Lecture 12: Classes and Files

Lecture 12
Classes and
Files

TCP1231 Computer Programming I 1

Objectives

To learn about classes and files
Explore how to declare and manipulate classes.
To illustrate the usage of classes with arrays,
structure, and functions.
To learn how to use text and binary files


Classes


Classes
A class is a logical method to organize data and functions in the
same structure. They are declared using keyword class, whose
functionality is similar to that of the C++ keyword struct, but
with the possibility of including functions as members, instead
of only data.

class class_name
{
permission_label_1:
member1;
permission_label_2:
member2;
...
} object_name;


Classes
where class_name is a name for the class (user defined type) and the
optional field object_name is one, or several, valid object
identifiers. The body of the declaration can contain members, that
can be either data or function declarations, and optionally
permission labels, that can be any of these three keywords:
private:, public: or protected:. They make reference to the
permission which the following members acquire:
• private members of a class are accessible only from other members
of their same class or from their "friend" classes.
• protected members are accessible from members of their same
class and friend classes, and also from members of their derived
classes.
• public members are accessible from anywhere the class is visible.

Example 1
class student{
int a;
char b;
int f1(int );
private:
int c;
float d;
void f2(int, int);
protected:
int e;
string addr;
float f3(float, char);
public:
int g;
string name;
void f4(int, int, string);
int f5(int, float, char);
};


Example 2
class A declares 3
class A{ variables
public: •i is public to all users
int i; of class A

protected: •j is protected, can

int j; only be used by
methods in class A or
private: its derived classes

int k; •k is private, can only

}; be used by methods in
class A.


Classes
If we declare members of a class before including any permission label, the
members are considered private, since it is the default permission that the
members of a class declared with the class keyword acquire
class CRectangle
{
int x, y;
public:
void set_values (int,int);
int area (void);
} rect;

Declares class CRectangle and an object called rect of this class (type). This
class contains four members: two variables of type int (x and y) in the private
section (because private is the default permission) and two functions in the public
section: set_values() and area(), of which we have only included the prototype.
Notice the difference between class name and object name: In the previous
example, CRectangle was the class name, whereas rect was an object of type
CRectangle.

#include <iostream> #include <iostream>
using namespace std; using namespace std;

struct info { class info {
public:
int id; int id;
float avg; float avg;
string name; string name;
}; };

int main () { int main () {
info s1; info c1;
s1.id=204; c1.id=204;
s1.avg=84.8; c1.avg=84.8;
s1.name=“qdah"; c1.name=“qdah";

system(“pause”); system(“pause”);
return 0; return 0;
} }



struct info { class info {
int id; public:
int avg; int id;
string name; int avg;
}; string name;
info read() void read() {
{ cin >> id;
info data; cin >> avg;
cin >> data.id; cin >> name;
cin >> data.avg; }
cin >> data.name; };
return data; int main () {
} info c1;
int main () { c1.read();
info s1;
s1=read (); return 0;
}
return 0;
}



class info { class info {
int id; int id;
int avg; int avg;
string name; string name;
public: public:
void read() { void read();
cin >> id; };
cin >> avg; void info:: read(){
cin >> name; cin >> id;
} cin >> avg;
}; cin >> name;
int main () { };
info c1; int main () {
c1.read(); info c1;
c1.read();
return 0;
} return 0;
}


#include <iostream>
using namespace std;

class CRectangle {
int x, y;
public:
int area (void) {return (x*y);}
};
void CRectangle::set_values (int a, int b) {
x = a;
y = b;
}

int main () {
CRectangle rect;
rect.set_values (3,4);
cout << "area: " << rect.area();
return 0; area: 12
}


#include <iostream>

class CRectangle {
int x, y;
public:
int area (void) {return (x*y);}
};
void CRectangle::set_values (int a, int b) {
x = a;
y = b;
}

int main () {
CRectangle rect, rectb; rect area: 12
rect.set_values (3,4); rectb area: 30
rectb.set_values (5,6);
cout << "rect area: " << rect.area() << endl;
cout << "rectb area: " << rectb.area() << endl;
}


Constructors and Destructors
Objects generally need to initialize variables or assign dynamic
memory during their process of creation to become totally operative
and to avoid returning unexpected values during their execution.
For example, what would happen if in the previous example we
called the function area() before having called function set_values?
Probably an indetermined result since the members x and y would
have never been assigned a value.
In order to avoid that, a class can include a special function: a
constructor, which can be declared by naming a member function
with the same name as the class. This constructor function will be
called automatically when a new instance of the class is created
(when declaring a new object or allocating an object of that class)
and only then. We are going to implement CRectangle including a
constructor:

