBreadth(7.0); Box1.setHeight(5.0); // box 2 specification Box2.setLength(12.0); Box2.setBreadth(13.0); Box2.setHeight(10.0); // volume of box 1 volume = Box1.getVolume(); cout << "Volume of Box1 : " << volume <<endl; // volume of box 2 volume = Box2.getVolume();
- 212. C++ 200 cout << "Volume of Box2 : " << volume <<endl; // Add two object as follows: Box3 = Box1 + Box2; // volume of box 3 volume = Box3.getVolume(); cout << "Volume of Box3 : " << volume <<endl; return 0; } When the above code is compiled and executed, it produces the following result: Volume of Box1 : 210 Volume of Box2 : 1560 Volume of Box3 : 5400 Overloadable/Non-overloadable Operators Following is the list of operators which can be overloaded: + - * / % ^ & | ~ ! , = < > <= >= ++ -- << >> == != && || += -= /= %= ^= &= |= *= <<= >>= [] () -> ->* new new [] delete delete [] Following is the list of operators, which cannot be overloaded:
- 213. C++ 201 :: .* . ?: Operator Overloading Examples Here are various operator overloading examples to help you in understanding the concept. S.N. Operators and Example 1 Unary operators overloading 2 Binary operators overloading 3 Relational operators overloading 4 Input/Output operators overloading 5 ++ and -- operators overloading 6 Assignment operators overloading 7 Function call () operator overloading 8 Subscripting [] operator overloading 9 Class member access operator -> overloading Unary Operators Overloading The unary operators operate on a single operand and following are the examples of Unary operators: The increment (++) and decrement (--) operators. The unary minus (-) operator. The logical not (!) operator. The unary operators operate on the object for which they were called and normally, this operator appears on the left side of the object, as in !obj, -obj, and ++obj but sometime they can be used as postfix as well like obj++ or obj--. Following example explain how minus (-) operator can be overloaded for prefix as well as postfix usage.
- 214. C++ 202 #include <iostream> using namespace std; class Distance { private: int feet; // 0 to infinite int inches; // 0 to 12 public: // required constructors Distance(){ feet = 0; inches = 0; } Distance(int f, int i){ feet = f; inches = i; } // method to display distance void displayDistance() { cout << "F: " << feet << " I:" << inches <<endl; } // overloaded minus (-) operator Distance operator- () { feet = -feet; inches = -inches; return Distance(feet, inches); } }; int main() { Distance D1(11, 10), D2(-5, 11);
- 215. C++ 203 -D1; // apply negation D1.displayDistance(); // display D1 -D2; // apply negation D2.displayDistance(); // display D2 return 0; } When the above code is compiled and executed, it produces the following result: F: -11 I:-10 F: 5 I:-11 Hope above example makes your concept clear and you can apply similar concept to overload Logical Not Operators (!). Increment (++) and Decrement (- -) Operators The increment (++) and decrement (--) operators are two important unary operators available in C++. Following example explain how increment (++) operator can be overloaded for prefix as well as postfix usage. Similar way, you can overload operator (--). #include <iostream> using namespace std; class Time { private: int hours; // 0 to 23 int minutes; // 0 to 59 public: // required constructors Time(){ hours = 0; minutes = 0; }
- 216. C++ 204 Time(int h, int m){ hours = h; minutes = m; } // method to display time void displayTime() { cout << "H: " << hours << " M:" << minutes <<endl; } // overloaded prefix ++ operator Time operator++ () { ++minutes; // increment this object if(minutes >= 60) { ++hours; minutes -= 60; } return Time(hours, minutes); } // overloaded postfix ++ operator Time operator++( int ) { // save the orignal value Time T(hours, minutes); // increment this object ++minutes; if(minutes >= 60) { ++hours; minutes -= 60; } // return old original value return T;
- 217. C++ 205 } }; int main() { Time T1(11, 59), T2(10,40); ++T1; // increment T1 T1.displayTime(); // display T1 ++T1; // increment T1 again T1.displayTime(); // display T1 T2++; // increment T2 T2.displayTime(); // display T2 T2++; // increment T2 again T2.displayTime(); // display T2 return 0; } When the above code is compiled and executed, it produces the following result: H: 12 M:0 H: 12 M:1 H: 10 M:41 H: 10 M:42 Binary Operators Overloading The unary operators take two arguments and following are the examples of Binary operators. You use binary operators very frequently like addition (+) operator, subtraction (-) operator and division (/) operator. Following example explains how addition (+) operator can be overloaded. Similar way, you can overload subtraction (-) and division (/) operators. #include <iostream> using namespace std;
- 218. C++ 206 class Box { double length; // Length of a box double breadth; // Breadth of a box double height; // Height of a box public: double getVolume(void) { return length * breadth * height; } void setLength( double len ) { length = len; } void setBreadth( double bre ) { breadth = bre; } void setHeight( double hei ) { height = hei; } // Overload + operator to add two Box objects. Box operator+(const Box& b) { Box box; box.length = this->length + b.length; box.breadth = this->breadth + b.breadth; box.height = this->height + b.height; return box;
- 219. C++ 207 } }; // Main function for the program int main( ) { Box Box1; // Declare Box1 of type Box Box Box2; // Declare Box2 of type Box Box Box3; // Declare Box3 of type Box double volume = 0.0; // Store the volume of a box here // box 1 specification Box1.setLength(6.0); Box1.setBreadth(7.0); Box1.setHeight(5.0); // box 2 specification Box2.setLength(12.0); Box2.setBreadth(13.0); Box2.setHeight(10.0); // volume of box 1 volume = Box1.getVolume(); cout << "Volume of Box1 : " << volume <<endl; // volume of box 2 volume = Box2.getVolume(); cout << "Volume of Box2 : " << volume <<endl; // Add two object as follows: Box3 = Box1 + Box2; // volume of box 3 volume = Box3.getVolume(); cout << "Volume of Box3 : " << volume <<endl;
- 220. C++ 208 return 0; } When the above code is compiled and executed, it produces the following result: Volume of Box1 : 210 Volume of Box2 : 1560 Volume of Box3 : 5400 Relational Operators Overloading There are various relational operators supported by C++ language like (<, >, <=, >=, ==, etc.) which can be used to compare C++ built-in data types. You can overload any of these operators, which can be used to compare the objects of a class. Following example explains how a < operator can be overloaded and similar way you can overload other relational operators. #include <iostream> using namespace std; class Distance { private: int feet; // 0 to infinite int inches; // 0 to 12 public: // required constructors Distance(){ feet = 0; inches = 0; } Distance(int f, int i){ feet = f; inches = i; } // method to display distance
- 221. C++ 209 void displayDistance() { cout << "F: " << feet << " I:" << inches <<endl; } // overloaded minus (-) operator Distance operator- () { feet = -feet; inches = -inches; return Distance(feet, inches); } // overloaded < operator bool operator <(const Distance& d) { if(feet < d.feet) { return true; } if(feet == d.feet && inches < d.inches) { return true; } return false; } }; int main() { Distance D1(11, 10), D2(5, 11); if( D1 < D2 ) { cout << "D1 is less than D2 " << endl; } else
- 222. C++ 210 { cout << "D2 is less than D1 " << endl; } return 0; } When the above code is compiled and executed, it produces the following result: D2 is less than D1 Input/Output Operators Overloading C++ is able to input and output the built-in data types using the stream extraction operator >> and the stream insertion operator <<. The stream insertion and stream extraction operators also can be overloaded to perform input and output for user-defined types like an object. Here, it is important to make operator overloading function a friend of the class because it would be called without creating an object. Following example explains how extraction operator >> and insertion operator <<. #include <iostream> using namespace std; class Distance { private: int feet; // 0 to infinite int inches; // 0 to 12 public: // required constructors Distance(){ feet = 0; inches = 0; } Distance(int f, int i){ feet = f; inches = i;
- 223. C++ 211 } friend ostream &operator<<( ostream &output, const Distance &D ) { output << "F : " << D.feet << " I : " << D.inches; return output; } friend istream &operator>>( istream &input, Distance &D ) { input >> D.feet >> D.inches; return input; } }; int main() { Distance D1(11, 10), D2(5, 11), D3; cout << "Enter the value of object : " << endl; cin >> D3; cout << "First Distance : " << D1 << endl; cout << "Second Distance :" << D2 << endl; cout << "Third Distance :" << D3 << endl; return 0; } When the above code is compiled and executed, it produces the following result: $./a.out Enter the value of object : 70 10 First Distance : F : 11 I : 10
- 224. C++ 212 Second Distance :F : 5 I : 11 Third Distance :F : 70 I : 10 ++ and - - Operators Overloading The increment (++) and decrement (--) operators are two important unary operators available in C++. Following example explain how increment (++) operator can be overloaded for prefix as well as postfix usage. Similar way, you can overload operator (--). #include <iostream> using namespace std; class Time { private: int hours; // 0 to 23 int minutes; // 0 to 59 public: // required constructors Time(){ hours = 0; minutes = 0; } Time(int h, int m){ hours = h; minutes = m; } // method to display time void displayTime() { cout << "H: " << hours << " M:" << minutes <<endl; } // overloaded prefix ++ operator Time operator++ () {
- 225. C++ 213 ++minutes; // increment this object if(minutes >= 60) { ++hours; minutes -= 60; } return Time(hours, minutes); } // overloaded postfix ++ operator Time operator++( int ) { // save the orignal value Time T(hours, minutes); // increment this object ++minutes; if(minutes >= 60) { ++hours; minutes -= 60; } // return old original value return T; } }; int main() { Time T1(11, 59), T2(10,40); ++T1; // increment T1 T1.displayTime(); // display T1 ++T1; // increment T1 again T1.displayTime(); // display T1 T2++; // increment T2
- 226. C++ 214 T2.displayTime(); // display T2 T2++; // increment T2 again T2.displayTime(); // display T2 return 0; } When the above code is compiled and executed, it produces the following result: H: 12 M:0 H: 12 M:1 H: 10 M:41 H: 10 M:42 Assignment Operators Overloading You can overload the assignment operator (=) just as you can other operators and it can be used to create an object just like the copy constructor. Following example explains how an assignment operator can be overloaded. #include <iostream> using namespace std; class Distance { private: int feet; // 0 to infinite int inches; // 0 to 12 public: // required constructors Distance(){ feet = 0; inches = 0; } Distance(int f, int i){ feet = f; inches = i; }
- 227. C++ 215 void operator=(const Distance &D ) { feet = D.feet; inches = D.inches; } // method to display distance void displayDistance() { cout << "F: " << feet << " I:" << inches << endl; } }; int main() { Distance D1(11, 10), D2(5, 11); cout << "First Distance : "; D1.displayDistance(); cout << "Second Distance :"; D2.displayDistance(); // use assignment operator D1 = D2; cout << "First Distance :"; D1.displayDistance(); return 0; } When the above code is compiled and executed, it produces the following result: First Distance : F: 11 I:10 Second Distance :F: 5 I:11 First Distance :F: 5 I:11 Function Call () Operator Overloading
- 228. C++ 216 The function call operator () can be overloaded for objects of class type. When you overload ( ), you are not creating a new way to call a function. Rather, you are creating an operator function that can be passed an arbitrary number of parameters. Following example explains how a function call operator () can be overloaded. #include <iostream> using namespace std; class Distance { private: int feet; // 0 to infinite int inches; // 0 to 12 public: // required constructors Distance(){ feet = 0; inches = 0; } Distance(int f, int i){ feet = f; inches = i; } // overload function call Distance operator()(int a, int b, int c) { Distance D; // just put random calculation D.feet = a + c + 10; D.inches = b + c + 100 ; return D; } // method to display distance void displayDistance() {
