Using C++ Coroutines for Web Scraping

15 Sep 2023

Web scraping is a powerful technique for gathering data from websites. It allows you to extract information from HTML pages and use it for various purposes like data analysis, machine learning, or building APIs. In this blog post, we will explore how to use C++ coroutines for web scraping, taking advantage of the C++20 coroutine feature to simplify the code and make it more readable.

What are C++ Coroutines?

Coroutines are a new language feature introduced in C++20 that enable writing asynchronous code in a more sequential and readable manner. They provide a way to suspend and resume execution at specific points, allowing for efficient and readable asynchronous programming.

Setting Up the Project

First, let’s set up our C++ project with the necessary libraries. We’ll use the cpprestsdk library, also known as Casablanca, for making HTTP requests. Make sure you have the cpprestsdk library installed on your system and included in your project.

Writing the Web Scraping Code

#include <cpprest/http_client.h>
#include <cpprest/json.h>
#include <cpprest/uri.h>

#include <iostream>
#include <experimental/coroutine>

using namespace web;
using namespace web::http;
using namespace web::http::client;

// Asynchronous task for making an HTTP request
pplx::task<http_response> make_request(const std::string& url)
{
    http_client client(utility::conversions::to_string_t(url));
    return client.request(methods::GET);
}

// Coroutine for web scraping
class WebScraper
{
public:
    WebScraper(const std::string& url) : m_url(url) {}

    struct awaitable
    {
        awaitable(const std::string& url) : m_url(url) {}

        bool await_ready() const noexcept
        {
            return false;
        }

        void await_suspend(std::experimental::coroutine_handle<> handle) const
        {
            m_handle = handle;

            // Start the asynchronous task for making an HTTP request
            m_task = make_request(m_url).then([this](http_response response) {
                if (response.status_code() == status_codes::OK)
                {
                    return response.extract_string();
                }

                return pplx::task_from_result(std::string());
            }).then([this](pplx::task<std::string> task) {
                try
                {
                    m_result = task.get();
                }
                catch (const std::exception& ex)
                {
                    std::cerr << "Error: " << ex.what() << std::endl;
                }

                m_handle.resume();
            });
        }

        std::string await_resume() const
        {
            return m_result;
        }

    private:
        const std::string& m_url;
        mutable std::string m_result;
        mutable std::experimental::coroutine_handle<> m_handle;
        mutable pplx::task<void> m_task;
    };

    awaitable scrape()
    {
        co_return co_await awaitable(m_url);
    }

private:
    std::string m_url;
};

Here, we define a make_request function that returns a task representing an asynchronous HTTP request. We also define a WebScraper class with a scrape method that acts as our coroutine for web scraping.

Using the Web Scraper

To use our web scraper, we can create an instance of the WebScraper class and call the scrape method. Here’s an example:

int main()
{
    std::string url = "https://example.com";

    WebScraper scraper(url);
    auto result = scraper.scrape();

    std::cout << "Scraped data: " << result.await_resume() << std::endl;

    return 0;
}

In the example above, we create a WebScraper instance with the URL of the website we want to scrape. We then call the scrape method and use the await_resume function to retrieve the scraped data.

Conclusion

C++ coroutines provide a powerful tool for writing asynchronous code in a more sequential and readable manner. By utilizing coroutines for web scraping, we can simplify our code and make it easier to understand. In this blog post, we’ve seen how to use C++ coroutines along with the cpprestsdk library to perform web scraping tasks.