std::noop_coroutine

 

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Defined in header <coroutine>
std::noop_coroutine_handle noop_coroutine() noexcept;		(since C++20)

Returns a coroutine handle referring to a no-op coroutine.

If there has been already a coroutine state of no-op coroutine, it is unspecified whether a subsequent call to noop_coroutine returns a previously obtained coroutine handle, or a coroutine handle referring to a new coroutine state of no-op coroutine.

Parameters

(none)

Return value

A std::noop_coroutine_handle referring to a no-op coroutine.

Notes

Return values from different calls to noop_coroutine may and may not compare equal.

noop_coroutine may only return a noop_coroutine_handle referring to a coroutine state object without starting a coroutine.

Example

Run this code

#include <coroutine>
#include <utility>
#include <iostream>
 
template<class T>
struct task {
    struct promise_type {
        auto get_return_object() {
            return task(std::coroutine_handle<promise_type>::from_promise(*this));
        }
        std::suspend_always initial_suspend() { return {}; }
        struct final_awaiter {
            bool await_ready() noexcept { return false; }
            void await_resume() noexcept {}
            std::coroutine_handle<> 
                await_suspend(std::coroutine_handle<promise_type> h) noexcept {
                // final_awaiter::await_suspend is called when the execution of the
                // current coroutine (referred to by 'h') is about to finish.
                // If the current coroutine was resumed by another coroutine via
                // co_await get_task(), a handle to that coroutine has been stored
                // as h.promise().previous. In that case, return the handle to resume
                // the previous coroutine.
                // Otherwise, return noop_coroutine(), whose resumption does nothing.
 
                auto previous = h.promise().previous;
                if (previous) {
                    return previous;
                } else {
                    return std::noop_coroutine();
                }
            }
        };
        final_awaiter final_suspend() noexcept { return {}; }
        void unhandled_exception() { throw; }
        void return_value(T value) { result = std::move(value); }
        T result;
        std::coroutine_handle<> previous;
    };
 
    task(std::coroutine_handle<promise_type> h) : coro(h) {}
    task(task&& t) = delete;
    ~task() { coro.destroy(); }
 
    struct awaiter {
        bool await_ready() { return false; }
        T await_resume() { return std::move(coro.promise().result); }
        auto await_suspend(std::coroutine_handle<> h) {
            coro.promise().previous = h;
            return coro;
        }
        std::coroutine_handle<promise_type> coro;
    };
    awaiter operator co_await() { return awaiter{coro}; }
    T operator()() {
        coro.resume();
        return std::move(coro.promise().result);
    }
private:
    std::coroutine_handle<promise_type> coro;
};
 
task<int> get_random() {
    std::cout << "in get_random()\n";
    co_return 4;
}
task<int> test() {
    task<int> v = get_random();
    task<int> u = get_random();
    std::cout << "in test()\n";
    int x = (co_await v + co_await u);
    co_return x;
}
 
int main() {
    task<int> t = test();
    int result = t();
    std::cout << result << '\n';
}

Output:

in test()
in get_random()
in get_random()
8

See also

noop_coroutine_promise (C++20)	used for coroutines with no observable effects (class)
noop_coroutine_handle (C++20)	std::coroutine_handle<std::noop_coroutine_promise>, intended to refer to a no-op coroutine (typedef)