Preprocessors

Preprocessors are instructions to the compiler.

They are processed before compilation (replacement is performed before the compilation).

They are used for:
- defining symbolic constants
- defining functions
- conditional operations

Preprocessor Directives are also called Macros.

Symbolic Directives

#define PI 3.14

int main()
{
    cout << PI; // 3.14
}

#define PI 3.14
#define PI 3

int main()
{
    cout << PI; // 3
}

#define maxi(a,b) (a > b ? a : b)

int main()
{
    cout << maxi(10, 12); // 12
}

#define msg(x) #x

int main()
{
    cout << msg(Hello); // Hello
}

# before x makes the value of x as string (It wraps the value of x in double quotes).

ifndef - If not defined

#define PI 3.14

#ifndef PI
    #define PI 3
#endif

int main()
{
    cout << PI; // 3.14
}

If value of PI is not defined already, then it will get executed.

#ifndef PI
    #define PI 3
#endif

int main()
{
    cout << PI; // 3
}

Last modified: 08 February 2026