#include <iostream>
using namespace std; Example  Constructor
class CRectangle {
int width, height;
public:
CRectangle () { width = 0; height = 0;};
CRectangle (int,int);
int area (void) {return (width*height);}
};
CRectangle::CRectangle (int a, int b) {
width = a;
height = b;
}

int main () {
CRectangle rect (3,4);
CRectangle rectb (5,6); rect area: 12
CRectangle rectc; rectb area: 30
cout << "rect area: " << rect.area() << endl;
cout << "rectb area: " << rectb.area() << endl; rectc area: 0
cout << "rectc area: " << rectc.area() << endl;
}

#include <iostream>
using namespace std; Example 
class CRectangle { Constructor & Destructors
int *width, *height;
public:
CRectangle (int,int);
rect area: 12
~CRectangle (); rectb area: 30
int area (void) {return (*width * *height);}
};

CRectangle::CRectangle (int a, int b) { CRectangle::~CRectangle () {
width = new int; delete width;
height = new int; delete height;
*width = a; }
*height = b;
} int main () {
CRectangle rect (3,4), rectb (5,6);
cout << "rect area: " << rect.area()
<< endl;
cout << "rectb area: " << rectb.area()
<< endl;
return 0;
}

#include <iostream> Point readPoint()
#include <math.h> {
using namespace std; Point p;
struct Point cout<< "enter point x,y : ";
{ int x; cin>> p.x >> p.y;
int y; return p;
}; }
void printPoint(Point );
float distPoint(Point , Point ); void printPoint(Point p)
Point readPoint(); {
cout<< "(x = "<< p.x<< ", y = "<<
int main(){ p.y<<")"<< endl;
Point p1, p2; }
float d;
p1=readPoint(); cout<< "p1 = "; float distPoint(Point p, Point q)
printPoint(p1); {
p2=readPoint(); cout<< "p2 = "; float dx = p.x - q.x;
printPoint(p2); float dy = p.y - q.y;
d= distPoint(p1,p2); float dsquared = dx*dx + dy*dy;
cout<< "distance p1 to p2 = "<< d;
d= distPoint(p2,p1); return sqrt(dsquared);
cout<< "distance p2 to p1 = "<< d; }
return 0;}

#include <iostream> int main()
#include <math.h> {
using namespace std; Point p1, p2;
class Point{ float d;
private:
int x; p1.readPoint();
int y; cout<< "p1 = ";
public: p1.printPoint();
void readPoint(); cout<< endl;
void printPoint(); p2.readPoint();
float distPoint(Point q); cout<< "p2 = ";
}; p2.printPoint();
void Point::readPoint(){ cout<< endl;
cout<< "enter point x,y : ";
cin>> x >> y; d = p1.distPoint(p2);
} cout<< "distance p1 to p2 = “ << d
void Point::printPoint(){ << endl;
cout<< "(x = "<< x<< ", y = "<< y <<")"<< endl; } d= p2.distPoint(p1);
float Point::distPoint(Point q){ cout<< "distance p2 to p1 = "<< d
float dx = x - q.x; << endl<< endl;
float dy = y - q.y;
float dsquared = dx*dx + dy*dy; system("pause");
return sqrt(dsquared); return 0;
} TCP1231 Computer Programming I } 18

Files


Streams and Basic File I/O
• Files for I/O are the same type of files used to
store programs

• A stream is a flow of data.
– Input stream: Data flows into the program
• If input stream flows from keyboard, the program
will accept data from the keyboard
• If input stream flows from a file, the program will
accept data from the file
– Output stream: Data flows out of the program
• To the screen
• To a file


Standard I/O

• cin – the standard input stream
• cout – the standard output stream
• cerr – the standard error stream

These streams are automatically created for you when
your program executes. To use them you only need to
#include <iostream> and the appropriate using directives.


Why use Files?
• Files allow you to store data permanently!
• Data output to a file lasts after the program ends
• An input file can be used over and over
– No typing of data again and again for testing
• Create a data file or read an output file at your
convenience
• Files allow you to deal with larger data sets


File I/O
When a program takes input from a file, we say that
it reads from the file.

When a program puts data into a file, we say that
it writes to the file.

To read or write to a file, we create a stream object,
and connect it to the file.


File I/O
• Reading from a file
– Taking input from a file
– Done from beginning to the end
• No backing up to read something again
• Just as done from the keyboard

• Writing to a file
– Sending output to a file
– Done from beginning to end
• No backing up to write something again
• Just as done to the screen


More on Files
 Files are used for permanent storage of data.
 Files are stored on secondary storage devices such as
magnetic disks, optical disks and tapes.
Bit: smallest data items in a computer ( 0 or 1)
Byte: Sequence of Bits (8 Bits)
 C++ views each file simple as a sequence of bytes.
 Each file ends with an end-of-file marker
 When a file is opened, an object is created and a
stream is associated with the object.