- 229. C++ 217 cout << "F: " << feet << " I:" << inches << endl; } }; int main() { Distance D1(11, 10), D2; cout << "First Distance : "; D1.displayDistance(); D2 = D1(10, 10, 10); // invoke operator() cout << "Second Distance :"; D2.displayDistance(); return 0; } When the above code is compiled and executed, it produces the following result: First Distance : F: 11 I:10 Second Distance :F: 30 I:120 Subscripting [ ] Operator Overloading The subscript operator [] is normally used to access array elements. This operator can be overloaded to enhance the existing functionality of C++ arrays. Following example explains how a subscript operator [] can be overloaded. #include <iostream> using namespace std; const int SIZE = 10; class safearay { private:
- 230. C++ 218 int arr[SIZE]; public: safearay() { register int i; for(i = 0; i < SIZE; i++) { arr[i] = i; } } int &operator[](int i) { if( i > SIZE ) { cout << "Index out of bounds" <<endl; // return first element. return arr[0]; } return arr[i]; } }; int main() { safearay A; cout << "Value of A[2] : " << A[2] <<endl; cout << "Value of A[5] : " << A[5]<<endl; cout << "Value of A[12] : " << A[12]<<endl; return 0; } When the above code is compiled and executed, it produces the following result: Value of A[2] : 2
- 231. C++ 219 Value of A[5] : 5 Index out of bounds Value of A[12] : 0 Class Member Access Operator - > Overloading The class member access operator (->) can be overloaded but it is bit trickier. It is defined to give a class type a "pointer-like" behavior. The operator -> must be a member function. If used, its return type must be a pointer or an object of a class to which you can apply. The operator-> is used often in conjunction with the pointer-dereference operator * to implement "smart pointers." These pointers are objects that behave like normal pointers except they perform other tasks when you access an object through them, such as automatic object deletion either when the pointer is destroyed, or the pointer is used to point to another object. The dereferencing operator-> can be defined as a unary postfix operator. That is, given a class: class Ptr{ //... X * operator->(); }; Objects of class Ptr can be used to access members of class X in a very similar manner to the way pointers are used. For example: void f(Ptr p ) { p->m = 10 ; // (p.operator->())->m = 10 } The statement p->m is interpreted as (p.operator->())->m. Using the same concept, following example explains how a class access operator -> can be overloaded. #include <iostream> #include <vector> using namespace std; // Consider an actual class. class Obj {
- 232. C++ 220 static int i, j; public: void f() const { cout << i++ << endl; } void g() const { cout << j++ << endl; } }; // Static member definitions: int Obj::i = 10; int Obj::j = 12; // Implement a container for the above class class ObjContainer { vector<Obj*> a; public: void add(Obj* obj) { a.push_back(obj); // call vector's standard method. } friend class SmartPointer; }; // implement smart pointer to access member of Obj class. class SmartPointer { ObjContainer oc; int index; public: SmartPointer(ObjContainer& objc) { oc = objc; index = 0; } // Return value indicates end of list: bool operator++() // Prefix version {
- 233. C++ 221 if(index >= oc.a.size()) return false; if(oc.a[++index] == 0) return false; return true; } bool operator++(int) // Postfix version { return operator++(); } // overload operator-> Obj* operator->() const { if(!oc.a[index]) { cout << "Zero value"; return (Obj*)0; } return oc.a[index]; } }; int main() { const int sz = 10; Obj o[sz]; ObjContainer oc; for(int i = 0; i < sz; i++) { oc.add(&o[i]); } SmartPointer sp(oc); // Create an iterator do { sp->f(); // smart pointer call sp->g(); } while(sp++); return 0;
- 234. C++ 222 } When the above code is compiled and executed, it produces the following result: 10 12 11 13 12 14 13 15 14 16 15 17 16 18 17 19 18 20 19 21
- 235. C++ 223 The word polymorphism means having many forms. Typically, polymorphism occurs when there is a hierarchy of classes and they are related by inheritance. C++ polymorphism means that a call to a member function will cause a different function to be executed depending on the type of object that invokes the function. Consider the following example where a base class has been derived by other two classes: #include <iostream> using namespace std; class Shape { protected: int width, height; public: Shape( int a=0, int b=0) { width = a; height = b; } int area() { cout << "Parent class area :" <<endl; return 0; } }; class Rectangle: public Shape{ public: Rectangle( int a=0, int b=0):Shape(a, b) { } int area () { cout << "Rectangle class area :" <<endl; 26. POLYMORPHISM
- 236. C++ 224 return (width * height); } }; class Triangle: public Shape{ public: Triangle( int a=0, int b=0):Shape(a, b) { } int area () { cout << "Triangle class area :" <<endl; return (width * height / 2); } }; // Main function for the program int main( ) { Shape *shape; Rectangle rec(10,7); Triangle tri(10,5); // store the address of Rectangle shape = &rec; // call rectangle area. shape->area(); // store the address of Triangle shape = &tri; // call triangle area. shape->area(); return 0; }
- 237. C++ 225 When the above code is compiled and executed, it produces the following result: Parent class area Parent class area The reason for the incorrect output is that the call of the function area() is being set once by the compiler as the version defined in the base class. This is called static resolution of the function call, or static linkage - the function call is fixed before the program is executed. This is also sometimes called early binding because the area() function is set during the compilation of the program. But now, let's make a slight modification in our program and precede the declaration of area() in the Shape class with the keyword virtual so that it looks like this: class Shape { protected: int width, height; public: Shape( int a=0, int b=0) { width = a; height = b; } virtual int area() { cout << "Parent class area :" <<endl; return 0; } }; After this slight modification, when the previous example code is compiled and executed, it produces the following result: Rectangle class area Triangle class area This time, the compiler looks at the contents of the pointer instead of its type. Hence, since addresses of objects of tri and rec classes are stored in *shape the respective area() function is called.
- 238. C++ 226 As you can see, each of the child classes has a separate implementation for the function area(). This is how polymorphism is generally used. You have different classes with a function of the same name, and even the same parameters, but with different implementations. Virtual Function A virtual function is a function in a base class that is declared using the keyword virtual. Defining in a base class a virtual function, with another version in a derived class, signals to the compiler that we don't want static linkage for this function. What we do want is the selection of the function to be called at any given point in the program to be based on the kind of object for which it is called. This sort of operation is referred to as dynamic linkage, or late binding. Pure Virtual Functions It is possible that you want to include a virtual function in a base class so that it may be redefined in a derived class to suit the objects of that class, but that there is no meaningful definition you could give for the function in the base class. We can change the virtual function area() in the base class to the following: class Shape { protected: int width, height; public: Shape( int a=0, int b=0) { width = a; height = b; } // pure virtual function virtual int area() = 0; }; The = 0 tells the compiler that the function has no body and above virtual function will be called pure virtual function.
- 239. C++ 227 Data abstraction refers to providing only essential information to the outside world and hiding their background details, i.e., to represent the needed information in program without presenting the details. Data abstraction is a programming (and design) technique that relies on the separation of interface and implementation. Let's take one real life example of a TV, which you can turn on and off, change the channel, adjust the volume, and add external components such as speakers, VCRs, and DVD players, BUT you do not know its internal details, that is, you do not know how it receives signals over the air or through a cable, how it translates them, and finally displays them on the screen. Thus, we can say a television clearly separates its internal implementation from its external interface and you can play with its interfaces like the power button, channel changer, and volume control without having zero knowledge of its internals. In C++, classes provides great level of data abstraction. They provide sufficient public methods to the outside world to play with the functionality of the object and to manipulate object data, i.e., state without actually knowing how class has been implemented internally. For example, your program can make a call to the sort() function without knowing what algorithm the function actually uses to sort the given values. In fact, the underlying implementation of the sorting functionality could change between releases of the library, and as long as the interface stays the same, your function call will still work. In C++, we use classes to define our own abstract data types (ADT). You can use the cout object of class ostream to stream data to standard output like this: #include <iostream> using namespace std; int main( ) { cout << "Hello C++" <<endl; return 0; } 27. DATA ABSTRACTION
- 240. C++ 228 Here, you don't need to understand how cout displays the text on the user's screen. You need to only know the public interface and the underlying implementation of ‘cout’ is free to change. Access Labels Enforce Abstraction In C++, we use access labels to define the abstract interface to the class. A class may contain zero or more access labels: Members defined with a public label are accessible to all parts of the program. The data-abstraction view of a type is defined by its public members. Members defined with a private label are not accessible to code that uses the class. The private sections hide the implementation from code that uses the type. There are no restrictions on how often an access label may appear. Each access label specifies the access level of the succeeding member definitions. The specified access level remains in effect until the next access label is encountered or the closing right brace of the class body is seen. Benefits of Data Abstraction Data abstraction provides two important advantages: Class internals are protected from inadvertent user-level errors, which might corrupt the state of the object. The class implementation may evolve over time in response to changing requirements or bug reports without requiring change in user-level code. By defining data members only in the private section of the class, the class author is free to make changes in the data. If the implementation changes, only the class code needs to be examined to see what affect the change may have. If data is public, then any function that directly access the data members of the old representation might be broken. Data Abstraction Example Any C++ program where you implement a class with public and private members is an example of data abstraction. Consider the following example: #include <iostream> using namespace std; class Adder{ public: // constructor Adder(int i = 0)
- 241. C++ 229 { total = i; } // interface to outside world void addNum(int number) { total += number; } // interface to outside world int getTotal() { return total; }; private: // hidden data from outside world int total; }; int main( ) { Adder a; a.addNum(10); a.addNum(20); a.addNum(30); cout << "Total " << a.getTotal() <<endl; return 0; } When the above code is compiled and executed, it produces the following result: Total 60 Above class adds numbers together, and returns the sum. The public members - addNum and getTotal are the interfaces to the outside world and a user needs to know them to use the class. The private member total is something that the user doesn't need to know about, but is needed for the class to operate properly.
- 242. C++ 230 Designing Strategy Abstraction separates code into interface and implementation. So while designing your component, you must keep interface independent of the implementation so that if you change underlying implementation then interface would remain intact. In this case whatever programs are using these interfaces, they would not be impacted and would just need a recompilation with the latest implementation.
