C++20 Modules: The Modern Replacement for Preprocessor Header Files

For five decades since 1972, C and C++ relied on textual header file inclusion via #include. C++20 Modules represent the biggest architectural upgrade to the compilation model in C++ history, eliminating macro pollution, header guards, and sluggish build times.

Creating and Exporting a C++20 Module (`math_module.ixx`)

// Declare the module name
export module MathModule;

// Export specific functions to consumers
export double calculateInterest(double principal, double rate, int years) {
    return principal * (rate / 100.0) * years;
}

Importing the Module in `main.cpp`

import MathModule;
import std; // C++23 standard library module

int main() {
    double interest = calculateInterest(10000, 7.5, 3);
    std::println("Calculated Interest: USD {:.2f}", interest); // 2250.00
    return 0;
}

Frequently Asked Questions (FAQ) - Student & Interview Review

Q1. Why are C++20 Modules vastly faster to compile than #include headers?

When you #include a header in multiple .cpp files, the preprocessor re-parses and tokenizes the exact same text hundreds of times. Modules are compiled ONCE into a binary Interface Unit (.ifc / .bmi), allowing subsequent translation units to import them in milliseconds without re-parsing.

Q2. Do macros leak out of C++20 modules?

No! Unlike header files where #define macros bleed into every file that includes them, modules provide strict boundary encapsulation. Macros defined inside a module do NOT leak outside unless explicitly exported.