C++20 modules are finally usable end to end, from your own modules to 'import std;' The functional build-system support with real diagnostics. This talk teaches modules the way #include was once taught: with small files, a build, and concrete use cases. Modules stopped being experimental somewhere around 2025. This session builds the mental model from scratch: what a primary module interface unit is, how partitions and implementation units fit together, and, crucially, how import differs from #include in ways that matter day to day. It uses a small library, exposes it as a module, splits it across partitions, consumes import std; and observes the compile-time effect. Then it answers the questions every team hits in week one: how modules interact with macros, with templates in headers, with header-only dependencies, and with a mixed codebase that can't convert everything at once. The examples will be using CMake, clang and recent gcc. You will leave able to structure a small library as a module, explain why a macro didn't cross a module boundary, and plan an incremental adoption that doesn't require converting the whole tree.
