std::noop_coroutine(3)

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 <iostream>
 #include <utility>
 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.
                 if (auto previous = h.promise().previous; 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 used for coroutines with no observable effects
   (C++20) (class)
   noop_coroutine_handle std::coroutine_handle<std::noop_coroutine_promise>, intended
   (C++20) to refer to a no-op coroutine
                        (typedef)