Coroutines

Coroutines in C++ are a powerful mechanism for writing asynchronous code that appears synchronous, simplifying complex asynchronous operations. They allow functions to suspend and resume execution, making them ideal for tasks like asynchronous I/O, event handling, and creating generators. By using coroutines, you can avoid complex callback structures and state machines, leading to more readable and maintainable code.

What are Coroutines?

Coroutines are functions that can suspend execution to be resumed later. Unlike traditional functions, which run to completion once called, coroutines can yield control back to the caller and retain their state, allowing them to pick up where they left off when resumed. This behavior is achieved through the use of the co_await, co_yield, and co_return keywords.

At a high level, a coroutine’s execution can be thought of as a state machine. When a coroutine is called, it doesn’t immediately execute like a regular function. Instead, it returns a coroutine handle. This handle encapsulates the coroutine’s state and allows it to be started, resumed, or destroyed. The coroutine only begins execution when its resume() method is called.

The real magic happens when the coroutine encounters a co_await expression. This expression suspends the coroutine’s execution and returns control to the caller. The co_await expression also handles scheduling the coroutine for resumption when the awaited operation completes. This is commonly used for asynchronous I/O operations.

co_yield is used within generator coroutines. It suspends execution and returns a value to the caller, allowing the coroutine to produce a sequence of values one at a time.

co_return is used to complete the execution of a coroutine and return a value to the caller.

Edge Cases & Considerations:

• Exception Handling: Exceptions thrown within a coroutine are propagated to the caller upon resumption. Proper exception handling within coroutines is crucial.
• Stack Management: Coroutines introduce a slight overhead due to the management of their state on the heap. However, this overhead is typically small compared to the complexity of managing asynchronous operations manually.
• Debugging: Debugging coroutines can be more challenging than debugging traditional functions due to their asynchronous nature. Debugging tools are improving to better support coroutines.
• Compiler Support: Coroutines require C++20 or later and a compiler that fully supports the coroutines feature.
• Performance: While coroutines generally improve the structure of asynchronous code, profiling is still important to identify performance bottlenecks. The overhead of suspending and resuming a coroutine should be considered, especially in performance-critical sections.

Syntax and Usage

To define a coroutine in C++, you must include the <coroutine> header. The coroutine must return a type that satisfies certain requirements, typically a promise type. The promise type provides the entry point and exit point for the coroutine, and handles the suspension and resumption. Common return types include std::future, custom awaitable types, and generator types.

The three primary keywords used with coroutines are:

• co_await: Suspends the coroutine until the awaitable object is ready.
• co_yield: Suspends the coroutine and returns a value to the caller (used in generator coroutines).
• co_return: Completes the coroutine and returns a value.

Here’s a basic example demonstrating the structure of a coroutine:

#include <iostream>
#include <coroutine>
#include <future>
 
struct Task {
  struct promise_type {
    Task get_return_object() { return Task{std::coroutine_handle<promise_type>::from_promise(*this)}; }
    std::suspend_never initial_suspend() noexcept { return {}; }
    std::suspend_never final_suspend() noexcept { return {}; }
    void return_void() {}
    void unhandled_exception() {}
  };
 
  std::coroutine_handle<promise_type> handle;
};
 
Task myCoroutine() {
  std::cout << "Coroutine started\n";
  co_await std::suspend_always{};
  std::cout << "Coroutine resumed\n";
  co_return;
}
 
int main() {
  Task t = myCoroutine();
  t.handle.resume();
  std::cout << "Main function continued\n";
  t.handle.resume();
  t.handle.destroy();
  return 0;
}

In this example, Task is a simple coroutine type. The promise_type nested struct defines the behavior of the coroutine. initial_suspend and final_suspend determine whether the coroutine suspends immediately upon entry or before exiting, respectively. co_await std::suspend_always{} will suspend the coroutine until resume is called.

Basic Example

This example demonstrates a more practical use of coroutines: simulating an asynchronous operation.