- 243. C++ 231 All C++ programs are composed of the following two fundamental elements: Program statements (code): This is the part of a program that performs actions and they are called functions. Program data: The data is the information of the program which gets affected by the program functions. Encapsulation is an Object Oriented Programming concept that binds together the data and functions that manipulate the data, and that keeps both safe from outside interference and misuse. Data encapsulation led to the important OOP concept of data hiding. Data encapsulation is a mechanism of bundling the data, and the functions that use them and data abstraction is a mechanism of exposing only the interfaces and hiding the implementation details from the user. C++ supports the properties of encapsulation and data hiding through the creation of user-defined types, called classes. We already have studied that a class can contain private, protected and public members. By default, all items defined in a class are private. For example: class Box { public: double getVolume(void) { return length * breadth * height; } private: double length; // Length of a box double breadth; // Breadth of a box double height; // Height of a box }; The variables length, breadth, and height are private. This means that they can be accessed only by other members of the Box class, and not by any other part of your program. This is one way encapsulation is achieved. To make parts of a class public (i.e., accessible to other parts of your program), you must declare them after the public keyword. All variables or functions 28. DATA ENCAPSULATION
- 244. C++ 232 defined after the public specifier are accessible by all other functions in your program. Making one class a friend of another, exposes the implementation details and reduces encapsulation. The ideal is to keep as many of the details of each class hidden from all other classes as possible. Data Encapsulation Example Any C++ program where you implement a class with public and private members is an example of data encapsulation and data abstraction. Consider the following example: #include <iostream> using namespace std; class Adder{ public: // constructor Adder(int i = 0) { total = i; } // interface to outside world void addNum(int number) { total += number; } // interface to outside world int getTotal() { return total; }; private: // hidden data from outside world int total; }; int main( ) {
- 245. C++ 233 Adder a; a.addNum(10); a.addNum(20); a.addNum(30); cout << "Total " << a.getTotal() <<endl; return 0; } When the above code is compiled and executed, it produces the following result: Total 60 Above class adds numbers together, and returns the sum. The public members - addNum and getTotal are the interfaces to the outside world and a user needs to know them to use the class. The private member total is something that is hidden from the outside world, but is needed for the class to operate properly. Designing Strategy Most of us have learnt to make class members private by default unless we really need to expose them. That's just good encapsulation. This is applied most frequently to data members, but it applies equally to all members, including virtual functions.
- 246. C++ 234 An interface describes the behavior or capabilities of a C++ class without committing to a particular implementation of that class. The C++ interfaces are implemented using abstract classes and these abstract classes should not be confused with data abstraction which is a concept of keeping implementation details separate from associated data. A class is made abstract by declaring at least one of its functions as pure virtual function. A pure virtual function is specified by placing "= 0" in its declaration as follows: class Box { public: // pure virtual function virtual double getVolume() = 0; private: double length; // Length of a box double breadth; // Breadth of a box double height; // Height of a box }; The purpose of an abstract class (often referred to as an ABC) is to provide an appropriate base class from which other classes can inherit. Abstract classes cannot be used to instantiate objects and serves only as an interface. Attempting to instantiate an object of an abstract class causes a compilation error. Thus, if a subclass of an ABC needs to be instantiated, it has to implement each of the virtual functions, which means that it supports the interface declared by the ABC. Failure to override a pure virtual function in a derived class, then attempting to instantiate objects of that class, is a compilation error. Classes that can be used to instantiate objects are called concrete classes. Abstract Class Example Consider the following example where parent class provides an interface to the base class to implement a function called getArea(): #include <iostream> 29. INTERFACES
- 247. C++ 235 using namespace std; // Base class class Shape { public: // pure virtual function providing interface framework. virtual int getArea() = 0; void setWidth(int w) { width = w; } void setHeight(int h) { height = h; } protected: int width; int height; }; // Derived classes class Rectangle: public Shape { public: int getArea() { return (width * height); } }; class Triangle: public Shape { public:
- 248. C++ 236 int getArea() { return (width * height)/2; } }; int main(void) { Rectangle Rect; Triangle Tri; Rect.setWidth(5); Rect.setHeight(7); // Print the area of the object. cout << "Total Rectangle area: " << Rect.getArea() << endl; Tri.setWidth(5); Tri.setHeight(7); // Print the area of the object. cout << "Total Triangle area: " << Tri.getArea() << endl; return 0; } When the above code is compiled and executed, it produces the following result: Total Rectangle area: 35 Total Triangle area: 17 You can see how an abstract class defined an interface in terms of getArea() and two other classes implemented same function but with different algorithm to calculate the area specific to the shape. Designing Strategy An object-oriented system might use an abstract base class to provide a common and standardized interface appropriate for all the external applications. Then, through inheritance from that abstract base class, derived classes are formed that operate similarly.
- 249. C++ 237 The capabilities (i.e., the public functions) offered by the external applications are provided as pure virtual functions in the abstract base class. The implementations of these pure virtual functions are provided in the derived classes that correspond to the specific types of the application. This architecture also allows new applications to be added to a system easily, even after the system has been defined.
- 250. C++ 238 So far, we have been using the iostream standard library, which provides cin and cout methods for reading from standard input and writing to standard output respectively. This tutorial will teach you how to read and write from a file. This requires another standard C++ library called fstream, which defines three new data types: Data Type Description ofstream This data type represents the output file stream and is used to create files and to write information to files. ifstream This data type represents the input file stream and is used to read information from files. fstream This data type represents the file stream generally, and has the capabilities of both ofstream and ifstream which means it can create files, write information to files, and read information from files. To perform file processing in C++, header files <iostream> and <fstream> must be included in your C++ source file. Opening a File A file must be opened before you can read from it or write to it. Either ofstream or fstream object may be used to open a file for writing. And ifstream object is used to open a file for reading purpose only. Following is the standard syntax for open() function, which is a member of fstream, ifstream, and ofstream objects. void open(const char *filename, ios::openmode mode); Here, the first argument specifies the name and location of the file to be opened and the second argument of the open() member function defines the mode in which the file should be opened. Mode Flag Description 30. FILES AND STREAMS
- 251. C++ 239 ios::app Append mode. All output to that file to be appended to the end. ios::ate Open a file for output and move the read/write control to the end of the file. ios::in Open a file for reading. ios::out Open a file for writing. ios::trunc If the file already exists, its contents will be truncated before opening the file. You can combine two or more of these values by ORing them together. For example if you want to open a file in write mode and want to truncate it in case that already exists, following will be the syntax: ofstream outfile; outfile.open("file.dat", ios::out | ios::trunc ); Similar way, you can open a file for reading and writing purpose as follows: fstream afile; afile.open("file.dat", ios::out | ios::in ); Closing a File When a C++ program terminates it automatically flushes all the streams, release all the allocated memory and close all the opened files. But it is always a good practice that a programmer should close all the opened files before program termination. Following is the standard syntax for close() function, which is a member of fstream, ifstream, and ofstream objects. void close(); Writing to a File While doing C++ programming, you write information to a file from your program using the stream insertion operator (<<) just as you use that operator to output information to the screen. The only difference is that you use an ofstream or fstream object instead of the cout object. Reading from a File
- 252. C++ 240 You read information from a file into your program using the stream extraction operator (>>) just as you use that operator to input information from the keyboard. The only difference is that you use an ifstream or fstream object instead of the cin object. Read & Write Example Following is the C++ program which opens a file in reading and writing mode. After writing information entered by the user to a file named afile.dat, the program reads information from the file and outputs it onto the screen: #include <fstream> #include <iostream> using namespace std; int main () { char data[100]; // open a file in write mode. ofstream outfile; outfile.open("afile.dat"); cout << "Writing to the file" << endl; cout << "Enter your name: "; cin.getline(data, 100); // write inputted data into the file. outfile << data << endl; cout << "Enter your age: "; cin >> data; cin.ignore(); // again write inputted data into the file. outfile << data << endl;
- 253. C++ 241 // close the opened file. outfile.close(); // open a file in read mode. ifstream infile; infile.open("afile.dat"); cout << "Reading from the file" << endl; infile >> data; // write the data at the screen. cout << data << endl; // again read the data from the file and display it. infile >> data; cout << data << endl; // close the opened file. infile.close(); return 0; } When the above code is compiled and executed, it produces the following sample input and output: $./a.out Writing to the file Enter your name: Zara Enter your age: 9 Reading from the file Zara 9 Above examples make use of additional functions from cin object, like getline() function to read the line from outside, and ignore() function to ignore the extra characters left by previous read statement.
- 254. C++ 242 File Position Pointers Both istream and ostream provide member functions for repositioning the file- position pointer. These member functions are seekg ("seek get") for istream and seekp ("seek put") for ostream. The argument to seekg and seekp normally is a long integer. A second argument can be specified to indicate the seek direction. The seek direction can be ios::beg (the default) for positioning relative to the beginning of a stream, ios::cur for positioning relative to the current position in a stream or ios::end for positioning relative to the end of a stream. The file-position pointer is an integer value that specifies the location in the file as a number of bytes from the file's starting location. Some examples of positioning the "get" file-position pointer are: // position to the nth byte of fileObject (assumes ios::beg) fileObject.seekg( n ); // position n bytes forward in fileObject fileObject.seekg( n, ios::cur ); // position n bytes back from end of fileObject fileObject.seekg( n, ios::end ); // position at end of fileObject fileObject.seekg( 0, ios::end );
- 255. C++ 243 An exception is a problem that arises during the execution of a program. A C++ exception is a response to an exceptional circumstance that arises while a program is running, such as an attempt to divide by zero. Exceptions provide a way to transfer control from one part of a program to another. C++ exception handling is built upon three keywords: try, catch, and throw. throw: A program throws an exception when a problem shows up. This is done using a throw keyword. catch: A program catches an exception with an exception handler at the place in a program where you want to handle the problem. The catch keyword indicates the catching of an exception. try: A try block identifies a block of code for which particular exceptions will be activated. It is followed by one or more catch blocks. Assuming a block will raise an exception, a method catches an exception using a combination of the try and catch keywords. A try/catch block is placed around the code that might generate an exception. Code within a try/catch block is referred to as protected code, and the syntax for using try/catch is as follows: try { // protected code }catch( ExceptionName e1 ) { // catch block }catch( ExceptionName e2 ) { // catch block }catch( ExceptionName eN ) { // catch block } You can list down multiple catch statements to catch different type of exceptions in case your try block raises more than one exception in different situations. 31. EXCEPTION HANDLING
- 256. C++ 244 Throwing Exceptions Exceptions can be thrown anywhere within a code block using throw statement. The operand of the throw statement determines a type for the exception and can be any expression and the type of the result of the expression determines the type of exception thrown. Following is an example of throwing an exception when dividing by zero condition occurs: double division(int a, int b) { if( b == 0 ) { throw "Division by zero condition!"; } return (a/b); } Catching Exceptions The catch block following the try block catches any exception. You can specify what type of exception you want to catch and this is determined by the exception declaration that appears in parentheses following the keyword catch. try { // protected code }catch( ExceptionName e ) { // code to handle ExceptionName exception } Above code will catch an exception of ExceptionName type. If you want to specify that a catch block should handle any type of exception that is thrown in a try block, you must put an ellipsis, ..., between the parentheses enclosing the exception declaration as follows: try { // protected code }catch(...)
