Preprocessor directives (#include, #define)
A preprocessor is a collection of special statements which are executed before the compilation process.
Almost every C++ program contains a preprocessor directive.The #include
preprocessor directives is commonly used to insert header file with extension ‘.h’.
These header files are already stored in computer in include directory. In the above program, two #include directives have been used, #include<iostream.h> and
#include<conio.h>
#include<iostream.h> is used for the C++ object and #include<conio.h> for the built-in function getch().
In C++, the other commonly used preprocessor directive is #define which is used
to define symbolic constant.Its general format is:
#define identifier value
For example:
#define PI 3.14159
#define NEWLINE ‘\n’
This define two new constant: 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 3.14159
#define NEWLINE ‘\n’
int main()
{
double r = 5.0; //radius
double circle;
circle = 2 * PI * r;
cout <<”Area of the Circle :”<<circle;
cout<<NEWLINE;
getch();
return 0;
}
Output of the program
Area of the Circle : 31.4159
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