理解 co_await 运算符

编译器与库的交互

The facilities the C++ Coroutines TS provides in the language can be thought of as a low-
level assembly-language for coroutines. These facilities can be difficult to use directly in
a safe way and are mainly intended to be used by library-writers to build higher-level
abstractions that application developers can work with safely.

The Promise interface specifies methods for customising the behaviour of the coroutine
itself. The library-writer is able to customise what happens when the coroutine is called,
what happens when the coroutine returns (either by normal means or via an unhandled exception)
and customise the behaviour of any co_await or co_yield expression within the
coroutine.

The Awaitable interface specifies methods that control the semantics of a co_await
expression. When a value is co_await-ed, the code is translated into a series of calls to
methods on the awaitable object that allow it to specify: whether to suspend the current
coroutine, execute some logic after it has suspended to schedule the coroutine for later
resumption, and execute some logic after the coroutine resumes to produce the result of the
co_await expression.

辨析 Awaiters & Awaitables

A type that supports the co_await operator is called an Awaitable type.