C++ Modules, introduced in C++20, are a significant feature that brings changes to the way code is organized, compiled, and linked in C++ programs. They offer benefits such as faster builds, improved compilation times, and better separation of interface and implementation. However, when it comes to unit testing and test coverage, C++ Modules introduce some considerations and challenges.

Separate compilation and testing

C++ Modules provide a new way of organizing code by separating the interface and implementation. This means that the private implementation details of a module are not visible to consumers of the module. While this improves encapsulation and reduces build times, it can also pose a challenge when writing unit tests.

In traditional C++ codebases without modules, unit tests often rely on including implementation headers directly to access internal functions or data structures. However, with modules, such direct access is not possible. Testing private implementation details becomes more difficult since the module interface only exposes public symbols.

Bridging the gap with module interface units

To address the challenge of testing private implementation details in C++ Modules, a common approach is to create “module interface units” in addition to the module implementation. These units act as bridges between the module and its unit tests.

A module interface unit is a separate translation unit that includes the implementation header of the module and exposes necessary internal symbols for testing purposes. By exposing the internal symbols required by the unit tests, developers can effectively test the private implementation details of a module.

Adapting test frameworks for module units

Most unit testing frameworks rely on header file inclusion to access and execute test functions. With C++ Modules, this approach needs to be adapted to work with module interface units. Instead of directly including the implementation headers, the test framework needs to include the module interface units that provide access to the necessary internal symbols.

This adaptation may require changes to the unit testing framework or the creation of custom macros or utilities to facilitate the inclusion and execution of tests against module interface units. The goal is to integrate seamlessly with the new module system while maintaining the ability to test private implementation details.

Adjusting test coverage analysis

Test coverage analysis tools often rely on instrumentation of source code to track which lines or branches are executed during tests. However, with C++ Modules, the traditional approach might not work as expected.

Since C++ Modules separate interface and implementation, code coverage analysis tools need to be aware of both the module’s interface units and implementation units. This means that the test coverage analysis tool needs to support the module system to accurately report coverage information across all relevant units.

Conclusion

C++ Modules bring many benefits in terms of code organization and compile-time improvements. However, when it comes to unit testing and test coverage, they introduce some challenges. By creating module interface units and adapting test frameworks, developers can overcome these challenges and effectively test and analyze the private implementation details of C++ Modules.

Keywords: C++ Modules, unit testing, test coverage