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A Mental Model for C++ Coroutine
C++ coroutine is not a library that is ready to go (e.g. std::vector). It is not even a trait (think of Rust’s Future trait) that library writers or users can implement (or the compiler generates for you in the case of Rust). C++ coroutine is a specification that defines a set of customization points, that requires library writers to implement in order to get a functional coroutine.
Let us pretend to be a compiler for a second, the only thing that can be used to customize the behavior of the coroutine is really just the return type Task<T>. An obvious option here is to make Task<T> a subclass of some language defined interface (which owns the coroutine frame for storing state across co_await points), where a set of customization points can be implemented as member functions to specify the behavior of this coroutine. As we observed earlier, the return type Task<T> is the best entry-point for users to customize coroutine behaviors.
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