Exploring the Power of Coroutines in C++20

作者

Coroutines, introduced in C++20, are a game‑changing feature that allows you to write asynchronous and lazy‑evaluation code in a natural, sequential style. Unlike callbacks or std::future, coroutines keep the state of a function across suspension points, enabling you to pause and resume execution with minimal overhead.

1. Basic Syntax

A coroutine function is marked with the co_await, co_yield, or co_return keywords. Here’s a minimal example:

#include <coroutine>
#include <iostream>

struct generator {
    struct promise_type;
    using handle_t = std::coroutine_handle <promise_type>;

    struct promise_type {
        int current_value;
        generator get_return_object() { return {handle_t::from_promise(*this)}; }
        std::suspend_always initial_suspend() { return {}; }
        std::suspend_always final_suspend() noexcept { return {}; }
        std::suspend_always yield_value(int v) {
            current_value = v;
            return {};
        }
        void return_void() {}
    };

    handle_t coro;
    explicit generator(handle_t h) : coro(h) {}
    ~generator() { coro.destroy(); }
    int next() {
        coro.resume();
        return coro.promise().current_value;
    }
};

generator count_to(int n) {
    for (int i = 1; i <= n; ++i)
        co_yield i;
}

Using the generator:

int main() {
    auto gen = count_to(5);
    for (int i = 0; i < 5; ++i) {
        std::cout << gen.next() << ' ';
    }
    // Output: 1 2 3 4 5
}

2. Practical Use Cases

  • Lazy Streams: Generate a sequence of values on demand without allocating a container.
  • Async IO: Wrap non‑blocking IO in a coroutine, letting the compiler manage the state machine.
  • Co‑routines in Game Loops: Represent game entity behaviors as coroutines that pause on events.

3. Under the Hood

When the compiler encounters a coroutine, it automatically transforms the function into a state machine. The promise_type encapsulates the coroutine’s state. co_await suspends the coroutine until the awaited expression becomes ready, while co_yield returns control to the caller and stores the yielded value.

The transformation preserves local variables across suspensions, making coroutines efficient. The only runtime cost is the allocation of the coroutine frame, which can be stack‑based or heap‑based depending on the promise’s initial_suspend and final_suspend behavior.

4. Advanced Features

  • Custom Awaiters: Define your own await_ready, await_suspend, and await_resume to integrate with networking libraries or GUI event loops.
  • co_return with Values: Coroutines can return values; just use co_return with a value and adjust the promise_type to store the return type.
  • Co‑operation: Combine multiple coroutines using co_await to orchestrate complex workflows.

5. Performance Considerations

  • Avoid Excessive Suspending: Each suspension introduces a tiny overhead. For tight loops, prefer loops over coroutines.
  • Stack Allocation: Use initial_suspend as std::suspend_always to allocate the coroutine frame on the heap only if necessary.
  • Inlining: The compiler can inline trivial coroutines, eliminating overhead.

6. Future Directions

C++23 extends coroutine support with std::task and better integration with the standard library. The community is actively developing coroutine‑aware containers like std::ranges::views::iota with lazy evaluation.

Coroutines in C++20 open a new paradigm for writing cleaner, more maintainable asynchronous code. By mastering their syntax and underlying mechanics, developers can unlock performance gains and more expressive code structures.

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