C++ coroutines provide a more structured approach to writing asynchronous code by allowing functions to be suspended and resumed at specific points. This enables developers to write asynchronous code in a more sequential and natural manner, similar to synchronous programming.

To implement asynchronous programming using C++ coroutines, you will need a compiler that supports the C++20 standard and its coroutine features. Visual Studio 2019 with the latest C++ features enabled is a suitable choice.

Let’s consider a simple example of fetching data asynchronously from a remote API using coroutines. We will use the Boost.Asio library, which provides excellent support for asynchronous I/O operations.

First, let’s set up our project and include the necessary headers:

#include <boost/asio.hpp>
#include <iostream>
#include <cppcoro/generator.hpp>

Next, let’s define a coroutine function that will fetch data asynchronously:

cppcoro::generator<std::string> fetchDataAsync(const std::string& url)
{
    boost::asio::io_context io;
    boost::asio::ip::tcp::socket socket(io);
    boost::asio::ip::tcp::resolver resolver(io);

    boost::asio::co_spawn(io, [&]() -> cppcoro::task<void>
    {
        co_await resolver.async_resolve({ "api.example.com", "http" });
        auto endpoint = *resolver.resolve(std::move(query));
        co_await socket.async_connect(endpoint);

        std::string request = "GET /data HTTP/1.1\r\nHost: api.example.com\r\nConnection: close\r\n\r\n";
        co_await boost::asio::async_write(socket, boost::asio::buffer(request));

        std::string response;
        char buffer[4096];

    read_loop:
        std::size_t bytesRead = co_await socket.async_read_some(boost::asio::buffer(buffer));
        response += std::string(buffer, buffer + bytesRead);
        if (bytesRead > 0)
            goto read_loop;

        co_return response;
    });

    io.run();
}

In the above code, we create an io_context object to handle I/O operations, a tcp::socket for communication, and a tcp::resolver to resolve the host name.

We then use co_spawn to launch a coroutine lambda that performs the actual async operations. Inside the lambda, we use co_await to suspend the function execution until the operation completes, making it appear like synchronous code.

Once the coroutine completes, it returns the fetched data using co_return.

To use the fetchDataAsync function, we need to define another coroutine that will iterate over the generator returned by fetchDataAsync and process each fetched piece of data:

cppcoro::task<void> processDataAsync()
{
    for co_await (const std::string& data : fetchDataAsync("http://api.example.com/data"))
    {
        // Process the fetched data.
        std::cout << data << std::endl;
    }
}

Finally, in the main function, we invoke the processDataAsync coroutine:

int main()
{
    processDataAsync().await_resume();
    return 0;
}

That’s it! We have implemented a basic example of asynchronous programming with C++ coroutines using Boost.Asio. By leveraging coroutines, our code is much more readable and resembles synchronous programming, making it easier to reason about and maintain.

#cpp #coroutines