Course Title: Modern C++ Programming: Mastering C++ with Best Practices and Advanced Techniques

Section Title: Advanced C++ Features: C++20 and Beyond

Topic: Coroutines in modern C++: Asynchronous programming and generators

Overview

Coroutines are a new feature in C++20 that allow for asynchronous programming, enabling developers to write efficient and scalable code without the need for threads or callbacks. This topic will introduce you to the concept of coroutines, explain how they work, and demonstrate how to use them in modern C++ programming.

What are Coroutines?

Coroutines are a special type of function that can suspend and resume its execution, allowing for cooperative multitasking. Unlike threads, which execute concurrently, coroutines execute sequentially, yielding control back to the caller when they need to wait for a resource or perform a time-consuming operation.

How Do Coroutines Work?

When a coroutine is called, it executes until it reaches a point where it needs to wait for a resource or perform a long-running operation. At this point, the coroutine yields control back to the caller by using the co_yield keyword. The caller can then resume the coroutine when the resource is available or the operation is complete.

Coroutine Basics

Here's a simple example of a coroutine that generates numbers:

#include <coroutine>
#include <iostream>

struct Generator {
    struct promise_type;
    using handle_type = std::coroutine_handle&lt;promise_type&gt;;

    struct promise_type {
        int current_value;
        auto get_return_object() { return Generator{handle_type::from_promise(*this)}; }
        auto initial_suspend() { return std::suspend_always{}; }
        auto final_suspend() noexcept { return std::suspend_never{}; }
        auto yield_value(int value) {
            current_value = value;
            return std::suspend_always{};
        }
        void return_void() {}
        void unhandled_exception() { std::exit(1); }
    };

    handle_type h;

    Generator(handle_type h) : h(h) {}

    ~Generator() { h.destroy(); }

    bool move_next() {
        return h.move_next();
    }

    int get() {
        return h.promise().current_value;
    }
};

Generator generate_numbers() {
    for (int i = 0; i &lt; 10; ++i) {
        co_yield i;
    }
}

int main() {
    Generator gen = generate_numbers();
    while (gen.move_next()) {
        std::cout &lt;&lt; "Generated: " &lt;&lt; gen.get() &lt;&lt; std::endl;
    }
    return 0;
}

This example demonstrates a coroutine that generates numbers from 0 to 9, using the co_yield keyword to yield control back to the caller.

Asynchronous Programming with Coroutines

Coroutines can be used for asynchronous programming, allowing developers to write efficient and scalable code. Here's an example of an asynchronous coroutine that performs a long-running operation:

#include <coroutine>
#include <iostream>

struct AsyncOperation {
    struct promise_type;
    using handle_type = std::coroutine_handle&lt;promise_type&gt;;

    struct promise_type {
        auto get_return_object() { return AsyncOperation{handle_type::from_promise(*this)}; }
        auto initial_suspend() { return std::suspend_always{}; }
        auto final_suspend() noexcept { return std::suspend_never{}; }
        auto yield_value() {
            return std::suspend_always{};
        }
        void return_void() {}
        void unhandled_exception() { std::exit(1); }
    };

    handle_type h;

    AsyncOperation(handle_type h) : h(h) {}

    ~AsyncOperation() { h.destroy(); }

    bool await_ready() {
        return h.done();
    }

    void await_resume() {
        h.resume();
    }
};

AsyncOperation perform_long_running_operation() {
    // Simulate a long-running operation
    for (int i = 0; i &lt; 1000000000; ++i) {
        // Do some work
    }
    co_yield;
}

int main() {
    AsyncOperation op = perform_long_running_operation();
    while (!op.await_ready()) {
        std::cout &lt;&lt; "Operation is running..." &lt;&lt; std::endl;
        // Perform some other work
    }
    op.await_resume();
    return 0;
}

This example demonstrates an asynchronous coroutine that performs a long-running operation, using the co_yield keyword to yield control back to the caller. The caller can then resume the coroutine when the operation is complete.

Best Practices

• Use coroutines when writing asynchronous code to improve efficiency and scalability.
• Use the co_yield keyword to yield control back to the caller when performing long-running operations.
• Use the std::coroutine_handle class to manage the coroutine handle.
• Use the std::suspend_always and std::suspend_never classes to control the suspension of the coroutine.

Conclusion

Coroutines are a powerful feature in C++20 that enable developers to write efficient and scalable asynchronous code. By understanding how coroutines work and using them effectively, developers can improve the performance and responsiveness of their applications.

Further Reading

• C++20 Coroutines on cppreference.com
• C++20 Coroutines Tutorial on Microsoft Learn
• Effective Modern C++: 42 Specific Ways to Improve Your Use of C++11 and C++14 by Scott Meyers

Do you have any questions or need help with coroutines? Leave a comment below.