0 1 2 3 4 5 6 7 … n-1

… End-of-file
marker


Input/Output with files
C++ has support both for input and output with files
through the following classes:
 ifstream: File class for reading operations (derived from istream)
 ofstream: File class for writing operations (derived from ostream)
 fstream: File class for both reading and writing operations
(derived from iostream)
#include <fstream>
int main()
{
ofstream outfile("myfile1.dat");
ifstream infile ("myfile2.dat");
fstream iofile ("myfile3.dat");


Input-file Stream (ifstream)
• Input-file streams are of type ifstream

• Type ifstream is defined in the fstream library
– You must use the include and using directives
#include <fstream>

• Declare an input-file stream variable using
ifstream in_stream;


Output-file Stream (ofstream)
• Output-file streams of are type ofstream

• Type ofstream is defined in the fstream library
– You must use these include and using directives
#include <fstream>

• Declare an input-file stream variable using
ofstream out_stream;


The fstreams Classes
We use objects of the ifstream class to read from
a file, and objects of the ofstream class to write to
a file.

These classes are defined in <fstream>. To use them
we must write

#include <fstream>
using std::ifstream;
using std::ofstream;


Connecting To a File

• Once a stream variable is declared, connect it to
a file
– Connecting a stream to a file is opening the file
– Use the open function of the stream object

in_stream.open("infile.dat");

Double quotes
Period
File name on the disk


How to open files?
To open a file with a stream object we use the function open()
void open ( filename, mode);
filename is a string of characters representing the name of the file
to be opened and mode is a combination of the following flags:
ios::in Open file for reading
ios::out Open file for writing
ios::ate Initial position: end of file
ios::app Every output is appended at the end of file
ios::trunc If the file already existed it is erased
ios::binary Binary mode

ofstream file;
OR file.open (“file1.abc", ios::out | ios::app | ios::binary);

ofstream file (“file1.abc", ios::out | ios::app | ios::binary);

More processes
To check if the file has been correctly opened we use is_open()
that returns a bool type value indicating True in case that indeed
the object has been correctly associated with an open file or False
otherwise

After we finish using the file we must close it, to do that the
function close() is used.
After we call close(), the stream object can be used to open
another file, and the file is available again to be opened by other
processes
Files
This for test !
“Hi and Bye” îyÑå▌┘┬µα╣
ÜÑå«ìæ
Text Files Binary Files


Closing A File
• After using a file, it should be closed
– This disconnects the stream from the file
– Close files to reduce the chance of a file being
corrupted if the program terminates abnormally

• It is important to close an output file if your
program later needs to read input
from the output file

• The system will automatically close files if you
forget as long as your program ends normally


Example


Text Files
We use the same members of classes that we used in communication
with the console (cin and cout). As in the following example, where
we use the overloaded insertion operator <<
#include <fstream> #include <iostream>
using namespace std; #include <fstream>
int main () { // writing to a text file int main () { // reading a text file
ofstream myfile ("file1.txt“, ios::out); char buffer;
if (myfile.is_open()) ifstream myfile ("file1.txt“, ios::in);
{ if (! myfile.is_open())
myfile << “Message from Belal.n"; { cout << "Error"; exit (1); }
myfile << “Just Hi and Bye.";
myfile.put(‘n’); while (! myfile.eof() ) {
myfile.close(); myfile.get (buffer);
} cout << buffer;
}
return 0; return 0;
} }
eof() TCP1231true if a file opened for reading has reached the end
Returns Computer Programming I 35

Text Files Read from a file “in.dat” then
Write to a file “out.dat”
#include <fstream>
#include <iostream>
int main()
{
char ch;

ifstream infile("IN.DAT");
ofstream outfile("OUT.DAT");
while (infile)
{
infile.get(ch);
outfile.put(ch);
}
infile.close();
}


The End of The File
• Input files used by a program may vary in length
– Programs may not be able to assume the number
of items in the file

• A way to know the end of the file is reached:
– The boolean expression (in_stream >> next)
• Reads a value from in_stream and stores it in next
• True if a value can be read and stored in next
• False if there is not a value to be read


End of The File - Example
• To calculate the average of the numbers in a file
– double next, sum = 0;
int count = 0;
while(in_stream >> next)
{
sum = sum + next;
count++;
}

double average = sum / count;


Binary Files
When we introduced C++ I/O, we said that input and output
streams converted between the programs internal representation
for data objects and characters (such as from the internal
representation for the number 15 to the characters ‘1’ and ‘5’ and
vice versa)
Reason for doing this:
To translate between the internal binary representation and a
format that is easier to understand for humans.

