Introduction 💡

C++ coroutines are a powerful language feature that allows developers to write asynchronous code in a more intuitive and readable way. The introduction of coroutines in C++20 has made it easier for developers to write asynchronous code without relying on external libraries or complex abstractions. However, not all compilers have full support for C++ coroutines, and some developers may need to use Boost C++ libraries to implement coroutines in their code.

In this blog post, we'll explore C++ coroutines with Boost C++ libraries. We'll start by introducing coroutines and their benefits, and then we'll dive into how coroutines work with Boost C++. Finally, we'll provide some code examples to help you get started with coroutines in your own projects.

What are Coroutines? 🤔

Coroutines are a programming language feature that allow a function to pause its execution at a certain point and resume it later from where it left off. This is especially useful for asynchronous programming, where a function may need to wait for some external event to occur before continuing its execution. In the absence of coroutines, asynchronous programming is often done using callbacks or by wrapping the code in complex abstractions like Promises or Futures.

Coroutines make asynchronous programming much more intuitive and readable by allowing you to write asynchronous code that looks like synchronous code. This means that you don't need to worry about callback functions or complex abstractions, and you can write your code in a more linear fashion.

How do Coroutines Work with Boost C++ Libraries? 🤔

Boost C++ libraries provide support for coroutines through the Boost.Coroutine2 library. This library allows developers to create coroutines in their C++ code, which can be used to write asynchronous code in a more intuitive and readable way.

The Boost.Coroutine2 library provides two main classes for creating coroutines: coroutine and symmetric_coroutine. The coroutine class is used to create a coroutine that can be resumed from the calling function, while the symmetric_coroutine class is used to create a coroutine that can be resumed from either the calling function or the coroutine itself.

Creating a Coroutine with Boost.Coroutine2

To create a coroutine with Boost.Coroutine2, you need to define a function that will act as the coroutine. This function should have the signature void function_name(boost::coroutines2::coroutine<return_type>::push_type& yield), where return_type is the type of value that the coroutine will return when it's done.

Here's an example of a simple coroutine that counts from 1 to 5:

#include <boost/coroutine2/all.hpp>
#include <iostream>

void count_to_five(boost::coroutines2::coroutine<int>::push_type& yield)
{
    for (int i = 1; i <= 5; ++i)
    {
        yield(i);
    }
}

int main()
{
    boost::coroutines2::coroutine<int>::pull_type coro(count_to_five);
    for (auto i : coro)
    {
        std::cout << i << std::endl;
    }
    return 0;
}

In this example, we define a function count_to_five that takes a boost::coroutines2::coroutine<int>::push_type& parameter. Inside this function, we loop from 1 to 5 and yield each value using the yield function provided by the push_type object.

In the main function, we create a boost::coroutines2::coroutine<int>::pull_type object using the count_to_five function as the coroutine. We then loop over the values returned by the coroutine using a range-based for loop, printing each value to the console.

When the coroutine reaches the end of its function, it automatically returns and the coroutine is considered finished.

Creating a Symmetric Coroutine with Boost.Coroutine2

In addition to the coroutine class, Boost.Coroutine2 also provides a symmetric_coroutine class that allows for more flexibility when working with coroutines. The symmetric_coroutine class can be resumed from either the calling function or the coroutine itself, which makes it useful for implementing coroutines that need to be resumed in different contexts.

To create a symmetric coroutine with Boost.Coroutine2, you need to define a function that will act as the coroutine. This function should have the signature void function_name(boost::coroutines2::coroutine<return_type>::symmetric_type& yield), where return_type is the type of value that the coroutine will return when it's done.

Here's an example of a symmetric coroutine that prints a message and then yields control back to the calling function:

#include <boost/coroutine2/all.hpp>
#include <iostream>

void print_message(boost::coroutines2::coroutine<std::string>::symmetric_type& yield)
{
    std::cout << "Coroutine started." << std::endl;
    yield("Message 1");
    std::cout << "Coroutine resumed." << std::endl;
    yield("Message 2");
    std::cout << "Coroutine finished." << std::endl;
}

int main()
{
    boost::coroutines2::coroutine<std::string>::symmetric_type coro(print_message);
    std::cout << coro.get() << std::endl;
    coro("Next message");
    std::cout << coro.get() << std::endl;
    coro("Last message");
    return 0;
}

In this example, we define a function print_message that takes a boost::coroutines2::coroutine<std::string>::symmetric_type& parameter. Inside this function, we print a message to the console, yield a value back to the calling function, print another message, yield a second value, and then print a final message.

In the main function, we create a boost::coroutines2::coroutine<std::string>::symmetric_type object using the print_message function as the coroutine. We then call the get function on the coroutine object to retrieve the initial value, and print it to the console. We then call the coroutine object with a value, which resumes the coroutine and yields the first value back to the calling function. We print the first value to the console and then call the coroutine object again with a second value, which resumes the coroutine and yields the second value back to the calling function. Finally, we print the second value to the console and return from the program.

Conclusion 🚀

C++ coroutines are a powerful language feature that make asynchronous programming more intuitive and readable. While C++20 provides built-in support for coroutines, not all compilers have full support for this feature. Boost C++ libraries provide an alternative way to implement coroutines in your code, using the Boost.Coroutine2 library. This library provides two main classes for creating coroutines: coroutine and symmetric_coroutine. These classes allow you to write asynchronous code in a more linear and readable fashion, without relying on complex abstractions or callback functions. We hope this post has helped you understand how to use coroutines with Boost C++ libraries and get started with writing asynchronous code in C++.