Preprocessor Directives in C++

 


1️⃣ What is a Preprocessor Directive?

A preprocessor directive in C++ is a command that starts with # and is executed before compilation. It helps with:
✔ Including header files (#include)
✔ Defining constants (#define)
✔ Conditional compilation (#ifdef, #ifndef, #endif)
✔ Macros and inline code substitution

⏩ Preprocessor directives do NOT end with a semicolon (;).

2️⃣ Common Preprocessor Directives in C++

Directive Purpose
#include Includes a file (like <iostream>, user-defined headers).
#define Defines macros/constants.
#undef Undefines a macro.
#ifdef Checks if a macro is defined.
#ifndef Checks if a macro is not defined.
#if, #elif, #else, #endif Conditional compilation.
#pragma Compiler-specific instructions.

3️⃣ #include: Including Header Files

The #include directive is used to import external files (standard libraries or user-defined headers).

🔹 Example: Including Standard Library

#include <iostream>  // Includes input-output stream
using namespace std;

int main() {
    cout << "Hello, World!" << endl;
    return 0;
}

✅ #include <iostream> allows the use of cout, cin, etc.

🔹 Example: Including a User-Defined Header File

File: myheader.h

#ifndef MYHEADER_H
#define MYHEADER_H

void greet() {
    std::cout << "Hello from My Header!" << std::endl;
}

#endif

File: main.cpp

#include <iostream>
#include "myheader.h"  // Includes user-defined header
using namespace std;

int main() {
    greet();
    return 0;
}

🔹 Output

Hello from My Header!

✅ Using #ifndef prevents multiple inclusions of myheader.h.

4️⃣ #define: Creating Macros & Constants

The #define directive is used to define constants or macros.

🔹 Example: Define a Constant

#include <iostream>
#define PI 3.14159

int main() {
    cout << "Value of PI: " << PI << endl;
    return 0;
}

🔹 Output

Value of PI: 3.14159

✅ Replaces PI with 3.14159 before compilation.

🔹 Example: Define a Macro Function

#include <iostream>
#define SQUARE(x) (x * x)  // Macro function

int main() {
    cout << "Square of 5: " << SQUARE(5) << endl;
    return 0;
}

🔹 Output

Square of 5: 25

✅ Replaces SQUARE(5) with (5 * 5) before compilation.

5️⃣ Conditional Compilation: #ifdef, #ifndef, #endif

🔹 #ifdef: Check if Macro is Defined

#include <iostream>
#define DEBUG

int main() {
    #ifdef DEBUG
        std::cout << "Debug Mode is ON" << std::endl;
    #endif
    return 0;
}

🔹 Output

Debug Mode is ON

✅ Since DEBUG is defined, the cout statement executes.

🔹 #ifndef: Check if Macro is NOT Defined

#include <iostream>

#ifndef VERSION
    #define VERSION 1.0
#endif

int main() {
    std::cout << "Software Version: " << VERSION << std::endl;
    return 0;
}

🔹 Output

Software Version: 1.0

✅ Defines VERSION only if it was not already defined.

6️⃣ #if, #elif, #else, #endif: Conditional Compilation

#include <iostream>
#define MODE 2  // Change this value to test

int main() {
    #if MODE == 1
        std::cout << "Mode 1 Selected" << std::endl;
    #elif MODE == 2
        std::cout << "Mode 2 Selected" << std::endl;
    #else
        std::cout << "Invalid Mode" << std::endl;
    #endif

    return 0;
}

🔹 Output

Mode 2 Selected

✅ Compiles different code based on MODE.

7️⃣ #pragma: Compiler-Specific Directives

🔹 #pragma once: Prevent Multiple Inclusions

Instead of using #ifndef guards, you can use:

#pragma once
void greet() {
    std::cout << "Hello from Pragma!" << std::endl;
}

✅ Ensures the file is included only once.

8️⃣ Summary

✔ #include → Includes standard/user-defined files.
✔ #define → Defines constants and macros.
✔ #ifdef, #ifndef → Checks if a macro is defined or not.
✔ #if, #elif, #else, #endif → Conditional compilation.
✔ #pragma → Compiler-specific instructions.

Would you like a real-world project using preprocessor directives? 🚀

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