Modern C++ has been moving fast, and with C++20, we’ve seen the introduction of a highly anticipated feature: coroutines. For those familiar with languages like Python or JavaScript, you might recognize them as a powerful tool to make asynchronous programming more straightforward.

Today, we’re diving deep into this feature, exploring how C++ coroutines can simplify complex tasks and improve code performance and readability.

What are Coroutines?

Traditional functions, when called, run from their beginning to their end, after which they return control to the caller. Coroutines, on the other hand, can be paused and resumed. They can yield control back to the caller without being done.

Key Concepts:

1. co_return: Instead of the traditional return, we use co_return in coroutines.
2. co_yield: This yields a value and can later resume execution.
3. co_await: Used to wait for another coroutine.

A Basic Example:

Let’s start with a simple example to generate a series of integers using coroutines.

#include <iostream>
#include <experimental/coroutine>
using namespace std;

generator<int> createRange(int start, int end) {
    for (int i = start; i <= end; ++i) {
        co_yield i;
    }
}

int main() {
    for (int value : createRange(1, 5)) {
        cout << value << endl; 
    }
    return 0;
}

In the example above, createRange doesn’t return a collection of numbers. Instead, it yields them one by one. Each time co_yield is encountered, the current state of the coroutine is saved, and the yielded value is returned. When the loop in main asks for the next value, the coroutine resumes just after where it last yielded.

Why Use Coroutines?

Imagine managing multiple I/O operations or complex tasks without getting bogged down in callback hell or threading complexities. Coroutines can:

1. Improve Readability: Code appears synchronous and logical, even if it’s doing asynchronous work.
2. Enhance Performance: Less overhead than threads and avoids callback complexities.
3. Simplify Code: Often leads to fewer lines of code compared to other asynchronous solutions.

Async Await with Coroutines:

Now, let’s look at a more advanced example involving asynchronous operations. For this, let’s mock an async function that simulates a delay:

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

struct Awaiter {
    std::chrono::milliseconds duration;
    bool await_ready() const noexcept { return duration.count() <= 0; }
    void await_suspend(std::experimental::coroutine_handle<> h) const {
        std::this_thread::sleep_for(duration);
    }
    void await_resume() const noexcept {}
};

Awaiter delay(std::chrono::milliseconds ms) { return Awaiter{ms}; }

async void asyncFunction() {
    cout << "Start\n";
    co_await delay(std::chrono::seconds(2));
    cout << "Finished after 2 seconds\n";
}

int main() {
    asyncFunction();
    std::this_thread::sleep_for(std::chrono::seconds(3)); // Give time for our async function to complete.
    return 0;
}

Here, the asyncFunction doesn’t block the main thread even though it has a delay. Instead, it runs asynchronously, allowing the main program to continue doing other tasks.

Closing Thoughts:

Coroutines are powerful, enabling C++ developers to write more readable, efficient, and elegant asynchronous code. However, like any other tool, they’re not always the best choice for every scenario. Always ensure to understand their implications on performance, readability, and maintainability, and choose the best tool for the task at hand.

Happy coding!