#include <iostream>
#include <coroutine>
#include <future>
#include <thread>
#include <chrono>
 
struct AsyncValue {
    struct promise_type {
        AsyncValue get_return_object() { return AsyncValue{std::coroutine_handle<promise_type>::from_promise(*this)}; }
        std::suspend_never initial_suspend() noexcept { return {}; }
        std::suspend_always final_suspend() noexcept { return {}; }
        void return_value(int value) { result = value; }
        void unhandled_exception() {}
 
        int result;
    };
 
    std::coroutine_handle<promise_type> handle;
 
    int get_result() {
        handle.resume(); // ensure coroutine finishes
        return handle.promise().result;
    }
};
 
AsyncValue simulateAsyncOperation() {
    std::cout << "Async operation started...\n";
    std::this_thread::sleep_for(std::chrono::seconds(2)); // Simulate work
    std::cout << "Async operation completed!\n";
    co_return 42;
}
 
int main() {
    AsyncValue task = simulateAsyncOperation();
    std::cout << "Waiting for result...\n";
    int result = task.get_result();
    std::cout << "Result: " << result << "\n";
    task.handle.destroy();
    return 0;
}

This example defines an AsyncValue type that represents an asynchronous operation. The simulateAsyncOperation coroutine simulates an asynchronous task by sleeping for 2 seconds. The co_return statement returns the value 42. The get_result method resumes the coroutine until it completes, then returns the result.

Advanced Example

This example demonstrates a generator coroutine that produces a sequence of Fibonacci numbers.

#include <iostream>
#include <coroutine>
#include <exception>
 
template <typename T>
struct Generator {
    struct promise_type {
        T value_;
        std::exception_ptr exception_;
 
        Generator get_return_object() {
            return Generator(std::coroutine_handle<promise_type>::from_promise(*this));
        }
        std::suspend_always initial_suspend() { return {}; }
        std::suspend_always final_suspend() noexcept { return {}; }
        void unhandled_exception() { exception_ = std::current_exception(); }
        std::suspend_always yield_value(T value) {
            value_ = value;
            return {};
        }
        void return_void() {}
    };
 
    std::coroutine_handle<promise_type> handle_;
 
    Generator(std::coroutine_handle<promise_type> handle) : handle_(handle) {}
    ~Generator() { if (handle_) handle_.destroy(); }
 
    // Prevent copy
    Generator(const Generator&) = delete;
    Generator& operator=(const Generator&) = delete;
 
    bool next() {
        handle_.resume();
        if (handle_.done()) {
            if (handle_.promise().exception_)
                std::rethrow_exception(handle_.promise().exception_);
            return false;
        } else {
            return true;
        }
    }
 
    T value() {
        return handle_.promise().value_;
    }
};
 
Generator<int> fibonacci_sequence(int n) {
    if (n <= 0)
        co_return;
 
    int a = 0;
    int b = 1;
 
    co_yield a;
    if (n == 1)
        co_return;
    co_yield b;
 
    for (int i = 2; i < n; ++i) {
        int next = a + b;
        a = b;
        b = next;
        co_yield next;
    }
}
 
int main() {
    for (int i : {1, 2, 5, 10}) {
        std::cout << "Fibonacci sequence (" << i << " elements): ";
        for (auto gen = fibonacci_sequence(i); gen.next();) {
            std::cout << gen.value() << " ";
        }
        std::cout << std::endl;
    }
    return 0;
}

This example defines a Generator template that can be used to create generator coroutines. The fibonacci_sequence coroutine generates a sequence of Fibonacci numbers up to a specified length. The co_yield keyword yields each number in the sequence. The main function iterates through the generator and prints the Fibonacci numbers.

Common Use Cases

• Asynchronous I/O: Handling asynchronous I/O operations without complex callbacks.
• Event Handling: Processing events in a sequential and readable manner.
• Generators: Creating sequences of values on demand.
• State Machines: Implementing complex state machines with clear state transitions.

Best Practices

• Keep Coroutines Short: Keep coroutines focused on a single task to improve readability.
• Handle Exceptions Properly: Ensure that exceptions are caught and handled within coroutines to prevent unexpected behavior.
• Avoid Blocking Operations: Avoid performing blocking operations within coroutines to prevent blocking the event loop.
• Use Clear Naming Conventions: Use clear and descriptive names for coroutines and their associated types.

Common Pitfalls

• Forgetting to Resume: Failing to resume a suspended coroutine can lead to deadlocks.
• Incorrect Promise Type: Using an incorrect promise type can result in unexpected behavior or compiler errors.
• Ignoring Exceptions: Ignoring exceptions within coroutines can lead to crashes or incorrect results.
• Overusing Coroutines: Coroutines introduce overhead; use them judiciously where they provide significant benefits in code structure and maintainability.

Key Takeaways

• Coroutines provide a powerful mechanism for writing asynchronous code in a synchronous style.
• The co_await, co_yield, and co_return keywords are essential for defining and using coroutines.
• Coroutines can be used for a variety of tasks, including asynchronous I/O, event handling, and creating generators.
• Proper exception handling and careful consideration of performance are crucial when working with coroutines.