Awaitables and Awaiters

Get a detailed understanding of crucial abstraction in the coroutine framework known as 'awaitables' and 'awaiters'.

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

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