C++ - Modules (C++20)

Overview

Estimated time: 45–65 minutes

Organize code with C++20 modules: faster builds, cleaner interfaces, and fewer macro/header pitfalls. Learn export/import basics, partitions, and build notes.

Learning Objectives

• Define a module interface and import it from a consumer.
• Use module partitions to split large interfaces.
• Understand practical build flags and migration tips.

Prerequisites

• C++ - Concepts & Constraints

Minimal module

// math.ixx (module interface)
export module math;
export int add(int a,int b){ return a+b; }

// main.cpp (consumer)
import math;
#include 
int main(){ std::cout << add(2,3) << "\n"; }

Partitioned module

// util.ixx
export module util;
export import :strings; // re-export partition
export int twice(int x){ return x*2; }

// util-strings.ixx (partition)
module util:strings;
export const char* greet(){ return "hello"; }

Build notes

• Compilers use special flags for scanning/compiling modules (varies by vendor and build system).
• Prefer using a modern CMake (3.26+) with modules support and toolchain integration.
• Avoid textual includes in module interfaces when possible; import standard libraries as they become modularized.

Common Pitfalls

• Mixing headers and modules without clear boundaries can cause ODR or visibility issues.
• Macros do not cross module boundaries; avoid relying on header-based macro tricks.

Checks for Understanding

1. What is exported in a module interface?
2. When would you use a partition?

Exercises

1. Create a small math module with add/sub/mul and import in a main program.
2. Add a partition for string helpers and re-export it from the main module.