source: doc/papers/concurrency/c++-cor/counter.cpp@ 7cfd6d4

#include <iostream>
#include <experimental/coroutine>

struct suspend_never {
        bool await_ready() noexcept {
                return true;
        }

        void await_suspend(std::experimental::coroutine_handle<>) noexcept {}
        void await_resume() noexcept {}
};

struct suspend_always {
        bool await_ready() noexcept {
                return false;
        }

        void await_suspend(std::experimental::coroutine_handle<>) noexcept {}
        void await_resume() noexcept {}
};

struct counter_cor {
        struct promise_type {
                counter_cor get_return_object() {
                        return counter_cor(std::experimental::coroutine_handle<promise_type>::from_promise(*this));
                }

                auto initial_suspend() { return suspend_never(); }
                auto final_suspend()   { return suspend_never(); }

                void return_void() {}

                void unhandled_exception() {}
        };

        std::experimental::coroutine_handle<promise_type> _coroutine = nullptr;

        explicit counter_cor(std::experimental::coroutine_handle<promise_type> coroutine)
                : _coroutine(coroutine)
        {}

        ~counter_cor() {
                if(_coroutine) { _coroutine.destroy(); }
        }

        counter_cor() = default;
        counter_cor(counter_cor const &) = delete;
        counter_cor& operator=(counter_cor const &) = delete;

        counter_cor(counter_cor&& other) {
                std::swap(_coroutine, other._coroutine);
        }

        counter_cor& operator=(counter_cor&& other) {
                if(&other != this) {
                        _coroutine = other._coroutine;
                        other._coroutine = nullptr;
                }
                return *this;
        }

        void resume() { _coroutine.resume(); }
};

counter_cor counter() {
        std::cout << "Counter: called\n";
        for(unsigned i = 1;; i++) {
                co_await suspend_always{};
                std::cout << "Counter: Resumed " << i << " time(s)\n";
        }
}

int main() {
        std::cout << "Main: calling counter\n";
        auto c = counter();
        std::cout << "Main: resumes\n";
        c.resume();
        c.resume();
        std::cout << "Main: done\n";
}