In C++20 we got the core language feature “coroutines”. Mostly oriented to the advanced C++ programmers, and not for the mere mortals, since it requires a lot of boilerplate one would not like to write. It provides the fundamentals to further build upon. Unfortunately the STL library did not build on it yet, more was to come in C++23. In C++23 we will only get one follow up: std::generator. So let’s revisit coroutines. What are they? A function that can be suspended and resumed. Easily said, but how to make use of it and write that boilerplate after all. We will look at the coroutine from 2 different angles: the user code and the compiler, and we will inspect what each wants or would like to have, and where we need to glue them together and what is needed for that purpose. We will give insights on topics like: coroutine frame, coroutine handle, promise_type, awaiters, awaitables, the interface or api.
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