C++20 modules have been in the standard for more than 5 years already. They promise to deliver a big change to how we write C++, but their adoption hasn't been as widespread as one would have expected. This talk is a deep dive into the practical aspects of C++20 modules, exploring the reality of the ecosystem as it is today.

We will introduce core concepts like interface and implementation units, the export and import keywords, BMIs and the 'std' module.

We will then move to more practical topics, like building modules with CMake, consuming libraries that only provide headers, handling macros and writing unit tests. We will also review the limitations of today's implementations.

You can follow the talk without any prior experience with C++20 modules.

About Rubén Pérez

Originally an industrial engineer, Ruben fell in love with C++ after reading Stroustrup's classic The C++ programming language. After eight years in the software industry, he has worked in a variety of sectors, including IoT, cyber security, finance and healthcare.

He is the proud author of Boost.MySQL and co-maintainer of Boost.Redis.

He is currently working for the C++ Alliance, performing regular contributions to Boost.

ISO C++ is the official website for the C++ programming language standardization committee, providing information, resources, and updates on the evolution of the C++ language. With a focus on language features, library enhancements, and standardization efforts, ISO C++ keeps developers informed about the latest developments in C++ programming. Developers can learn about upcoming language features, review proposals for language changes, and contribute to the evolution of the C++ language through active participation in the standardization process.

A Boost library author (Boost.MySQL, Boost.Beast) shares hands-on experience attempting to support `import boost` using C++20 modules. The talk covers core module concepts (primary module interfaces, implementation units, partitions, BMIs, visibility vs reachability, module attachment, global module fragment discards), practical CMake integration with Clang, and the real-world challenges of modularizing existing header-heavy libraries like Boost.Asio. Build time measurements reveal that naively wrapping headers in modules can make things worse due to increased translation units and reduced parallelism. Three modularization styles are compared (re-export via using, ABI-breaking, export extern C++), with the conclusion that while modules offer real benefits (75% build time reduction for lightweight libraries), the tooling ecosystem is not yet production-ready and modules are not a silver bullet for template-heavy code.

The road to 'import boost': a library developer's journey into C++20 modules - Rubén Pérez Hidalgo