source: doc/papers/concurrency/examples/Format.cpp@ d06707e

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

struct fmt_cor {
        struct promise_type {
                char _value;

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

                auto initial_suspend() { return std::experimental::suspend_never(); }
                auto final_suspend()   { return std::experimental::suspend_always(); }

                void return_void() {}
                void unhandled_exception() {}
        };

        struct get {
                promise_type * _promise = nullptr;

                bool await_ready() noexcept {
                        return false;
                }

                void await_suspend(std::experimental::coroutine_handle<promise_type> _coroutine) noexcept {
                        _promise = &_coroutine.promise();
                }
                char await_resume() noexcept {
                        assert(_promise);
                        return _promise->_value;
                }
        };

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

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

        ~fmt_cor() {
                if(_coroutine) {
                        _coroutine.destroy();

                }
        }

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

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

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

        void send(char a) {
                _coroutine.promise()._value = a;
                _coroutine.resume();
        }
};

fmt_cor Fmt() {
        struct locals {
                int g, b;

                ~locals() {
                        if (g != 0 | b != 0) {
                                std::cout << "\n";
                        }
                }
        } l;
        for(;;) {
                for(l.g = 0; l.g < 5; l.g++) {
                        for(l.b = 0; l.b < 4; l.b++) {
                                std::cout << co_await fmt_cor::get();
                        }
                        std::cout << "  ";
                }
                std::cout << std::endl;
        }
}

int main() {
        auto fmt = Fmt();
        for(int i = 0; i < 41; i++) {
                fmt.send('a');
        }
}