In modern C++, coroutines have become a powerful tool for writing asynchronous and concurrent code in a more sequential and readable manner. One of the practical use cases of coroutines is task scheduling, where we can schedule tasks to run concurrently and await their completion without blocking threads. In this blog post, we will explore how to implement coroutine-based task scheduling in C++.

What are coroutines?

Coroutines allow us to write code that can be suspended and resumed at specific points, without breaking the flow of execution. With coroutines, we can write asynchronous code that resembles synchronous code, making it easier to understand and maintain.

Implementing a task scheduler using coroutines

To implement a coroutine-based task scheduler, we can use a combination of coroutines and an event loop. The event loop will be responsible for managing the execution of tasks and scheduling them to run concurrently.

Let’s start by defining a simple Task class that will represent our tasks:

class Task {
public:
    virtual ~Task() {}
    virtual void execute() = 0;
};

Next, we can create a Scheduler class that will manage the execution of tasks:

class Scheduler {
public:
    Scheduler() {}

    void schedule(std::shared_ptr<Task> task) {
        tasks.push_back(task);
    }

    void run() {
        while (!tasks.empty()) {
            auto task = tasks.front();
            tasks.pop_front();
            task->execute();
        }
    }

private:
    std::list<std::shared_ptr<Task>> tasks;
};

Now, let’s define a coroutine-based task using the <coroutine> header:

#include <coroutine>

class CoroutineTask : public Task {
public:
    struct promise_type {
        CoroutineTask get_return_object() { return CoroutineTask(std::coroutine_handle<promise_type>::from_promise(*this)); }
        std::suspend_never initial_suspend() { return {}; }
        std::suspend_never final_suspend() noexcept { return {}; }
        void unhandled_exception() {}
        void return_void() {}
    };

    CoroutineTask(std::coroutine_handle<promise_type> handle) : handle(handle) {}

    void execute() override {
        handle.resume();
        if (!handle.done()) {
            Scheduler::getInstance().schedule(std::make_shared<CoroutineTask>(handle));
        }
    }

private:
    std::coroutine_handle<promise_type> handle;
};

In this example, we define a coroutine-based task using the coroutine_handle type. The execute function resumes the coroutine and schedules it to run again if it’s not done yet.

Finally, let’s put everything together and use our coroutine-based task scheduler:

#include <iostream>

class MyTask : public CoroutineTask {
public:
    MyTask() : CoroutineTask(nullptr) {}

    void execute() override {
        std::cout << "Task executed!" << std::endl;
    }
};

int main() {
    Scheduler& scheduler = Scheduler::getInstance();

    std::shared_ptr<MyTask> task = std::make_shared<MyTask>();
    scheduler.schedule(task);

    scheduler.run();

    return 0;
}

In this example, we define a simple MyTask class that inherits from CoroutineTask. We schedule an instance of MyTask using the Scheduler and then run the scheduler.

Conclusion

Coroutine-based task scheduling in C++ provides an elegant and efficient way to write concurrent code that is more readable and maintainable. By leveraging the power of coroutines and an event loop, we can easily schedule and execute tasks asynchronously without using blocking threads.