Cpp Modules

This is your reference when touching C++20 modules, which are still rough around the edges despite being standardized. It walks through named modules versus header units, module partitions, and the Global Module Fragment trick for preserving macro access. The CMake section is solid (requires 3.28+ with FILE_SET CXX_MODULES) and the compiler flag examples for Clang and GCC will save you from the usual "module not found" spiral. The error table is honest about common gotchas like stale BMI caches and ODR violations across partitions. Use this when migrating from headers or debugging why your build system can't find the precompiled module files.