co_await

Learn await points with co_await.

What does co_await do?

co_await creates an await point inside a coroutine. The awaited object (awaitable) must be ready; until then the coroutine suspends, and when ready it continues.

An awaitable type must have three members: await_ready (is it ready?), await_suspend (how to suspend?) and await_resume (the value on resume). Example: int v = co_await Awaiter{};. Here Awaiter is an awaitable; the value returned by await_resume is assigned to v. This way asynchronous flow is written with synchronous-looking code.

Awaitable type

An awaitable is a type with await_ready, await_suspend and await_resume members. In its simplest form, a ready awaitable looks like this:

struct Awaiter { bool await_ready() { return true; } void await_suspend(std::coroutine_handle<>) { } int await_resume() { return 7; } };. Here await_ready returns true, so the coroutine is not suspended; await_resume returns 7. The expression co_await Awaiter{} yields the value 7. This structure represents the result of an asynchronous operation.

Exercise Get a value with co_await

Write int v = co_await Awaiter{}; in the body of the run coroutine and print the value with std::cout << v;. In main call run();.

Show example solution

#include <iostream>
#include <coroutine>

struct Awaiter {
    bool await_ready() { return true; }
    void await_suspend(std::coroutine_handle<>) { }
    int await_resume() { return 7; }
};

struct Task {
    struct promise_type {
        Task get_return_object() { return {std::coroutine_handle<promise_type>::from_promise(*this)}; }
        std::suspend_never initial_suspend() { return {}; }
        std::suspend_always final_suspend() noexcept { return {}; }
        void return_void() { }
        void unhandled_exception() { }
    };
    std::coroutine_handle<promise_type> handle;
};

Task run() {
    int v = co_await Awaiter{};
    std::cout << v;
}

int main() {
    run();
    return 0;
}

Test yourself

the co_await usage

int v = ____ Awaiter{};

Show answer

int v = co_await Awaiter{};

Match each member to its role

await_ready

Is it ready?
await_suspend

How to suspend
await_resume

Resume value
co_await

Await expression
What does co_await Awaiter{} return?
- The Awaiter struct itself
- A coroutine handle
- Nothing
- The value returned by await_resume
Show answer

The value returned by await_resume — co_await yields the value returned by the awaitable's await_resume member.

Next lesson: co_return

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