Awaitable types revisited
We already talked a bit about awaitable types in the previous chapter. But now we need to get a little bit more specific about what co_await does and what an awaitable type is. The keyword co_await is a unary operator, meaning that it takes a single argument. The argument we pass to co_await needs to fulfill some requirements that we will explore in this section.
When we say co_await in our code, we express that we are waiting for something that may or may not be ready for us. If it's not ready, co_await suspends the currently executing coroutine and returns control back to its caller. When the asynchronous task has completed, it should transfer the control back to the coroutine originally waiting for the task to finish. From here on, I will typically refer to the awaiting function as the continuation.
Now consider the following expression:
co_await X{};
For this code to compile, X needs to be an awaitable type. So...