The simplest example of coroutines in C++20

Coroutines were introduced in C++20 standard. Unfortunately, the implementation isn't such friendly even to experienced developers because the standard library provides only interfaces and leaves the event loop, awaiters, promises etc. on programmers.

This is a minimal working example which could help people to start learning:

#include <iostream>
#include <coroutine>
#include <thread>
#include <queue>
#include <functional>

std::queue<std::function<bool()>> task_queue;

struct sleep {
    sleep(int n) : delay{n} {}

    constexpr bool await_ready() const noexcept { return false; }

    void await_suspend(std::coroutine_handle<> h) const noexcept {
        auto start = std::chrono::steady_clock::now();
        task_queue.push([start, h, d = delay] {
            if (decltype(start)::clock::now() - start > d) {
                h.resume();
                return true;
            } else {
                return false;
            }
        });
    }

    void await_resume() const noexcept {}

    std::chrono::milliseconds delay;
};


struct Task {
    struct promise_type {
        promise_type() = default;
        Task get_return_object() { return {}; }
        std::suspend_never initial_suspend() { return {}; } 
        std::suspend_always final_suspend() noexcept { return {}; }
        void unhandled_exception() {}
    };
};

Task foo1() noexcept {
    std::cout << "1. hello from foo1" << std::endl;
    for (int i = 0; i < 10; ++i) {
        co_await sleep{10};
        std::cout << "2. hello from foo1" << std::endl;
    }
}

Task foo2() noexcept {
    std::cout << "1. hello from foo2" << std::endl;
    for (int i = 0; i < 10; ++i) {
        co_await sleep{10};
        std::cout << "2. hello from foo2" << std::endl;
    }
}

//call foo
int main() {
    foo1();
    foo2();

    while (!task_queue.empty()) {
        auto task = task_queue.front();
        if (!task()) {
            task_queue.push(task);
        }
        task_queue.pop();

        std::this_thread::sleep_for(std::chrono::milliseconds(10));
    }
}

The magic happens here co_await sleep{10};. The current coroutine suspends and the awaiter takes its handler to resume it later. In this example, the awaiter defers a task into an event loop to checks if the sleep time expires.

This is a simple event loop with a queue of tasks:

while (!task_queue.empty()) {
    auto task = task_queue.front();
    if (!task()) {
        task_queue.push(task);
    }
    task_queue.pop();
    std::this_thread::sleep_for(std::chrono::milliseconds(10));
}

Play with this example a bit before starting to read about the coroutines in detail. Hope, it was helpful!

Comments

[Utkarsh Saxena]

Due to std::suspend_always final_suspend() coroutines frames are never destroyed. This is memory leak :) godbolt.org/z/K7n8GKhjK Vs godbolt.org/z/r87M5qof4

[Alexey Timin]

Thank you for pointing this. You're right -stackoverflow.com/questions/760395... . For the simplest example, it is better to use std::suspend_never of course.