Thus, C++ allows us to perform binary file I/O, in which we store
and retrieve the original binary representation for data objects


Binary Files
The advantages of binary file are:
 No need to convert between the internal representation and
a character-based representation
 Reduces the associated time to store/retrieve data
 Possible conversion and round-off errors are avoided
 Storing data objects in binary format usually takes less
space (though this depends on the nature of the data)
Disadvantages:
 The file is not easily readable by humans
 Portability becomes an issue, as different operating
systems (and even different compilers) may use different
internal representations


Creating Binary Files
To open a file with a stream object we use the function open()
void open ( filename, mode | ios::binary);

Creating a stream that will handle binary I/O is as simple
as setting a flag
Create a binary file output stream:

ofstream fout (“myFile.txt”, ios::out |ios::trunc | ios::binary);

Create a binary file input stream:
ifstream fin (“myFile2.txt”, ios::in | ios::binary);


Binary Files (read and write)
Two functions are used : READ & WRITE
write()
–Lets us write a specified number of characters to an output
stream
–does a straight byte-by-byte copy
fout.write(reinterpret_cast<char *>(&obj), sizeof obj);

read ( )
–Lets us read a specified number of characters to an output
stream
–does a straight byte-by-byte read
fin.read(reinterpret_cast<char *>(&obj), sizeof obj);


Example
#include <iostream> ofstream output;
#include <fstream> output.open("indata.dat", ios::out|ios::binary);
#include <cstring> output.write(reinterpret_cast<char *>(&people),
using namespace std; sizeof(person) );
output.close();
typedef struct
{ char gender; person newperson;
int age; ifstream input;
string name; input.open("indata.dat", ios::in|ios::binary);
} person; input.read(reinterpret_cast<char *> (&newperson),
sizeof(person) );
int main(void) cout << "n Name is ==> " << newperson.name;
{ cout << "n Gender is ==> " << newperson.gender;
person people; cout << "n Age is ==> " << newperson.age;
people.name= "Ali"; input.close();
people.age= 22;
people.gender= 'm'; return 0;
}

File Seek
The C++ I/O system supports four functions for seeking
seekg() ifstream Moves get file pointer to a specific location
seekp() ofstream Moves put file pointer to a specific location

tellg() ifstream Returns the current position of the get pointer
tellp() ofstream Returns the current position of the put pointer
moves the file pointer to the 20th byte in
Origin value Seek From … the file, infile. After this, if a read
ios::beg seek from beginning of file operation is initiated, the reading starts
ios::cur seek from current location from the 21st item within the file.
ios::end seek from end of file

moves the file pointer to the
infile.seekg(20, ios::beg); or infile.seekg(20); 20th byte in the file outfile.
After this, if write operation is
outfile.seekp(20, ios::beg); or outfile.seekp(20); initiated, the writing starts
from the 21st item within the
file.


Seek

Seek call Action performed

outfile.seekg(0, ios::beg) Go to the beginning of the file
outfile.seekg(0, ios::cur) Stay at the current file
outfile.seekg(0, ios::end) Go to the end of the file
outfile.seekg(n, ios::beg) Move to (n+1) byte location in the file
outfile.seekg(n, ios::cur) Move forward by n bytes from current position
outfile.seekg(-n, ios::cur) Move backward by n bytes from current position
outfile.seekg(-n, ios::end) Move forward by n bytes from the end
infile.seekg(n, ios::beg) Move write pointer to (n+1) byte location
infile.seekg(-n, ios::cur) Move write pointer backward by n bytes


Examples,,, seekp, tellp
ofstream fout(“myFile.txt”);
if (fout.is_open()) {
fout << “Hello, this is a file.” << endl;
fout.seekp(15);
fout << “not a file“;

if (fout.is_open() ) {
fout << “Hello,this is a file.” << endl;
fout.seekp(fout.tellp()– static_cast<streampos>(8));

if (fout.is_open()) {
fout << “Hello, this is a file.” << endl;
fout.seekp(-8, ios_base::cur);


Examples,,, seekg, tellg
ifstream fin(“myFile.txt”);
if (fin.is_open()) {
char c;
fin.seekg(5, ios_base::cur);
fin >> c;
…
}

ifstream fin(“myFile.txt”);
if (fin.is_open() ) {
char c;
fin.seekg(fin.tellg()–static_cast<streampos>(5) );
fin >> c;
…
}


The End


Lecture 12: Classes and Files

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