The preprocessor are the directives, which gives instruction to the compiler to do something before execution.

C++  supports various preprocessors like #include, #define, #if, #else, #line, etc.

The #define Preprocessor

This directive creates the symbolic constants.

#include <iostream>
using namespace std;

#define a 40;

int main ()
{
 
    cout << "Value of a :" << a << endl; 

    return 0;
}

 Macros:

You can use #define to define a macro used in the entire program.

#include <iostream>
using namespace std;

#define MIN(a,b) (((a)<(b)) ? a : b)

int main ()
{
   int i, j;
   i = 100;
   j = 30;
   cout <<"The minimum is " << MIN(i, j) << endl;

    return 0;
}

Conditional Directives:

These directives are used to check a certain condition you want.

#ifndef NULL
   #define NULL 0
#endif

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

#define MIN(a,b) (((a)<(b)) ? a : b)

int main ()
{
   int i, j;
   i = 100;
   j = 30;
#ifdef DEBUG
   cerr <<"Trace: Inside main function" << endl;
#endif

#if 0
   /* This is commented part */
   cout << MKSTR(HELLO C++) << endl;
#endif

   cout <<"The minimum is " << MIN(i, j) << endl;

#ifdef DEBUG
   cerr <<"Trace: Coming out of main function" << endl;
#endif
    return 0;
}