Awaitables and Awaiters

The three functions of a promise object: yield_value, initial_suspend, and final_suspend return awaitables.

Awaitables

An Awaitable is something you can await on. The awaitable determines if the coroutine pauses or not.

Essentially, the compiler generates the three functions calls using the promise prom and the co_await operator.

Call	Compiler generated call
yield value	co_await prom.yield_value(value)
prom.initial_suspend()	co_await prom.initial_suspend()
prom.final_suspend()	co_await prom.final_suspend()

The co_await operator needs an awaitable as argument. Awaitables have to implement the concept Awaitable.

The concept Awaitable

The concept Awaitable requires three functions.

Function	Description
await_ready	Indicates if the result is ready. When it returns false, await_suspend is called.
await_suspend	Schedule the coroutine for resumption or destruction.
await_resume	Provides the result for the co_await exp expression.

The C++20 standard already defines two basic awaitables: std::suspend_always, and std::suspend_never.