Coroutine generator

coro3.cpp

/*
 * A generator. Initial code taken from N4649 p15
 * http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/n4649.pdf
 */


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


struct generator {
    struct promise_type;
    using handle = std::experimental::coroutine_handle<promise_type>;
    struct promise_type {
        int current_value;
        static auto get_return_object_on_allocation_failure() {
            return generator{nullptr};
        }
        auto get_return_object() {
            return generator{handle::from_promise(*this)};
        }
        auto initial_suspend() {
            return std::experimental::suspend_always{};
        }
        auto final_suspend() {
            return std::experimental::suspend_always{};
        }
        auto yield_value(int value) {
            std::cout << "Yielded " << value << std::endl;
            current_value = value;
            return std::experimental::suspend_always{};
        }
        auto return_void() {
            return std::experimental::suspend_never{};
        }
        void unhandled_exception() {
            std::exit(1);
        }
    };
    bool move_next() {
        if ( coro ) {
            std::cout << "Moving to next" << std::endl;;
            coro.resume();
            std::cout << "Are we done? ";
            auto still_going = not coro.done();
            std::cout << (still_going ? "There is another" : "We're done") << std::endl;
            return still_going;
        } else {
            return false;
        }
    }
    int current_value() {
        return coro.promise().current_value;
    }
    generator(const generator &) = delete;
    generator(generator &&g)
    : coro(g.coro) {
        g.coro = nullptr;
    };
    ~generator() {
        if ( coro ) coro.destroy();
    }
private:
    generator(handle h)
    : coro(h) {
    }
    handle coro;
};


generator f() {
    std::cout << "Going to yield 1" << std::endl;
    co_yield 1;
    std::cout << "Going to yield 2" << std::endl;
    co_yield 2;
    std::cout << "Going to yield 3" << std::endl;
    co_yield 3;
    std::cout << "f() is done" << std::endl;
}


int main() {
    std::cout << "coro3" << std::endl;
    auto g = f();
    while ( g.move_next() )
        std::cout << ">> " << g.current_value() << std::endl;
    return 0;
}

There is a subtle bug in the code, to do with the way the generator is destructed. Because the destructor will destroy the coroutine we have to be careful about copying the generator instance itself. Making it movable and not copyable fixes this:

generator(const generator &) = delete;
generator(generator &&g)
: coro(g.coro) {
    g.coro = nullptr;
};