- 257. C++ 245 { // code to handle any exception } The following is an example, which throws a division by zero exception and we catch it in catch block. #include <iostream> using namespace std; double division(int a, int b) { if( b == 0 ) { throw "Division by zero condition!"; } return (a/b); } int main () { int x = 50; int y = 0; double z = 0; try { z = division(x, y); cout << z << endl; }catch (const char* msg) { cerr << msg << endl; } return 0; }
- 258. C++ 246 Because we are raising an exception of type const char*, so while catching this exception, we have to use const char* in catch block. If we compile and run above code, this would produce the following result: Division by zero condition! C++ Standard Exceptions C++ provides a list of standard exceptions defined in <exception> which we can use in our programs. These are arranged in a parent-child class hierarchy shown below: Here is the small description of each exception mentioned in the above hierarchy: Exception Description std::exception An exception and parent class of all the standard C++ exceptions.
- 259. C++ 247 std::bad_alloc This can be thrown by new. std::bad_cast This can be thrown by dynamic_cast. std::bad_exception This is useful device to handle unexpected exceptions in a C++ program. std::bad_typeid This can be thrown by typeid. std::logic_error An exception that theoretically can be detected by reading the code. std::domain_error This is an exception thrown when a mathematically invalid domain is used. std::invalid_argument This is thrown due to invalid arguments. std::length_error This is thrown when a too big std::string is created. std::out_of_range This can be thrown by the ‘at’ method, for example a std::vector and std::bitset<>::operator[](). std::runtime_error An exception that theoretically cannot be detected by reading the code. std::overflow_error This is thrown if a mathematical overflow occurs. std::range_error This is occurred when you try to store a value which is out of range. std::underflow_error This is thrown if a mathematical underflow occurs. Define New Exceptions You can define your own exceptions by inheriting and overriding exception class functionality. Following is the example, which shows how you can use std::exception class to implement your own exception in standard way: #include <iostream> #include <exception> using namespace std;
- 260. C++ 248 struct MyException : public exception { const char * what () const throw () { return "C++ Exception"; } }; int main() { try { throw MyException(); } catch(MyException& e) { std::cout << "MyException caught" << std::endl; std::cout << e.what() << std::endl; } catch(std::exception& e) { //Other errors } } This would produce the following result: MyException caught C++ Exception Here, what() is a public method provided by exception class and it has been overridden by all the child exception classes. This returns the cause of an exception.
- 261. C++ 249 A good understanding of how dynamic memory really works in C++ is essential to becoming a good C++ programmer. Memory in your C++ program is divided into two parts: The stack: All variables declared inside the function will take up memory from the stack. The heap: This is unused memory of the program and can be used to allocate the memory dynamically when program runs. Many times, you are not aware in advance how much memory you will need to store particular information in a defined variable and the size of required memory can be determined at run time. You can allocate memory at run time within the heap for the variable of a given type using a special operator in C++ which returns the address of the space allocated. This operator is called new operator. If you are not in need of dynamically allocated memory anymore, you can use delete operator, which de-allocates memory previously allocated by new operator. The new and delete Operators There is following generic syntax to use new operator to allocate memory dynamically for any data-type. new data-type; Here, data-type could be any built-in data type including an array or any user defined data types include class or structure. Let us start with built-in data types. For example we can define a pointer to type double and then request that the memory be allocated at execution time. We can do this using the new operator with the following statements: double* pvalue = NULL; // Pointer initialized with null pvalue = new double; // Request memory for the variable The memory may not have been allocated successfully, if the free store had been used up. So it is good practice to check if new operator is returning NULL pointer and take appropriate action as below: 32. DYNAMIC MEMORY
- 262. C++ 250 double* pvalue = NULL; if( !(pvalue = new double )) { cout << "Error: out of memory." <<endl; exit(1); } The malloc() function from C, still exists in C++, but it is recommended to avoid using malloc() function. The main advantage of new over malloc() is that new doesn't just allocate memory, it constructs objects which is prime purpose of C++. At any point, when you feel a variable that has been dynamically allocated is not anymore required, you can free up the memory that it occupies in the free store with the delete operator as follows: delete pvalue; // Release memory pointed to by pvalue Let us put above concepts and form the following example to show how new and delete work: #include <iostream> using namespace std; int main () { double* pvalue = NULL; // Pointer initialized with null pvalue = new double; // Request memory for the variable *pvalue = 29494.99; // Store value at allocated address cout << "Value of pvalue : " << *pvalue << endl; delete pvalue; // free up the memory. return 0; } If we compile and run above code, this would produce the following result:
- 263. C++ 251 Value of pvalue : 29495 Dynamic Memory Allocation for Arrays Consider you want to allocate memory for an array of characters, i.e., string of 20 characters. Using the same syntax what we have used above we can allocate memory dynamically as shown below. char* pvalue = NULL; // Pointer initialized with null pvalue = new char[20]; // Request memory for the variable To remove the array that we have just created the statement would look like this: delete [] pvalue; // Delete array pointed to by pvalue Following is the syntax of new operator for a multi-dimensional array as follows: int ROW = 2; int COL = 3; double **pvalue = new double* [ROW]; // Allocate memory for rows // Now allocate memory for columns for(int i = 0; i < COL; i++) { pvalue[i] = new double[COL]; } The syntax to release the memory for multi-dimensional will be as follows: for(int i = 0; i < COL; i++) { delete[] pvalue[i]; } delete [] pvalue; Dynamic Memory Allocation for Objects Objects are no different from simple data types. For example, consider the following code where we are going to use an array of objects to clarify the concept: #include <iostream> using namespace std;
- 264. C++ 252 class Box { public: Box() { cout << "Constructor called!" <<endl; } ~Box() { cout << "Destructor called!" <<endl; } }; int main( ) { Box* myBoxArray = new Box[4]; delete [] myBoxArray; // Delete array return 0; } If you were to allocate an array of four Box objects, the Simple constructor would be called four times and similarly while deleting these objects, destructor will also be called same number of times. If we compile and run above code, this would produce the following result: Constructor called! Constructor called! Constructor called! Constructor called! Destructor called! Destructor called! Destructor called! Destructor called!
- 265. C++ 253 Consider a situation, when we have two persons with the same name, Zara, in the same class. Whenever we need to differentiate them definitely we would have to use some additional information along with their name, like either the area, if they live in different area or their mother’s or father’s name, etc. Same situation can arise in your C++ applications. For example, you might be writing some code that has a function called xyz() and there is another library available which is also having same function xyz(). Now the compiler has no way of knowing which version of xyz() function you are referring to within your code. A namespace is designed to overcome this difficulty and is used as additional information to differentiate similar functions, classes, variables etc. with the same name available in different libraries. Using namespace, you can define the context in which names are defined. In essence, a namespace defines a scope. Defining a Namespace A namespace definition begins with the keyword namespace followed by the namespace name as follows: namespace namespace_name { // code declarations } To call the namespace-enabled version of either function or variable, prepend (::) the namespace name as follows: name::code; // code could be variable or function. Let us see how namespace scope the entities including variable and functions: #include <iostream> using namespace std; // first name space namespace first_space{ void func(){ cout << "Inside first_space" << endl; } } 33. NAMESPACES
- 266. C++ 254 // second name space namespace second_space{ void func(){ cout << "Inside second_space" << endl; } } int main () { // Calls function from first name space. first_space::func(); // Calls function from second name space. second_space::func(); return 0; } If we compile and run above code, this would produce the following result: Inside first_space Inside second_space The using directive You can also avoid prepending of namespaces with the using namespace directive. This directive tells the compiler that the subsequent code is making use of names in the specified namespace. The namespace is thus implied for the following code: #include <iostream> using namespace std; // first name space namespace first_space{ void func(){ cout << "Inside first_space" << endl; }
- 267. C++ 255 } // second name space namespace second_space{ void func(){ cout << "Inside second_space" << endl; } } using namespace first_space; int main () { // This calls function from first name space. func(); return 0; } If we compile and run above code, this would produce the following result: Inside first_space The ‘using’ directive can also be used to refer to a particular item within a namespace. For example, if the only part of the std namespace that you intend to use is cout, you can refer to it as follows: using std::cout; Subsequent code can refer to cout without prepending the namespace, but other items in the std namespace will still need to be explicit as follows: #include <iostream> using std::cout; int main () { cout << "std::endl is used with std!" << std::endl;
- 268. C++ 256 return 0; } If we compile and run above code, this would produce the following result: std::endl is used with std! Names introduced in a using directive obey normal scope rules. The name is visible from the point of the using directive to the end of the scope in which the directive is found. Entities with the same name defined in an outer scope are hidden. Discontiguous Namespaces A namespace can be defined in several parts and so a namespace is made up of the sum of its separately defined parts. The separate parts of a namespace can be spread over multiple files. So, if one part of the namespace requires a name defined in another file, that name must still be declared. Writing a following namespace definition either defines a new namespace or adds new elements to an existing one: namespace namespace_name { // code declarations } Nested Namespaces Namespaces can be nested where you can define one namespace inside another namespace as follows: namespace namespace_name1 { // code declarations namespace namespace_name2 { // code declarations } } You can access members of nested namespace by using resolution operators as follows: // to access members of namespace_name2 using namespace namespace_name1::namespace_name2;
- 269. C++ 257 // to access members of namespace:name1 using namespace namespace_name1; In the above statements if you are using namespace_name1, then it will make elements of namespace_name2 available in the scope as follows: #include <iostream> using namespace std; // first name space namespace first_space{ void func(){ cout << "Inside first_space" << endl; } // second name space namespace second_space{ void func(){ cout << "Inside second_space" << endl; } } } using namespace first_space::second_space; int main () { // This calls function from second name space. func(); return 0; } If we compile and run above code, this would produce the following result: Inside second_space
