NAME

std::noop_coroutine - std::noop_coroutine

Synopsis

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