C++ Preprocessor

🧠 C++ Preprocessor – Do Things Before Compilation Starts

The preprocessor in C++ runs before your code is actually compiled. Think of it as a helper that sets the stage — like arranging the props before a play begins! 🎭

It handles special instructions called directives that begin with a # symbol. These are not C++ statements — they’re commands to the compiler.

📌 Common Preprocessor Directives

• #include – includes another file (like libraries)
• #define – defines constants or macros
• #undef – undefines something previously defined
• #ifdef, #ifndef – conditionally compile parts of the code
• #if, #else, #elif, #endif – more conditional checks

🔧 Example: #include and #define

#include <iostream>
#define PI 3.14159

using namespace std;

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

🧪 Example: Conditional Compilation

This helps when your code depends on certain conditions.

#include <iostream>

#define DEBUG_MODE

using namespace std;

int main() {
    #ifdef DEBUG_MODE
        cout << "Debug mode is ON" << endl;
    #endif

    cout << "Program is running!" << endl;
    return 0;
}
  

🎯 Summary

• Preprocessor prepares your code before it’s compiled
• Use #include to add files
• #define to create constants or macros
• Conditional directives make your code smarter!