Why Many Professional C++ Developers Find C++20 Coroutines Complex

The introduction of coroutines in C++20 was met with excitement, as they offer a powerful mechanism for writing asynchronous code, allowing functions to suspend and resume execution efficiently. However, despite the potential benefits, many professional C++ developers view coroutines as complex and difficult to grasp. In this article, we’ll dive into the reasons behind this perception, explore the core challenges of C++20 coroutines, and provide examples to clarify how they work.

Understanding Coroutines in C++20

At their core, coroutines are functions that can be suspended and resumed. Unlike regular functions, which execute linearly from start to finish, coroutines allow the program to pause execution (using co_await, co_yield, or co_return) and resume later from where it left off.

Here's a basic coroutine example in C++20:

#include <iostream>
#include <coroutine>

struct SimpleTask {
    struct promise_type {
        SimpleTask get_return_object() { return {}; }
        std::suspend_always initial_suspend() { return {}; }
        std::suspend_always final_suspend() noexcept { return {}; }
        void return_void() {}
        void unhandled_exception() {}
    };
};

SimpleTask myCoroutine() {
    std::cout << "Coroutine start\n";
    co_await std::suspend_always{};
    std::cout << "Coroutine resumed\n";
}

int main() {
    auto task = myCoroutine();
    std::cout << "Main function continues\n";
}

In this simple example, the coroutine myCoroutine starts, suspends execution, and then resumes. However, to many developers, the complexity of this mechanism lies beneath the surface.

Reasons Why C++20 Coroutines Are Considered Complicated

1. Manual Management of Coroutines' Lifecycle

Coroutines, unlike regular functions, require developers to manually manage their lifecycle. This means understanding promise types, suspend points, and the overall interaction between the coroutine and the system. The concept of "promises" and "suspend points" can be difficult to grasp for those unfamiliar with the low-level mechanics of how functions interact with the stack and program flow.

For example, a coroutine’s lifecycle is determined by the interaction between promise_type and the special keywords co_await, co_yield, and co_return. Understanding this interaction and how it impacts control flow is critical but nontrivial.

Here's a skeleton of a coroutine with lifecycle control:

struct MyTask {
    struct promise_type {
        MyTask get_return_object() { return {}; }
        std::suspend_always initial_suspend() { return {}; }
        std::suspend_always final_suspend() noexcept { return {}; }
        void return_void() {}
        void unhandled_exception() {}
    };
};

MyTask example() {
    co_await std::suspend_always{};
}

This example shows the basic structure needed to implement even the simplest coroutine, but for many developers, the interaction between the promise_type and the main coroutine logic can feel unwieldy.

2. Steep Learning Curve for Promise Types and Return Types

One of the key challenges with C++20 coroutines is the promise type. Every coroutine must have a custom promise type that determines how it interacts with the caller. This can involve extensive boilerplate code, adding complexity for developers who are used to more straightforward function constructs.

Additionally, coroutines must return a special type (e.g., std::future, std::generator, or a custom type), which is not a typical return value like int or void. This non-traditional return mechanism introduces a mental shift for developers and makes coroutines feel less intuitive than traditional functions.

Example of using std::future with coroutines:

#include <iostream>
#include <future>

std::future<int> asyncFunction() {
    co_return 42;
}

int main() {
    auto future = asyncFunction();
    std::cout << "Result: " << future.get() << std::endl;  // Output: Result: 42
}

While this example is simple, more complex coroutines involve custom promise types and manual control over the state of the coroutine, adding to the learning curve.

3. Complex Error Handling Mechanisms

Another complication arises from error handling in coroutines. Since coroutines can suspend at various points, developers need to ensure proper handling of exceptions and failures. This often requires more advanced techniques such as ensuring that suspended coroutines are safely resumed or destroyed, and properly propagating exceptions from within a coroutine to the caller.

Example with error handling:

#include <iostream>
#include <coroutine>

struct ErrorTask {
    struct promise_type {
        ErrorTask get_return_object() { return {}; }
        std::suspend_always initial_suspend() { return {}; }
        std::suspend_always final_suspend() noexcept { return {}; }
        void return_void() {}
        void unhandled_exception() {
            std::cout << "Exception caught in coroutine\n";
        }
    };
};

ErrorTask coroutineWithError() {
    throw std::runtime_error("Error in coroutine");
    co_return;
}

int main() {
    auto task = coroutineWithError();
}

In this example, exceptions must be handled within the coroutine’s promise_type, adding a layer of complexity not typically present in traditional functions.

4. Asynchronous Execution with co_await

co_await is one of the core features of coroutines, but it can be difficult to use effectively. The behavior of co_await is highly dependent on the type being awaited and the promise type used, which requires developers to understand asynchronous programming and the awaitable concept deeply. For professionals not accustomed to this kind of programming, this poses a significant barrier.

A simple co_await example:

std::future<void> asyncTask() {
    std::cout << "Starting async task\n";
    co_await std::suspend_always{};
    std::cout << "Resuming async task\n";
}

int main() {
    auto task = asyncTask();
    task.get();
}

The asynchronous execution here, with suspensions and resumptions, can become hard to trace in more complex codebases. Tracking the coroutine's state and understanding when and where it is suspended/resumed makes debugging and reasoning about the code much harder compared to synchronous code.

5. Performance Considerations

Coroutines are designed to be more lightweight than traditional thread-based concurrency models, but that doesn’t mean they are always easy to optimize. Developers need to be aware of heap allocations, stateful management, and other runtime overheads. For example, every co_await or co_yield introduces a state machine in the background that tracks the coroutine’s execution state. Understanding when and how coroutines allocate memory and manage state is crucial for optimizing performance.

Comparing C++ Coroutines to Other Languages

One major reason many professional C++ developers find coroutines complex is the language's low-level nature. Coroutines in languages like Python, JavaScript, and Kotlin are much simpler, requiring fewer explicit lifecycle management steps.

Python Coroutine Example:

async def my_coroutine():
    print("Start")
    await asyncio.sleep(1)
    print("End")

asyncio.run(my_coroutine())

In Python, the coroutine concept is built-in to the language and much easier to use. There’s no need to deal with low-level details like promise types or managing the coroutine’s lifecycle. In contrast, C++ coroutines provide more control but at the cost of increased complexity.

C++20 coroutines introduce significant new capabilities to the language, but the complexity stems from the language’s requirement for manual control over coroutine lifecycles, state management, and error handling. The steep learning curve for promise types, combined with the requirement to manage asynchronous execution, makes coroutines feel far more complex in C++ than in many higher-level languages.

For developers accustomed to simpler languages or paradigms, mastering C++20 coroutines involves embracing this increased control, which comes with added complexity. However, once understood, coroutines can become a powerful tool for writing efficient and scalable asynchronous programs.

If you're new to coroutines, it’s important to start with simple examples, gradually working your way up to more complex use cases while thoroughly understanding the underlying mechanisms that make coroutines work.