Coroutines in C++ are a powerful feature that allow you to write asynchronous code in a more readable and maintainable way. When working with coroutines, you may often need to handle the return values from asynchronous operations. In this blog post, we will explore how to work with return values in C++ coroutines.

Return values in coroutines

In C++ coroutines, return values are handled using the co_await keyword. When you use co_await on a coroutine or a future object, it suspends the execution of the current coroutine until the awaited operation completes.

To retrieve the return value from the awaited operation, you can use the co_await expression inside a coroutine. Let’s look at an example:

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

std::future<int> asyncOperation()
{
    // Simulating an asynchronous operation
    std::promise<int> promise;
    std::future<int> future = promise.get_future();

    std::thread([promise = std::move(promise)]() mutable {
        // Performing some time-consuming task
        std::this_thread::sleep_for(std::chrono::seconds(2));
        promise.set_value(42);
    }).detach();

    return future;
}

std::experimental::coroutine_handle<> coroutineExample()
{
    // Using co_await to suspend coroutine execution
    int result = co_await asyncOperation();

    std::cout << "Result: " << result << std::endl;

    // Resuming the coroutine
    co_return;
}

int main()
{
    auto coroutine = coroutineExample();

    // Executing the coroutine until completion
    coroutine.resume();

    return 0;
}

In this example, we have an asyncOperation() function that returns a std::future<int>. Inside the coroutineExample() function, we use co_await to suspend the execution of the coroutine until the asyncOperation() completes.

Once the awaited operation completes and returns a value, we retrieve it by assigning it to a variable. In this case, we store the returned value in the result variable and print it to the console.

Conclusion

Working with return values in C++ coroutines is straightforward. By using the co_await keyword, you can easily retrieve the return value from an asynchronous operation. This makes it easier to handle the results of asynchronous tasks and write more readable and maintainable code.

#C++ #coroutines