C++20 Modules

Issues Migrating to Modules

What are some common issues you might face when migrating a large codebase to use C++20 modules?

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Migrating a large codebase to use C++20 modules can bring significant benefits, but it also comes with challenges. Here are some common issues you might face:

Compatibility with Existing Code

Modules are a new feature, and existing codebases often rely on traditional header files and #include directives. Ensuring compatibility between modules and non-module code can be tricky, requiring careful planning and incremental changes.

Tooling and Compiler Support

Not all compilers and build tools fully support C++20 modules yet. You might encounter issues with:

  • Limited or inconsistent support for modules across different compilers.
  • Build systems that do not natively support modules, necessitating workarounds or custom scripts.
  • Debuggers and static analyzers that may not yet be fully compatible with modules.

Hidden Dependencies

Large codebases often have complex interdependencies that are not immediately apparent. Converting headers to modules can uncover hidden dependencies, leading to compile-time errors that need to be resolved.

Incremental Build Issues

Modules can change how incremental builds work. Traditional #include directives allow for fine-grained dependency tracking, but modules require a different approach, which might initially slow down the build process as the build system adapts.

Refactoring Challenges

Refactoring large codebases to use modules often involves:

  • Identifying logical groupings of code that can be modularized.
  • Deciding what to export and what to keep internal.
  • Managing the transition in a way that avoids breaking existing functionality.

Example: Refactoring a Header to a Module

Consider a header file Utilities.h that we want to convert to a module:

// Utilities.h
#pragma once
#include <iostream>
#include <string>

void printMessage(const std::string &message) {
  std::cout << message << std::endl;

The refactored module might look something like this:

// Utilities.cppm
export module Utilities;
import <iostream>;
import <string>;

export void printMessage(
  const std::string &message) {
  std::cout << message << std::endl;

Managing Legacy Code

During the migration, you might need to mix modules with traditional headers. This requires careful management to avoid conflicts and ensure consistent behavior. Using header units can help bridge the gap between modules and non-modular code.

Testing and Validation

Extensive testing is necessary to ensure that the migration does not introduce new bugs. Automated tests, continuous integration, and thorough review processes are essential.


Migrating a large codebase to C++20 modules involves dealing with compatibility issues, tooling support, hidden dependencies, and refactoring challenges.

Careful planning, incremental changes, and extensive testing can help manage these issues effectively.

Answers to questions are automatically generated and may not have been reviewed.

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