Raymond Chen: All of C++'s async machinery runs on one hidden class

C++ futures part 1: The mental model

Raymond Chen breaks down std::promise, std::packaged_task, std::async, std::future, and std::shared_future by revealing they all rest on a single unnamed shared state he calls the "source" — a struct with a mutex, a condition variable, and a union holding either a value or an exception. He sketches a simplified implementation showing how values and exceptions go in, how waiters block, and how results get consumed, then promises to build the rest on top next time.