Lesson 3of 5

C++ Preprocessor Directives: #include, #define & Examples

Learn what C++ preprocessor directives do and how they work before compilation. Explore #include, #define, and conditional directives with a simple code example.

What is a Preprocessor Directive in C++?

A preprocessor directive is a special instruction in a C++ program that is processed before the actual compilation of the program.

It starts with the # symbol.
It does not end with a semicolon (;).
It tells the preprocessor to perform a specific task before the compiler compiles the code.

Example

#include <iostream>
using namespace std;
 
int main() {
    cout << "Hello World";
    return 0;
}

Here:

#include <iostream>

is a preprocessor directive.

It tells the preprocessor to include the contents of the iostream header file before compilation.

Common Preprocessor Directives

Directive	Purpose
`#include`	Includes a header file
`#define`	Defines a constant or macro
`#if`	Starts a conditional section
`#ifdef`	Checks whether a macro is defined
`#ifndef`	Checks whether a macro is not defined

Simple definition for students:

A preprocessor directive is an instruction beginning with # that is processed before the C++ program is compiled.

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