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Structure in C++ Program

Structure in C++Program:

General Syntax of A C++ program is given below:

Preprocessor directives < header file> 

int main () 

{ 

Body of the program 

} 

Each C++ program has three main components. These are: Preprocessor directives, main function and body of the main function. Now, "hello world" example to explain these components:

# include<iostream.h> 

# include<conio.h> 

int main () 

{ 

cout<<" hello world!"; 

getch (); 

Return 0; 

}

Output of the program 
Hello world! 

The result of the above program is that it prints " hello world!" on the screen. It is one of the simplest programs that can be written in C++, but it contains components of almost every C++ program.

1. Preprocessor Directives (#Include, #Define):

A processor is a collection of special statements which are executed before the compilation process. Almost every C++ program contains a processor. The #include preprocessor directives is commonly used load header files with extension (.h). In the above program two #Include directives have been used a stick food <iostream.h> and #include <conio.h>. #include <iostream.h> is used for the C++ objects cout and #include <conio.h> for the built-in function getch ().

In C++, The Other commonly used processor directive is #define which is used to define symbolic constants. Its general format is:

#define identifier value: 

For Example:

#define PI 3.14159
#define NEWLINE '\n'

This defines to new constants: PI and NEWLINE.

Once they are defined, they can be used in the rest of the program as if they were any other regular constant, for Example:

#include <iostream.h>
#include <conio.h>
#define PI 3.1459
#define NEWlINE '\n'
int main( )
{
Double R=5.0; //radius
double circle;
Circle is equal to 2*5*r;
cout<<"area of the circle:"<<circle;
cout<< NEWLINE;
getch ( );
Return 0;
}


Output of the program
area of the circle: 31.4159

The #define directive is not a C++ therefore it assumes the entire line as the directive and does not require a (;) at its end.

2. main( ) Function:

The main function is the point by where all C++ program start their execution. It does not matter whether there are other functions other name is defined before or after it, the