- 270. C++ 258 Templates are the foundation of generic programming, which involves writing code in a way that is independent of any particular type. A template is a blueprint or formula for creating a generic class or a function. The library containers like iterators and algorithms are examples of generic programming and have been developed using template concept. There is a single definition of each container, such as vector, but we can define many different kinds of vectors for example, vector <int> or vector <string>. You can use templates to define functions as well as classes, let us see how they work: Function Template The general form of a template function definition is shown here: template <class type> ret-type func-name(parameter list) { // body of function } Here, type is a placeholder name for a data type used by the function. This name can be used within the function definition. The following is the example of a function template that returns the maximum of two values: #include <iostream> #include <string> using namespace std; template <typename T> inline T const& Max (T const& a, T const& b) { return a < b ? b:a; } int main () { 34. TEMPLATES
- 271. C++ 259 int i = 39; int j = 20; cout << "Max(i, j): " << Max(i, j) << endl; double f1 = 13.5; double f2 = 20.7; cout << "Max(f1, f2): " << Max(f1, f2) << endl; string s1 = "Hello"; string s2 = "World"; cout << "Max(s1, s2): " << Max(s1, s2) << endl; return 0; } If we compile and run above code, this would produce the following result: Max(i, j): 39 Max(f1, f2): 20.7 Max(s1, s2): World Class Template Just as we can define function templates, we can also define class templates. The general form of a generic class declaration is shown here: template <class type> class class-name { . . . } Here, type is the placeholder type name, which will be specified when a class is instantiated. You can define more than one generic data type by using a comma- separated list. Following is the example to define class Stack<> and implement generic methods to push and pop the elements from the stack:
- 272. C++ 260 #include <iostream> #include <vector> #include <cstdlib> #include <string> #include <stdexcept> using namespace std; template <class T> class Stack { private: vector<T> elems; // elements public: void push(T const&); // push element void pop(); // pop element T top() const; // return top element bool empty() const{ // return true if empty. return elems.empty(); } }; template <class T> void Stack<T>::push (T const& elem) { // append copy of passed element elems.push_back(elem); } template <class T> void Stack<T>::pop () { if (elems.empty()) { throw out_of_range("Stack<>::pop(): empty stack");
- 273. C++ 261 } // remove last element elems.pop_back(); } template <class T> T Stack<T>::top () const { if (elems.empty()) { throw out_of_range("Stack<>::top(): empty stack"); } // return copy of last element return elems.back(); } int main() { try { Stack<int> intStack; // stack of ints Stack<string> stringStack; // stack of strings // manipulate int stack intStack.push(7); cout << intStack.top() <<endl; // manipulate string stack stringStack.push("hello"); cout << stringStack.top() << std::endl; stringStack.pop(); stringStack.pop(); } catch (exception const& ex) { cerr << "Exception: " << ex.what() <<endl; return -1;
- 274. C++ 262 } } If we compile and run above code, this would produce the following result: 7 hello Exception: Stack<>::pop(): empty stack
- 275. C++ 263 The preprocessors are the directives, which give instructions to the compiler to preprocess the information before actual compilation starts. All preprocessor directives begin with #, and only white-space characters may appear before a preprocessor directive on a line. Preprocessor directives are not C++ statements, so they do not end in a semicolon (;). You already have seen a #include directive in all the examples. This macro is used to include a header file into the source file. There are number of preprocessor directives supported by C++ like #include, #define, #if, #else, #line, etc. Let us see important directives: The #define Preprocessor The #define preprocessor directive creates symbolic constants. The symbolic constant is called a macro and the general form of the directive is: #define macro-name replacement-text When this line appears in a file, all subsequent occurrences of macro in that file will be replaced by replacement-text before the program is compiled. For example: #include <iostream> using namespace std; #define PI 3.14159 int main () { cout << "Value of PI :" << PI << endl; return 0; } Now, let us do the preprocessing of this code to see the result assuming we have the source code file. So let us compile it with -E option and redirect the result to 35. PREPROCESSOR
- 276. C++ 264 test.p. Now, if you check test.p, it will have lots of information and at the bottom, you will find the value replaced as follows: $gcc -E test.cpp > test.p ... int main () { cout << "Value of PI :" << 3.14159 << endl; return 0; } Function-Like Macros You can use #define to define a macro which will take argument as follows: #include <iostream> using namespace std; #define MIN(a,b) (((a)<(b)) ? a : b) int main () { int i, j; i = 100; j = 30; cout <<"The minimum is " << MIN(i, j) << endl; return 0; } If we compile and run above code, this would produce the following result: The minimum is 30 Conditional Compilation
- 277. C++ 265 There are several directives, which can be used to compile selective portions of your program's source code. This process is called conditional compilation. The conditional preprocessor construct is much like the ‘if’ selection structure. Consider the following preprocessor code: #ifndef NULL #define NULL 0 #endif You can compile a program for debugging purpose. You can also turn on or off the debugging using a single macro as follows: #ifdef DEBUG cerr <<"Variable x = " << x << endl; #endif This causes the cerr statement to be compiled in the program if the symbolic constant DEBUG has been defined before directive #ifdef DEBUG. You can use #if 0 statement to comment out a portion of the program as follows: #if 0 code prevented from compiling #endif Let us try the following example: #include <iostream> using namespace std; #define DEBUG #define MIN(a,b) (((a)<(b)) ? a : b) int main () { int i, j; i = 100; j = 30; #ifdef DEBUG cerr <<"Trace: Inside main function" << endl; #endif
- 278. C++ 266 #if 0 /* This is commented part */ cout << MKSTR(HELLO C++) << endl; #endif cout <<"The minimum is " << MIN(i, j) << endl; #ifdef DEBUG cerr <<"Trace: Coming out of main function" << endl; #endif return 0; } If we compile and run above code, this would produce the following result: Trace: Inside main function The minimum is 30 Trace: Coming out of main function The # and # # Operators The # and ## preprocessor operators are available in C++ and ANSI/ISO C. The # operator causes a replacement-text token to be converted to a string surrounded by quotes. Consider the following macro definition: #include <iostream> using namespace std; #define MKSTR( x ) #x int main () { cout << MKSTR(HELLO C++) << endl; return 0;
- 279. C++ 267 } If we compile and run above code, this would produce the following result: HELLO C++ Let us see how it worked. It is simple to understand that the C++ preprocessor turns the line: cout << MKSTR(HELLO C++) << endl; Above line will be turned into the following line: cout << "HELLO C++" << endl; The ## operator is used to concatenate two tokens. Here is an example: #define CONCAT( x, y ) x ## y When CONCAT appears in the program, its arguments are concatenated and used to replace the macro. For example, CONCAT(HELLO, C++) is replaced by "HELLO C++" in the program as follows. #include <iostream> using namespace std; #define concat(a, b) a ## b int main() { int xy = 100; cout << concat(x, y); return 0; } If we compile and run above code, this would produce the following result: 100 Let us see how it worked. It is simple to understand that the C++ preprocessor transforms: cout << concat(x, y);
- 280. C++ 268 Above line will be transformed into the following line: cout << xy; Predefined C++ Macros C++ provides a number of predefined macros mentioned below: Macro Description __LINE__ This contains the current line number of the program when it is being compiled. __FILE__ This contains the current file name of the program when it is being compiled. __DATE__ This contains a string of the form month/day/year that is the date of the translation of the source file into object code. __TIME__ This contains a string of the form hour:minute:second that is the time at which the program was compiled. Let us see an example for all the above macros: #include <iostream> using namespace std; int main () { cout << "Value of __LINE__ : " << __LINE__ << endl; cout << "Value of __FILE__ : " << __FILE__ << endl; cout << "Value of __DATE__ : " << __DATE__ << endl; cout << "Value of __TIME__ : " << __TIME__ << endl; return 0; } If we compile and run above code, this would produce the following result:
- 281. C++ 269 Value of __LINE__ : 6 Value of __FILE__ : test.cpp Value of __DATE__ : Feb 28 2011 Value of __TIME__ : 18:52:48
- 282. C++ 270 Signals are the interrupts delivered to a process by the operating system which can terminate a program prematurely. You can generate interrupts by pressing Ctrl+C on a UNIX, LINUX, Mac OS X or Windows system. There are signals which cannot be caught by the program but there is a following list of signals which you can catch in your program and can take appropriate actions based on the signal. These signals are defined in C++ header file <csignal>. Signal Description SIGABRT Abnormal termination of the program, such as a call to abort. SIGFPE An erroneous arithmetic operation, such as a divide by zero or an operation resulting in overflow. SIGILL Detection of an illegal instruction. SIGINT Receipt of an interactive attention signal. SIGSEGV An invalid access to storage. SIGTERM A termination request sent to the program. The signal() Function C++ signal-handling library provides function signal to trap unexpected events. Following is the syntax of the signal() function: void (*signal (int sig, void (*func)(int)))(int); Keeping it simple, this function receives two arguments: first argument as an integer, which represents signal number and second argument as a pointer to the signal-handling function. Let us write a simple C++ program where we will catch SIGINT signal using signal() function. Whatever signal you want to catch in your program, you must register that signal using signal function and associate it with a signal handler. Examine the following example: #include <iostream> 36. SIGNAL HANDLING
- 283. C++ 271 #include <csignal> using namespace std; void signalHandler( int signum ) { cout << "Interrupt signal (" << signum << ") received.n"; // cleanup and close up stuff here // terminate program exit(signum); } int main () { // register signal SIGINT and signal handler signal(SIGINT, signalHandler); while(1){ cout << "Going to sleep...." << endl; sleep(1); } return 0; } When the above code is compiled and executed, it produces the following result: Going to sleep.... Going to sleep.... Going to sleep.... Now, press Ctrl+C to interrupt the program and you will see that your program will catch the signal and would come out by printing something as follows:
- 284. C++ 272 Going to sleep.... Going to sleep.... Going to sleep.... Interrupt signal (2) received. The raise() Function You can generate signals by function raise(), which takes an integer signal number as an argument and has the following syntax. int raise (signal sig); Here, sig is the signal number to send any of the signals: SIGINT, SIGABRT, SIGFPE, SIGILL, SIGSEGV, SIGTERM, SIGHUP. Following is the example where we raise a signal internally using raise() function as follows: #include <iostream> #include <csignal> using namespace std; void signalHandler( int signum ) { cout << "Interrupt signal (" << signum << ") received.n"; // cleanup and close up stuff here // terminate program exit(signum); } int main () { int i = 0; // register signal SIGINT and signal handler signal(SIGINT, signalHandler);
- 285. C++ 273 while(++i){ cout << "Going to sleep...." << endl; if( i == 3 ){ raise( SIGINT); } sleep(1); } return 0; } When the above code is compiled and executed, it produces the following result and would come out automatically: Going to sleep.... Going to sleep.... Going to sleep.... Interrupt signal (2) received.
