Exploring the Power of C++20 Coroutines: Async Programming Simplified

作者

Coroutines, introduced in C++20, bring a new paradigm to asynchronous programming, allowing developers to write code that looks synchronous while operating non-blockingly under the hood. This feature is especially valuable for I/O-bound applications, such as network servers or GUI event loops, where you want to avoid thread contention while maintaining readable code.

What Is a Coroutine?

A coroutine is a function that can suspend its execution at a co_await, co_yield, or co_return point and resume later. Unlike threads, coroutines are lightweight and share the same stack frame, making them far cheaper to create and switch between.

The basic building blocks are:

  • std::suspend_always and std::suspend_never – traits that dictate when the coroutine should suspend.
  • std::coroutine_handle – a handle to control the coroutine’s state.
  • std::future or custom awaitables – objects that provide the await_ready, await_suspend, and await_resume functions.

A Minimal Example

#include <iostream>
#include <coroutine>
#include <thread>
#include <chrono>

struct simple_task {
    struct promise_type {
        simple_task get_return_object() { return {}; }
        std::suspend_never initial_suspend() noexcept { return {}; }
        std::suspend_never final_suspend() noexcept { return {}; }
        void return_void() noexcept {}
        void unhandled_exception() { std::terminate(); }
    };
};

simple_task async_print(int x) {
    std::cout << "Before suspend: " << x << '\n';
    co_await std::suspend_always{}; // Suspend here
    std::cout << "After resume: " << x << '\n';
}

Running async_print(42) will pause after printing the first line; resuming the coroutine (via its handle) continues execution.

Integrating with std::async

Although std::async itself is not a coroutine, you can combine them to offload heavy work to background threads while keeping the main flow simple.

std::future <int> compute(int a, int b) {
    return std::async(std::launch::async, [=]{
        std::this_thread::sleep_for(std::chrono::seconds(2));
        return a + b;
    });
}

co_await compute(10, 20);

Here the coroutine yields control until the future completes, freeing the calling thread to do other tasks.

Awaitable Types

A type is awaitable if it provides:

  • await_ready() – returns true if ready immediately.
  • await_suspend(std::coroutine_handle<>) – called when the coroutine suspends; can schedule resumption.
  • await_resume() – returns the result when resumed.

A simple example of an awaitable that simulates a timer:

struct timer {
    std::chrono::milliseconds delay;
    bool await_ready() const noexcept { return delay.count() == 0; }
    void await_suspend(std::coroutine_handle<> h) {
        std::thread([h, delay=delay]{
            std::this_thread::sleep_for(delay);
            h.resume();
        }).detach();
    }
    void await_resume() const noexcept {}
};

Using it:

co_await timer{std::chrono::milliseconds(500)};

Practical Use Cases

  1. Network Servers – Each connection can be handled by a coroutine, suspending on I/O operations without blocking the entire event loop.
  2. Game Loops – Coroutine-based animation sequences allow for clean sequencing of actions over frames.
  3. GUI Frameworks – UI callbacks can be coroutine-friendly, enabling asynchronous file loading or background computations.

Challenges and Tips

  • Error Propagation: If an exception is thrown inside a coroutine, the promise’s unhandled_exception() is called. Ensure proper exception handling or propagate via std::exception_ptr.
  • Lifetime Management: The coroutine must outlive any references it captures. Prefer move semantics or store data on the heap.
  • Debugging: Coroutines can be harder to trace. Using tools like std::coroutine_handle::address() can help identify specific coroutine instances.

Conclusion

C++20 coroutines open a door to elegant, efficient asynchronous programming. By embracing co_await and custom awaitables, developers can write code that feels imperative while leveraging non-blocking execution patterns. Whether building high-performance servers or responsive UI applications, coroutines provide a powerful addition to the modern C++ toolkit.

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