What are the C++20 modules?

I wanted to write about how to use C++ modules over three years ago. Despite the fact that modules were already accepted as part of the C++20 specification, the support of the C++ ecosystem was still nowhere near ready to use this feature. Fortunately, a lot has changed since the first edition of this book, and with the release of CMake 3.28, the C++20 modules are officially supported (although experimental support has been available since 3.26).

Three years may seem like a long time to implement a single feature, but we need to remember that it’s not only up to CMake. Many pieces of the puzzle have to come together and work well. First, we need compilers to understand how to deal with modules, then buildsystems like GNU Make or Ninja have to be able to work with modules, and only then can CMake use these new mechanisms to provide its support for modules.

This tells us one thing: not everyone will have the latest compatible tooling, and...