🔵 Core C++ · Lesson 34

Preprocessor Directives

What is Preprocessor Directives?

Preprocessor Directives
Preprocessor directives begin with # and are processed before compilation. They include headers, macros and conditional compilation.

Level
🔵 Core C++ Features

Example File
preprocessors.cpp

Main Focus
Concept + syntax + practical C++ program

Why should you learn this?

It helps you write correct and readable C++ programs.
It is used repeatedly in school practicals, projects and competitive programming.
It builds the base for advanced topics such as OOP, STL and data structures.

Important Terms

Term	Meaning / Use
#include	#include used in Preprocessor Directives programming.
#define	#define used in Preprocessor Directives programming.
#ifdef	#ifdef used in Preprocessor Directives programming.
macro	Macro used in Preprocessor Directives programming.
preprocessor	Preprocessor used in Preprocessor Directives programming.

Syntax / Pattern

#define NAME value

Example Program

#include <iostream>
#define MAX_MARKS 100
using namespace std;
int main(){
    cout << MAX_MARKS;
    return 0;
}

Expected Output

100

Program Explanation

Before compilation, MAX_MARKS is replaced by 100.
Modern C++ often prefers const or constexpr over macros.

Exam Tip: In C++ practical answers, write the logic first, then the program, then expected output. For theory, always include one suitable example.

Where will you use it?

header inclusion
compile-time configuration
simple constants

Common Mistakes

Using macros for everything.
Forgetting that macros do not respect type checking.

Practice Tasks

Use #define for PI and compare with const.
Explain header guards.

Summary

Preprocessor Directives is an important C++ topic. Learn the definition, understand the syntax, run the example program and then solve the practice tasks to make the concept strong.