A Sample Program

The following program illustrates the usage of objects and classes in C++:
//The program uses member functions to input two numbers, add two numbers and display the result
#include<iostream.h>
class add //Specifies the class
{
private:
int iNum1, iNum2, iNum3; //Member data
public:
void input(int iVar1, int iVar2) //Member function
{
cout<<”Functions to assign values to the member data”<<endl;
iNum1=iVar1;
iNum2=iVar2;
}
void sum(void) //Member function
{
cout<<”Functions to find the sum of two numbers”<<endl;
iNum3=iNum1+iNum2;
}
void disp(void) //Member function
{
cout<<”The sum of the two numbers is”<<iNum3<<endl;
}
};

void main()
{
add A1;
int iX, iY;
cout<<”Input two numbers”<<endl;
cin>>iX;
cin>>iY;
A1.input(iX, iY);
A1.sum();
A1.disp();
}

The sample output of Program 10.2 is:
Input two number
3
2
Function to assign values to the member data
Function to find the sum of two numbers
The sum of the two numbers is 5

Class Specifier
The specifier for a class starts with the keyword class followed by the class name. In Program 10.2, the class specifier is:
Class add
{
private:
int iNum1, iNum2, iNum3; Member data
};
As in the case of a structure, the body of the class is delimited by braces and is terminated by a semicolon.

Defining the Object

In Program 10.2, the statement:
Add A1;
defines an object A1, which is an instance of a class called add. It is this definition that actually creates objects and allocates memory for them.

Access Specifiers

The body of the class contains two keywords:
• Private
• Public

The keywords private and public are called access specifiers. The primary mechanism used to incorporate data hiding in C++ is by using the private keyword. Private data or functions can only be accesses from within the class. Public data or functions, on the other hand, are accessible outside the class. This is shown in Figure 10.4


Therefore, in Program 10.2, it is not possible to access the member data iNum1, iNum2 and iNum3 directly from function main() as they are private o the add class. This data can be accessed only from the member functions of that class.

Usually the data within the class is declared as private and the functions are declared as public. Therefore, the data is hidden and safe from accidental manipulation, whereas functions that operate on this data can be accessed from outside the class. The outcome of abstracting the internal implementation of a class with the use of access specifiers produces what is known as Abstract Data Type.

Invoking Member Functions

In Program 10.2, the statements:
Add A1;
A1.input(ix, iy);
A1.sum();
A1.disp();

Are used to invoke the member functions of the class add :

The dot operator, also called the class member access operator, is used to associate the object name with the member function.

In Program 10.2 the dot operator associates the object A1 with the function input(), sum() and disp().

Scope Resolution Operator

In Program 10.2, the member functions are defined within the class specifier. These functions can also be defined outside the boundaries of the class using the scope resolution operator(::).

Program 10.2 can be modified using the scope resolution operator in the following way:

//This program uses the scope resolution operator to define the functions outside the class
#include<iostream.h>
class add
{
private:
int iNum1, iNum, iNum3;
public:
void input(int, int);
void sum(void);
void disp(void);
};
void add::input(int iVar1, int iVar2)
{
iNum1=iVar1;
iNum2=iVar2;
}
void add::sum(void)
{
iNum3=iNum1+iNum2;
}
void add:disp(void)
{
cout<<”The sum of two numbers is”<<iNum3<<endl;
}
void main()
{
add A1;
int iX, iY;
cout<<”Input two numbers”<<endl;
cin>>iX;
cin>>iY;
A1.input(iX, iY);
A1.sum();
A1.disp();
}


The member functions of the class add are defined outside the class using the scope resolution operators (::). In Program 10.3, the statement:
Void add::input(int iVar1, int iVar2)

uses the scope resolution operator to define the function outside the boundary of the class.

In the above statement, void is the return type of the function, add is the class name, :: is the scope resolution operator, input is the function name, and int iVar1 and int iVar2 are the function parameters.

The declaration:
Void input(int, int);
in the class informs the compiler that this function is a member of the class but will be defined outside the class.