- 286. C++ 274 Multithreading is a specialized form of multitasking and a multitasking is the feature that allows your computer to run two or more programs concurrently. In general, there are two types of multitasking: process-based and thread-based. Process-based multitasking handles the concurrent execution of programs. Thread-based multitasking deals with the concurrent execution of pieces of the same program. A multithreaded program contains two or more parts that can run concurrently. Each part of such a program is called a thread, and each thread defines a separate path of execution. C++ does not contain any built-in support for multithreaded applications. Instead, it relies entirely upon the operating system to provide this feature. This tutorial assumes that you are working on Linux OS and we are going to write multi-threaded C++ program using POSIX. POSIX Threads, or Pthreads provides API which are available on many Unix-like POSIX systems such as FreeBSD, NetBSD, GNU/Linux, Mac OS X and Solaris. Creating Threads The following routine is used to create a POSIX thread: #include <pthread.h> pthread_create (thread, attr, start_routine, arg) Here, pthread_create creates a new thread and makes it executable. This routine can be called any number of times from anywhere within your code. Here is the description of the parameters: Parameter Description thread An opaque, unique identifier for the new thread returned by the subroutine. attr An opaque attribute object that may be used to set thread attributes. You can specify a thread attributes object, or NULL for the default values. start_routine The C++ routine that the thread will execute once it is created. 37. MULTITHREADING
- 287. C++ 275 arg A single argument that may be passed to start_routine. It must be passed by reference as a pointer cast of type void. NULL may be used if no argument is to be passed. The maximum number of threads that may be created by a process is implementation dependent. Once created, threads are peers, and may create other threads. There is no implied hierarchy or dependency between threads. Terminating Threads There is following routine which we use to terminate a POSIX thread: #include <pthread.h> pthread_exit (status) Here pthread_exit is used to explicitly exit a thread. Typically, the pthread_exit() routine is called after a thread has completed its work and is no longer required to exist. If main() finishes before the threads it has created, and exits with pthread_exit(), the other threads will continue to execute. Otherwise, they will be automatically terminated when main() finishes. Example: This simple example code creates 5 threads with the pthread_create() routine. Each thread prints a "Hello World!" message, and then terminates with a call to pthread_exit(). #include <iostream> #include <cstdlib> #include <pthread.h> using namespace std; #define NUM_THREADS 5 void *PrintHello(void *threadid) { long tid; tid = (long)threadid; cout << "Hello World! Thread ID, " << tid << endl;
- 288. C++ 276 pthread_exit(NULL); } int main () { pthread_t threads[NUM_THREADS]; int rc; int i; for( i=0; i < NUM_THREADS; i++ ){ cout << "main() : creating thread, " << i << endl; rc = pthread_create(&threads[i], NULL, PrintHello, (void *)i); if (rc){ cout << "Error:unable to create thread," << rc << endl; exit(-1); } } pthread_exit(NULL); } Compile the following program using -lpthread library as follows: $gcc test.cpp -lpthread Now, execute your program which gives the following output: main() : creating thread, 0 main() : creating thread, 1 main() : creating thread, 2 main() : creating thread, 3 main() : creating thread, 4 Hello World! Thread ID, 0 Hello World! Thread ID, 1 Hello World! Thread ID, 2 Hello World! Thread ID, 3 Hello World! Thread ID, 4
- 289. C++ 277 Passing Arguments to Threads This example shows how to pass multiple arguments via a structure. You can pass any data type in a thread callback because it points to void as explained in the following example: #include <iostream> #include <cstdlib> #include <pthread.h> using namespace std; #define NUM_THREADS 5 struct thread_data{ int thread_id; char *message; }; void *PrintHello(void *threadarg) { struct thread_data *my_data; my_data = (struct thread_data *) threadarg; cout << "Thread ID : " << my_data->thread_id ; cout << " Message : " << my_data->message << endl; pthread_exit(NULL); } int main () { pthread_t threads[NUM_THREADS]; struct thread_data td[NUM_THREADS]; int rc;
- 290. C++ 278 int i; for( i=0; i < NUM_THREADS; i++ ){ cout <<"main() : creating thread, " << i << endl; td[i].thread_id = i; td[i].message = "This is message"; rc = pthread_create(&threads[i], NULL, PrintHello, (void *)&td[i]); if (rc){ cout << "Error:unable to create thread," << rc << endl; exit(-1); } } pthread_exit(NULL); } When the above code is compiled and executed, it produces the following result: main() : creating thread, 0 main() : creating thread, 1 main() : creating thread, 2 main() : creating thread, 3 main() : creating thread, 4 Thread ID : 3 Message : This is message Thread ID : 2 Message : This is message Thread ID : 0 Message : This is message Thread ID : 1 Message : This is message Thread ID : 4 Message : This is message Joining and Detaching Threads There are following two routines which we can use to join or detach threads: pthread_join (threadid, status) pthread_detach (threadid)
- 291. C++ 279 The pthread_join() subroutine blocks the calling thread until the specified ‘threadid’ thread terminates. When a thread is created, one of its attributes defines whether it is joinable or detached. Only threads that are created as joinable can be joined. If a thread is created as detached, it can never be joined. This example demonstrates how to wait for thread completions by using the Pthread join routine. #include <iostream> #include <cstdlib> #include <pthread.h> #include <unistd.h> using namespace std; #define NUM_THREADS 5 void *wait(void *t) { int i; long tid; tid = (long)t; sleep(1); cout << "Sleeping in thread " << endl; cout << "Thread with id : " << tid << " ...exiting " << endl; pthread_exit(NULL); } int main () { int rc; int i; pthread_t threads[NUM_THREADS]; pthread_attr_t attr; void *status;
- 292. C++ 280 // Initialize and set thread joinable pthread_attr_init(&attr); pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_JOINABLE); for( i=0; i < NUM_THREADS; i++ ){ cout << "main() : creating thread, " << i << endl; rc = pthread_create(&threads[i], NULL, wait, (void *)i ); if (rc){ cout << "Error:unable to create thread," << rc << endl; exit(-1); } } // free attribute and wait for the other threads pthread_attr_destroy(&attr); for( i=0; i < NUM_THREADS; i++ ){ rc = pthread_join(threads[i], &status); if (rc){ cout << "Error:unable to join," << rc << endl; exit(-1); } cout << "Main: completed thread id :" << i ; cout << " exiting with status :" << status << endl; } cout << "Main: program exiting." << endl; pthread_exit(NULL); } When the above code is compiled and executed, it produces the following result: main() : creating thread, 0 main() : creating thread, 1 main() : creating thread, 2
- 293. C++ 281 main() : creating thread, 3 main() : creating thread, 4 Sleeping in thread Thread with id : 0 .... exiting Sleeping in thread Thread with id : 1 .... exiting Sleeping in thread Thread with id : 2 .... exiting Sleeping in thread Thread with id : 3 .... exiting Sleeping in thread Thread with id : 4 .... exiting Main: completed thread id :0 exiting with status :0 Main: completed thread id :1 exiting with status :0 Main: completed thread id :2 exiting with status :0 Main: completed thread id :3 exiting with status :0 Main: completed thread id :4 exiting with status :0 Main: program exiting.
- 294. C++ 282 What is CGI? The Common Gateway Interface, or CGI, is a set of standards that define how information is exchanged between the web server and a custom script. The CGI specs are currently maintained by the NCSA and NCSA defines CGI is as follows: The Common Gateway Interface, or CGI, is a standard for external gateway programs to interface with information servers such as HTTP servers. The current version is CGI/1.1 and CGI/1.2 is under progress. Web Browsing To understand the concept of CGI, let's see what happens when we click a hyperlink to browse a particular web page or URL. Your browser contacts the HTTP web server and demand for the URL i.e. filename. Web Server will parse the URL and will look for the filename. If it finds the requested file then web server sends that file back to the browser otherwise sends an error message indicating that you have requested a wrong file. Web browser takes response from web server and displays either the received file or error message based on the received response. However, it is possible to set up the HTTP server in such a way that whenever a file in a certain directory is requested, that file is not sent back; instead it is executed as a program, and produced output from the program is sent back to your browser to display. The Common Gateway Interface (CGI) is a standard protocol for enabling applications (called CGI programs or CGI scripts) to interact with Web servers and with clients. These CGI programs can be a written in Python, PERL, Shell, C or C++ etc. CGI Architecture Diagram The following simple program shows a simple architecture of CGI: 38. WEB PROGRAMMING
- 295. C++ 283 Web Server Configuration Before you proceed with CGI Programming, make sure that your Web Server supports CGI and it is configured to handle CGI Programs. All the CGI Programs to be executed by the HTTP server are kept in a pre-configured directory. This directory is called CGI directory and by convention it is named as /var/www/cgi- bin. By convention CGI files will have extension as .cgi, though they are C++ executable. By default, Apache Web Server is configured to run CGI programs in /var/www/cgi-bin. If you want to specify any other directory to run your CGI scripts, you can modify the following section in the httpd.conf file: <Directory "/var/www/cgi-bin"> AllowOverride None Options ExecCGI Order allow,deny Allow from all </Directory>
- 296. C++ 284 <Directory "/var/www/cgi-bin"> Options All </Directory> Here, I assume that you have Web Server up and running successfully and you are able to run any other CGI program like Perl or Shell etc. First CGI Program Consider the following C++ Program content: #include <iostream> using namespace std; int main () { cout << "Content-type:text/htmlrnrn"; cout << "<html>n"; cout << "<head>n"; cout << "<title>Hello World - First CGI Program</title>n"; cout << "</head>n"; cout << "<body>n"; cout << "<h2>Hello World! This is my first CGI program</h2>n"; cout << "</body>n"; cout << "</html>n"; return 0; } Compile above code and name the executable as cplusplus.cgi. This file is being kept in /var/www/cgi-bin directory and it has following content. Before running your CGI program make sure you have change mode of file using chmod 755 cplusplus.cgi UNIX command to make file executable. Now if you click cplusplus.cgi then this produces the following output: My First CGI program The above C++ program is a simple program which is writing its output on STDOUT file i.e. screen. There is one important and extra feature available which is first line printing Content-type:text/htmlrnrn. This line is sent back to the browser and specify the content type to be displayed on the browser screen.
- 297. C++ 285 Now you must have understood the basic concept of CGI and you can write many complicated CGI programs using Python. A C++ CGI program can interact with any other external system, such as RDBMS, to exchange information. HTTP Header The line Content-type:text/htmlrnrn is a part of HTTP header, which is sent to the browser to understand the content. All the HTTP header will be in the following form: HTTP Field Name: Field Content For Example Content-type: text/htmlrnrn There are few other important HTTP headers, which you will use frequently in your CGI Programming. Header Description Content-type: A MIME string defining the format of the file being returned. Example is Content-type:text/html. Expires: Date The date the information becomes invalid. This should be used by the browser to decide when a page needs to be refreshed. A valid date string should be in the format 01 Jan 1998 12:00:00 GMT. Location: URL The URL that should be returned instead of the URL requested. You can use this field to redirect a request to any file. Last-modified: Date The date of last modification of the resource. Content-length: N The length, in bytes, of the data being returned. The browser uses this value to report the estimated download time for a file. Set-Cookie: String Set the cookie passed through the string. CGI Environment Variables All the CGI program will have access to the following environment variables. These variables play an important role while writing any CGI program.
- 298. C++ 286 Variable Name Description CONTENT_TYPE The data type of the content, used when the client is sending attached content to the server. For example file upload etc. CONTENT_LENGTH The length of the query information that is available only for POST requests. HTTP_COOKIE Returns the set cookies in the form of key & value pair. HTTP_USER_AGENT The User-Agent request-header field contains information about the user agent originating the request. It is a name of the web browser. PATH_INFO The path for the CGI script. QUERY_STRING The URL-encoded information that is sent with GET method request. REMOTE_ADDR The IP address of the remote host making the request. This can be useful for logging or for authentication purpose. REMOTE_HOST The fully qualified name of the host making the request. If this information is not available then REMOTE_ADDR can be used to get IR address. REQUEST_METHOD The method used to make the request. The most common methods are GET and POST. SCRIPT_FILENAME The full path to the CGI script. SCRIPT_NAME The name of the CGI script. SERVER_NAME The server's hostname or IP Address. SERVER_SOFTWARE The name and version of the software the server is running.
