Just as any variable is declared in a program before it is used, it is necessary to declare or prototype a function to inform the compiler that the function would be referenced at a later point in the program.

In Program 7.1, the statement:

void add();

is a function declaratoin or a prototype.

Defining a Function

The function definition contains the code for the function. The function definition for add() in Program 7.1 is:

//Function definition for add
void add()
{
int iNum1, iNum2, iNum3;
cin>>iNum1;
cin>>iNum2;
iNum3 = iNum1 + iNum2;
cout<<"The sum of the two numbers is"<<iNum3<<endl;
}

A function declaration can be avoided if the function definition appears before the first call to the function.

Program 7.1 can be modified to avoid function declaration in the following way:

//Program 7.2
//This program uses a function to calculate the sum of two numbers
#include<iostream.h>
void add()
{
int iNum1, iNum2, iNum3;
cin>>iNum1;
cin>>iNum2;
iNum3 = iNum1 + iNum2;
cout<<"The sum of the two numbers is"<<iNum3<<endl;
}
void main()
{
cout<<"Calling function add()"<<endl;
add(); //A call to the function add(), while calling a function the command is terminated with a ';'
cout<<"A return from add() function"<<endl;
}

Function Arguments

Argument(s) of a function is(are) the data the function receives when called from another function. It is not always necessary for a function to have arguments. The function add() in Program 7.1 did not contain any arguments. The add() function in Program 7.1 can be modified to accept two arguments in the following way:

//Program 7.2
#include<iostream.h>
vodi main(int iVar1, int iVar2) //Function containing two arguments
{
int iVar3 = iVar1 + iVar2;
cout<<"The sum of the two numbers is: "<<iVar3<<endl;
}
void main()
{
int iNum1, iNum2;
cout<<"Input tow numbers "<<endl;
cin>>iNum1;
cin>>iNum2;
add(iNum1, iNum2); //Passing two values to the function add()
}