CPP Language Basics - Reference

Objectives
In this chapter you will learn about
•History of C++
•Some drawbacks of C
•A simple C++ program
•Input and Output operators
•Variable declaration
•bool Datatype
•Typecasting
•The Reference –‘&’operator

C++
1979 - First Release It was called C with Classes
1983 Renamed to C++
1998 ANSI C++
2003 STL was added
2011 C++ 11
2014 C++14
2017 C++17

 C++ is a superset of C language.
 It supports everything what is there in C
and provides extra features.

Before we start with C++,
lets see some C questions

int a ;
int a ;
int main( )
{
printf("a = %d " , a);
}
int a = 5;
int a = 10;
int main( )
{
printf("a = %d " , a);
}

 An external declaration for an object is a definition if it has
an initializer.
 An external object declaration that does not have an
initializer, and does not contain the extern specifier, is a
tentative definition.
 If a definition for an object appears in a translation unit, any
tentative definitions are treated merely as redundant
declarations.
 If no definition for the object appears in the translation unit,
all its tentative definitions become a single definition with
initializer 0.

int main( )
{
const int x ;
printf("x = %d n",x);
}
C: It is not compulsory to initialize const;
C++: Constants should be initialized in C++.

int main( )
{
const int x = 5;
printf("Enter a number : ");
scanf(" %d" , &x);
printf("x = %d n",x);
}

 Scanf cannot verify if the argument is
const variable or not.
 It allows us to modify the value of const
variable.

#include <iostream>
int main( )
{
std::cout << "Hello World";
return 0;
}

#include <iostream>
int main( )
{
int num1 = 0;
int num2 = 0;
std::cout << "Enter two numbers :";
std::cin >> num1;
std::cin >> num2;
int sum = num1 + num2;
std::cout << sum;
return 0;
}

#include <iostream>
using namespace std;
int main( )
{
int num1 = 0;
int num2 = 0;
cout << "Enter two numbers :";
cin >> num1 >> num2;
int sum = num1 + num2;
cout << “Sum = “ << sum;
return 0;
}

#include <iostream>
int main()
{
int regno;
char name[20];
cout << “Enter your reg no. :“ ;
cin >> regno;
cout << "Enter your name :“ ;
cin >> name;
cout << "REG NO : " << regno << endl ;
cout << "Name :" << name << endl;
return 0;
}

 The ‘cout’is an object of the class ‘ostream’
 It inserts data into the console output stream
 << - called the Insertion Operator or put to operator, It
directs the contents of the variable on its right to the
object on its left.
 The ‘cin’is an object of the class ‘istream’
 It extracts data from the console input stream
 >> - called the Extraction or get from operator, it takes
the value from the stream object on its left and places it
in the variable on its right.

 Use <iostream> instead of using <iostream.h>
 The <iostream> file provides support for all
console-related input –output stream based
operations
 using namespace std;
 ‘std’is known as the standard namespace and is
pre-defined

#include <iostream>
int main( )
{
const int x = 5;
cout << "Enter a number : ";
cin >> x;
cout << "x =" << x;
}
#include <stdio.h>
int main( )
{
const int x = 5;
printf("Enter a number : “);
scanf(“ %d”,&x);
printf("x = %d” , x);
}

 C allows a variable to be declared only at the
beginning of a function (or to be more precise,
beginning of a block)
 C++ supports ‘anywhere declaration’ –before
the first use of the variable
int x;
cin>>x;
int y;
cin>>y;
int res = x + y;
cout<<res<<endl;
for(int i=0;i<10;++i)
cout<<i<<endl;

 “bool” is a new C++ data type
 It is used like a flag for signifying occurrence of
some condition
 Takes only two values defined by two new
keywords
• true – 1
• false - 0
bool powerof2(int
num)
{ // if only 1 bit is set
if(!(num & num-1))
return true;
else
return false;
}
bool search(int a[],int n,int key)
{
for(int i = 0 ; i < n ; i++)
if(a[i] == key)
return true;
return false;
}

 The following types of type-casting are supported
in C++
 double x = 10.5;
 int y1 = (int) x; // c -style casting
 cout<<y1<<endl;
 int y2 = int(x); // c++ -style casting(function style)
 cout<<y2<<endl;
 int y3 = static_cast<int>(x); //C++ 98

void update(int *a)
{
*++a;
printf("Update %d " , *a);
}
int main( )
{
int a = 5;
update(&a);
printf("Main %d n" , a);
}
void update(int a)
{
++a;
printf("Update %d " , a);
}
int main( )
{
int a = 5;
update(a);
printf("Main %d n" , a);
}

void myswap(int *a,int *b)
{
int *temp = a;
a = b;
b = temp;
printf("myswap a=%d b = %d n" , *a , *b);
}
int main( )
{
int a = 5 , b = 10;
myswap(&a , & b);
printf("Main a=%d b = %d n" , a , b);
}

 The previous two code snippets showed
us how easily we can go wrong with
pointer.

 The Reference ‘&’operator is used in C++ to
create aliases to other existing variables /
objects
TYPE &refName= varName;
int x = 10;
int &y = x;
cout<<x<<‘ ‘<<y<<endl;
++y;
10
x
y

int x = 10;
int y = x;
++y;
10
x
y
int x = 10;
int &y = x;
++y;
10
10
x
y

 A reference should always be initialized.
 A reference cannot refer to constant
literals. It can only refer to
objects(variables).
• int &reference;
• int &reference = 5;
• int a = 5, b = 10;
• int &reference = a + b;

• Once a reference is created, we can refer to that location using
either of the names.
• & is only used to create a reference.
• To refer to value of x using reference, we do not use &

CPP Language Basics - Reference

void myswap(int a,int b)
{
a = a ^ b;
b = a ^ b;
a = a ^ b;
}
int main()
{
int a = 10,b = 20;
myswap(a,b);
cout<<a<<'t'<<b<<endl;
return 0;
}
void myswap(int *a,int *b)
{
*a = *a ^ *b;
*b = *a ^ *b;
*a = *a ^ *b;
}
int main()
{
int a = 10,b = 20;
myswap(&a,&b);
return 0;
}
Call by Value Call by Address

void myswap(int a,int b)
{
a = a ^ b;
b = a ^ b;
a = a ^ b;
}
int main()
{
int a = 10,b = 20;
myswap(a,b);
return 0;
}
void myswap(int &a,int &b)
{
a = a ^ b;
b = a ^ b;
a = a ^ b;
}
int main()
{
int a = 10,b = 20;
myswap(a,b);
return 0;
}
Call by Value Call by Reference

 The function call is made in the same
manner as Call By Value
• Whether a reference is taken for the actual
arguments
OR
• a new variable is created and the value is copied
is made transparent to the invoker of the function

 Reference should always be initialised,
pointers need not be initialised.
 No NULL reference - A reference must always
refer to some object
 Pointers may be reassigned to refer to different
objects. A reference, however, always refers to
the object with which it is initialized

CPP Language Basics - Reference

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