- 299. C++ 287 Here is small CGI program to list out all the CGI variables. #include <iostream> #include <stdlib.h> using namespace std; const string ENV[ 24 ] = { "COMSPEC", "DOCUMENT_ROOT", "GATEWAY_INTERFACE", "HTTP_ACCEPT", "HTTP_ACCEPT_ENCODING", "HTTP_ACCEPT_LANGUAGE", "HTTP_CONNECTION", "HTTP_HOST", "HTTP_USER_AGENT", "PATH", "QUERY_STRING", "REMOTE_ADDR", "REMOTE_PORT", "REQUEST_METHOD", "REQUEST_URI", "SCRIPT_FILENAME", "SCRIPT_NAME", "SERVER_ADDR", "SERVER_ADMIN", "SERVER_NAME","SERVER_PORT","SERVER_PROTOCOL", "SERVER_SIGNATURE","SERVER_SOFTWARE" }; int main () { cout << "Content-type:text/htmlrnrn"; cout << "<html>n"; cout << "<head>n"; cout << "<title>CGI Environment Variables</title>n"; cout << "</head>n"; cout << "<body>n"; cout << "<table border = "0" cellspacing = "2">"; for ( int i = 0; i < 24; i++ ) { cout << "<tr><td>" << ENV[ i ] << "</td><td>"; // attempt to retrieve value of environment variable char *value = getenv( ENV[ i ].c_str() );
- 300. C++ 288 if ( value != 0 ){ cout << value; }else{ cout << "Environment variable does not exist."; } cout << "</td></tr>n"; } cout << "</table><n"; cout << "</body>n"; cout << "</html>n"; return 0; } The output is as follows: COMSPEC Environment variable does not exist. DOCUMENT_ROOT /var/www/tutorialspoint GATEWAY_INTERFACE CGI/1.1 HTTP_ACCEPT text/html, application/xhtml+xml, */* HTTP_ACCEPT_ENCODING gzip, deflate HTTP_ACCEPT_LANGUAGE en-US HTTP_CONNECTION Keep-Alive HTTP_HOST www.tutorialspoint.com HTTP_USER_AGENT Mozilla/5.0 (Windows NT 6.3; WOW64; Trident/7.0; rv:11.0) like Gecko PATH /sbin:/usr/sbin:/bin:/usr/bin QUERY_STRING REMOTE_ADDR 183.82.104.71 REMOTE_PORT 50902 REQUEST_METHOD GET REQUEST_URI /cgi-bin/cpp_env.cgi SCRIPT_FILENAME /var/www/cgi-bin/cpp_env.cgi SCRIPT_NAME /cgi-bin/cpp_env.cgi SERVER_ADDR 66.135.33.172
- 301. C++ 289 SERVER_ADMIN webmaster@tutorialspoint.com SERVER_NAME www.tutorialspoint.com SERVER_PORT 80 SERVER_PROTOCOL HTTP/1.1 SERVER_SIGNATURE SERVER_SOFTWARE Apache < C++ CGI Library For real examples, you would need to do many operations by your CGI program. There is a CGI library written for C++ program which you can download from ftp://ftp.gnu.org/gnu/cgicc/ and follow the steps to install the library: $tar xzf cgicc-X.X.X.tar.gz $cd cgicc-X.X.X/ $./configure --prefix=/usr $make $make install You can check related documentation available at ‘C++ CGI Lib Documentation’. GET and POST Methods You must have come across many situations when you need to pass some information from your browser to web server and ultimately to your CGI Program. Most frequently browser uses two methods to pass this information to web server. These methods are GET Method and POST Method. Passing Information Using GET Method The GET method sends the encoded user information appended to the page request. The page and the encoded information are separated by the ‘?’ character as follows: http://www.test.com/cgi-bin/cpp.cgi?key1=value1&key2=value2 The GET method is the default method to pass information from browser to web server and it produces a long string that appears in your browser's Location:box. Never use the GET method if you have password or other sensitive information to pass to the server. The GET method has size limitation and you can pass up to 1024 characters in a request string. When using GET method, information is passed using QUERY_STRING http header and will be accessible in your CGI Program through QUERY_STRING environment variable.
- 302. C++ 290 You can pass information by simply concatenating key and value pairs along with any URL or you can use HTML <FORM> tags to pass information using GET method. Simple URL Example: Get Method Here is a simple URL which will pass two values to hello_get.py program using GET method. /cgi-bin/cpp_get.cgi?first_name=ZARA&last_name=ALI Below is a program to generate cpp_get.cgi CGI program to handle input given by web browser. We are going to use C++ CGI library which makes it very easy to access passed information: #include <iostream> #include <vector> #include <string> #include <stdio.h> #include <stdlib.h> #include <cgicc/CgiDefs.h> #include <cgicc/Cgicc.h> #include <cgicc/HTTPHTMLHeader.h> #include <cgicc/HTMLClasses.h> using namespace std; using namespace cgicc; int main () { Cgicc formData; cout << "Content-type:text/htmlrnrn"; cout << "<html>n"; cout << "<head>n"; cout << "<title>Using GET and POST Methods</title>n"; cout << "</head>n"; cout << "<body>n";
- 303. C++ 291 form_iterator fi = formData.getElement("first_name"); if( !fi->isEmpty() && fi != (*formData).end()) { cout << "First name: " << **fi << endl; }else{ cout << "No text entered for first name" << endl; } cout << "<br/>n"; fi = formData.getElement("last_name"); if( !fi->isEmpty() &&fi != (*formData).end()) { cout << "Last name: " << **fi << endl; }else{ cout << "No text entered for last name" << endl; } cout << "<br/>n"; cout << "</body>n"; cout << "</html>n"; return 0; } Now, compile the above program as follows: $g++ -o cpp_get.cgi cpp_get.cpp -lcgicc Generate cpp_get.cgi and put it in your CGI directory and try to access using following link: /cgi-bin/cpp_get.cgi?first_name=ZARA&last_name=ALI This would generate following result: First name: ZARA Last name: ALI Simple FORM Example: GET Method Here is a simple example which passes two values using HTML FORM and submit button. We are going to use same CGI script cpp_get.cgi to handle this input. <form action="/cgi-bin/cpp_get.cgi" method="get">
- 304. C++ 292 First Name: <input type="text" name="first_name"> <br /> Last Name: <input type="text" name="last_name" /> <input type="submit" value="Submit" /> </form> Here is the actual output of the above form. You enter First and Last Name and then click submit button to see the result. First Name: Last Name: Submit Passing Information Using POST Method A generally more reliable method of passing information to a CGI program is the POST method. This packages the information in exactly the same way as GET methods, but instead of sending it as a text string after a ‘?’ in the URL it sends it as a separate message. This message comes into the CGI script in the form of the standard input. The same cpp_get.cgi program will handle POST method as well. Let us take same example as above, which passes two values using HTML FORM and submit button but this time with POST method as follows: <form action="/cgi-bin/cpp_get.cgi" method="post"> First Name: <input type="text" name="first_name"><br /> Last Name: <input type="text" name="last_name" /> <input type="submit" value="Submit" /> </form> Here is the actual output of the above form. You enter First and Last Name and then click submit button to see the result. First Name: Last Name: Submit Passing Checkbox Data to CGI Program Checkboxes are used when more than one option is required to be selected.
- 305. C++ 293 Here is example HTML code for a form with two checkboxes: <form action="/cgi-bin/cpp_checkbox.cgi" method="POST" target="_blank"> <input type="checkbox" name="maths" value="on" /> Maths <input type="checkbox" name="physics" value="on" /> Physics <input type="submit" value="Select Subject" /> </form> The result of this code is the following form: Maths Physics Select Subject Below is C++ program, which will generate cpp_checkbox.cgi script to handle input given by web browser through checkbox button. #include <iostream> #include <vector> #include <string> #include <stdio.h> #include <stdlib.h> #include <cgicc/CgiDefs.h> #include <cgicc/Cgicc.h> #include <cgicc/HTTPHTMLHeader.h> #include <cgicc/HTMLClasses.h> using namespace std; using namespace cgicc; int main () { Cgicc formData; bool maths_flag, physics_flag; cout << "Content-type:text/htmlrnrn"; cout << "<html>n";
- 306. C++ 294 cout << "<head>n"; cout << "<title>Checkbox Data to CGI</title>n"; cout << "</head>n"; cout << "<body>n"; maths_flag = formData.queryCheckbox("maths"); if( maths_flag ) { cout << "Maths Flag: ON " << endl; }else{ cout << "Maths Flag: OFF " << endl; } cout << "<br/>n"; physics_flag = formData.queryCheckbox("physics"); if( physics_flag ) { cout << "Physics Flag: ON " << endl; }else{ cout << "Physics Flag: OFF " << endl; } cout << "<br/>n"; cout << "</body>n"; cout << "</html>n"; return 0; } Passing Radio Button Data to CGI Program Radio Buttons are used when only one option is required to be selected. Here is example HTML code for a form with two radio button: <form action="/cgi-bin/cpp_radiobutton.cgi" method="post" target="_blank"> <input type="radio" name="subject" value="maths" checked="checked"/> Maths
- 307. C++ 295 <input type="radio" name="subject" value="physics" /> Physics <input type="submit" value="Select Subject" /> </form> The result of this code is the following form: Maths Physics Select Subject Below is C++ program, which will generate cpp_radiobutton.cgi script to handle input given by web browser through radio buttons. #include <iostream> #include <vector> #include <string> #include <stdio.h> #include <stdlib.h> #include <cgicc/CgiDefs.h> #include <cgicc/Cgicc.h> #include <cgicc/HTTPHTMLHeader.h> #include <cgicc/HTMLClasses.h> using namespace std; using namespace cgicc; int main () { Cgicc formData; cout << "Content-type:text/htmlrnrn"; cout << "<html>n"; cout << "<head>n"; cout << "<title>Radio Button Data to CGI</title>n"; cout << "</head>n"; cout << "<body>n"; form_iterator fi = formData.getElement("subject");
- 308. C++ 296 if( !fi->isEmpty() && fi != (*formData).end()) { cout << "Radio box selected: " << **fi << endl; } cout << "<br/>n"; cout << "</body>n"; cout << "</html>n"; return 0; } Passing Text Area Data to CGI Program TEXTAREA element is used when multiline text has to be passed to the CGI Program. Here is example HTML code for a form with a TEXTAREA box: <form action="/cgi-bin/cpp_textarea.cgi" method="post" target="_blank"> <textarea name="textcontent" cols="40" rows="4"> Type your text here... </textarea> <input type="submit" value="Submit" /> </form> The result of this code is the following form: Submit Below is C++ program, which will generate cpp_textarea.cgi script to handle input given by web browser through text area. #include <iostream> #include <vector> #include <string> #include <stdio.h>
- 309. C++ 297 #include <stdlib.h> #include <cgicc/CgiDefs.h> #include <cgicc/Cgicc.h> #include <cgicc/HTTPHTMLHeader.h> #include <cgicc/HTMLClasses.h> using namespace std; using namespace cgicc; int main () { Cgicc formData; cout << "Content-type:text/htmlrnrn"; cout << "<html>n"; cout << "<head>n"; cout << "<title>Text Area Data to CGI</title>n"; cout << "</head>n"; cout << "<body>n"; form_iterator fi = formData.getElement("textcontent"); if( !fi->isEmpty() && fi != (*formData).end()) { cout << "Text Content: " << **fi << endl; }else{ cout << "No text entered" << endl; } cout << "<br/>n"; cout << "</body>n"; cout << "</html>n"; return 0; }
- 310. C++ 298 Passing Dropdown Box Data to CGI Program Dropdown Box is used when we have many options available but only one or two will be selected. Here is example HTML code for a form with one dropdown box: <form action="/cgi-bin/cpp_dropdown.cgi" method="post" target="_blank"> <select name="dropdown"> <option value="Maths" selected>Maths</option> <option value="Physics">Physics</option> </select> <input type="submit" value="Submit"/> </form> The result of this code is the following form: Maths Submit Below is C++ program, which will generate cpp_dropdown.cgi script to handle input given by web browser through drop down box. #include <iostream> #include <vector> #include <string> #include <stdio.h> #include <stdlib.h> #include <cgicc/CgiDefs.h> #include <cgicc/Cgicc.h> #include <cgicc/HTTPHTMLHeader.h> #include <cgicc/HTMLClasses.h> using namespace std; using namespace cgicc; int main () { Cgicc formData;
- 311. C++ 299 cout << "Content-type:text/htmlrnrn"; cout << "<html>n"; cout << "<head>n"; cout << "<title>Drop Down Box Data to CGI</title>n"; cout << "</head>n"; cout << "<body>n"; form_iterator fi = formData.getElement("dropdown"); if( !fi->isEmpty() && fi != (*formData).end()) { cout << "Value Selected: " << **fi << endl; } cout << "<br/>n"; cout << "</body>n"; cout << "</html>n"; return 0; } Using Cookies in CGI HTTP protocol is a stateless protocol. But for a commercial website it is required to maintain session information among different pages. For example one user registration ends after completing many pages. But how to maintain user's session information across all the web pages. In many situations, using cookies is the most efficient method of remembering and tracking preferences, purchases, commissions, and other information required for better visitor experience or site statistics. How It Works Your server sends some data to the visitor's browser in the form of a cookie. The browser may accept the cookie. If it does, it is stored as a plain text record on the visitor's hard drive. Now, when the visitor arrives at another page on your site, the cookie is available for retrieval. Once retrieved, your server knows/remembers what was stored. Cookies are a plain text data record of 5 variable-length fields: Expires: This showsthe date the cookie will expire. If this is blank, the cookie will expire when the visitor quits the browser.
- 312. C++ 300 Domain: This is the domain name of your site. Path: This is the path to the directory or web page that sets the cookie. This may be blank if you want to retrieve the cookie from any directory or page. Secure: If this field contains the word "secure" then the cookie may only be retrieved with a secure server. If this field is blank, no such restriction exists. Name=Value: Cookies are set and retrieved in the form of key and value pairs. Setting up Cookies It is very easy to send cookies to browser. These cookies will be sent along with HTTP Header before the Content-type filed. Assuming you want to set UserID and Password as cookies. So cookies setting will be done as follows: #include <iostream> using namespace std; int main () { cout << "Set-Cookie:UserID=XYZ;rn"; cout << "Set-Cookie:Password=XYZ123;rn"; cout << "Set-Cookie:Domain=www.tutorialspoint.com;rn"; cout << "Set-Cookie:Path=/perl;n"; cout << "Content-type:text/htmlrnrn"; cout << "<html>n"; cout << "<head>n"; cout << "<title>Cookies in CGI</title>n"; cout << "</head>n"; cout << "<body>n"; cout << "Setting cookies" << endl; cout << "<br/>n"; cout << "</body>n";
- 313. C++ 301 cout << "</html>n"; return 0; } From this example, you must have understood how to set cookies. We use Set- Cookie HTTP header to set cookies. Here, it is optional to set cookies attributes like Expires, Domain, and Path. It is notable that cookies are set before sending magic line "Content- type:text/htmlrnrn. Compile above program to produce setcookies.cgi, and try to set cookies using following link. It will set four cookies at your computer: /cgi-bin/setcookies.cgi Retrieving Cookies It is easy to retrieve all the set cookies. Cookies are stored in CGI environment variable HTTP_COOKIE and they will have following form. key1=value1;key2=value2;key3=value3.... Here is an example of how to retrieve cookies. #include <iostream> #include <vector> #include <string> #include <stdio.h> #include <stdlib.h> #include <cgicc/CgiDefs.h> #include <cgicc/Cgicc.h> #include <cgicc/HTTPHTMLHeader.h> #include <cgicc/HTMLClasses.h> using namespace std; using namespace cgicc; int main () {
- 314. C++ 302 Cgicc cgi; const_cookie_iterator cci; cout << "Content-type:text/htmlrnrn"; cout << "<html>n"; cout << "<head>n"; cout << "<title>Cookies in CGI</title>n"; cout << "</head>n"; cout << "<body>n"; cout << "<table border = "0" cellspacing = "2">"; // get environment variables const CgiEnvironment& env = cgi.getEnvironment(); for( cci = env.getCookieList().begin(); cci != env.getCookieList().end(); ++cci ) { cout << "<tr><td>" << cci->getName() << "</td><td>"; cout << cci->getValue(); cout << "</td></tr>n"; } cout << "</table><n"; cout << "<br/>n"; cout << "</body>n"; cout << "</html>n"; return 0; } Now, compile above program to produce getcookies.cgi, and try to get a list of all the cookies available at your computer: /cgi-bin/getcookies.cgi
- 315. C++ 303 This will produce a list of all the four cookies set in previous section and all other cookies set in your computer: UserID XYZ Password XYZ123 Domain www.tutorialspoint.com Path /perl File Upload Example To upload a file the HTML form must have the enctype attribute set to multipart/form-data. The input tag with the file type will create a "Browse" button. <html> <body> <form enctype="multipart/form-data" action="/cgi-bin/cpp_uploadfile.cgi" method="post"> <p>File: <input type="file" name="userfile" /></p> <p><input type="submit" value="Upload" /></p> </form> </body> </html> The result of this code is the following form: File: Upload Note: Above example has been disabled intentionally to stop people uploading files on our server. But you can try above code with your server. Here is the script cpp_uploadfile.cpp to handle file upload: #include <iostream> #include <vector> #include <string> #include <stdio.h> #include <stdlib.h>
- 316. C++ 304 #include <cgicc/CgiDefs.h> #include <cgicc/Cgicc.h> #include <cgicc/HTTPHTMLHeader.h> #include <cgicc/HTMLClasses.h> using namespace std; using namespace cgicc; int main () { Cgicc cgi; cout << "Content-type:text/htmlrnrn"; cout << "<html>n"; cout << "<head>n"; cout << "<title>File Upload in CGI</title>n"; cout << "</head>n"; cout << "<body>n"; // get list of files to be uploaded const_file_iterator file = cgi.getFile("userfile"); if(file != cgi.getFiles().end()) { // send data type at cout. cout << HTTPContentHeader(file->getDataType()); // write content at cout. file->writeToStream(cout); } cout << "<File uploaded successfully>n"; cout << "</body>n"; cout << "</html>n"; return 0; }
- 317. C++ 305 The above example is for writing content at cout stream but you can open your file stream and save the content of uploaded file in a file at desired location. Hope you have enjoyed this tutorial. If yes, please send us your feedback.
- 318. C++ 306 Hope you have already understood the concept of C++ Template which we have discussed earlier. The C++ STL (Standard Template Library) is a powerful set of C++ template classes to provide general-purpose classes and functions with templates that implement many popular and commonly used algorithms and data structures like vectors, lists, queues, and stacks. At the core of the C++ Standard Template Library are following three well- structured components: Component Description Containers Containers are used to manage collections of objects of a certain kind. There are several different types of containers like deque, list, vector, map etc. Algorithms Algorithms act on containers. They provide the means by which you will perform initialization, sorting, searching, and transforming of the contents of containers. Iterators Iterators are used to step through the elements of collections of objects. These collections may be containers or subsets of containers. We will discuss about all the three C++ STL components in next chapter while discussing C++ Standard Library. For now, keep in mind that all the three components have a rich set of pre-defined functions which help us in doing complicated tasks in very easy fashion. Let us take the following program that demonstrates the vector container (a C++ Standard Template) which is similar to an array with an exception that it automatically handles its own storage requirements in case it grows: #include <iostream> #include <vector> using namespace std; int main() { 39. STL TUTORIAL
- 319. C++ 307 // create a vector to store int vector<int> vec; int i; // display the original size of vec cout << "vector size = " << vec.size() << endl; // push 5 values into the vector for(i = 0; i < 5; i++){ vec.push_back(i); } // display extended size of vec cout << "extended vector size = " << vec.size() << endl; // access 5 values from the vector for(i = 0; i < 5; i++){ cout << "value of vec [" << i << "] = " << vec[i] << endl; } // use iterator to access the values vector<int>::iterator v = vec.begin(); while( v != vec.end()) { cout << "value of v = " << *v << endl; v++; } return 0; } When the above code is compiled and executed, it produces the following result: vector size = 0 extended vector size = 5 value of vec [0] = 0
- 320. C++ 308 value of vec [1] = 1 value of vec [2] = 2 value of vec [3] = 3 value of vec [4] = 4 value of v = 0 value of v = 1 value of v = 2 value of v = 3 value of v = 4 Here are following points to be noted related to various functions we used in the above example: The push_back( ) member function inserts value at the end of the vector, expanding its size as needed. The size( ) function displays the size of the vector. The function begin( ) returns an iterator to the start of the vector. The function end( ) returns an iterator to the end of the vector.
- 321. C++ 309 The C++ Standard Library can be categorized into two parts: The Standard Function Library: This library consists of general- purpose, stand-alone functions that are not part of any class. The function library is inherited from C. The Object Oriented Class Library: This is a collection of classes and associated functions. Standard C++ Library incorporates all the Standard C libraries also, with small additions and changes to support type safety. The Standard Function Library The standard function library is divided into the following categories: I/O, String and character handling, Mathematical, Time, date, and localization, Dynamic allocation, Miscellaneous, Wide-character functions The Object Oriented Class Library Standard C++ Object Oriented Library defines an extensive set of classes that provide support for a number of common activities, including I/O, strings, and numeric processing. This library includes the following: The Standard C++ I/O Classes The String Class The Numeric Classes The STL Container Classes The STL Algorithms The STL Function Objects The STL Iterators The STL Allocators 40. STANDARD LIBRARY
- 322. C++ 310 The Localization library Exception Handling Classes Miscellaneous Support Library