Web scraping is a powerful technique used to automatically extract data from websites. In this blog post, we will explore how to build web scrapers using C++ coroutines - a feature introduced in C++20 that simplifies asynchronous programming.

Why C++ Coroutines?

C++ coroutines provide an elegant solution to manage asynchronous operations, making them ideal for web scraping. They allow us to write code that looks like synchronous programming, but is actually asynchronous under the hood. Coroutines enable us to pause and resume execution, which is crucial for handling network requests and processing HTML responses.

Prerequisites

Before diving into web scraping with C++ coroutines, make sure you have the following set up:

1. A C++20-compatible compiler (such as GCC or Clang)
2. A basic understanding of C++ coroutines

Setting up the Project

To get started, let’s create a new C++ project to build our web scraper.

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

cppcoro::generator<std::string> fetchData() {
    // TODO: Implement fetching data from a website
    // and yield the results
}

int main() {
    for (const auto& data : fetchData()) {
        std::cout << data << std::endl;
    }

    return 0;
}

In the above code snippet, we define a fetchData function that will be responsible for fetching data from a website. We use the cppcoro::generator type to define a coroutine that yields strings to be processed later.

Fetching Data

To fetch data from a website, we can use networking libraries such as Boost.Asio or libcurl. In this example, let’s use libcurl to make a simple HTTP GET request.

cppcoro::generator<std::string> fetchData() {
    // Create a libcurl handle
    CURL* curl = curl_easy_init();
    if (!curl) {
        co_return;
    }

    // Set the URL to fetch
    curl_easy_setopt(curl, CURLOPT_URL, "https://example.com");

    // Set the write callback function
    curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, [](char* ptr, size_t size, size_t nmemb, std::string* data) {
        data->append(ptr, size * nmemb);
        return size * nmemb;
    });

    // Set the write data parameter
    std::string content;
    curl_easy_setopt(curl, CURLOPT_WRITEDATA, &content);

    // Perform the HTTP request
    CURLcode res = curl_easy_perform(curl);
    if (res != CURLE_OK) {
        co_return;
    }

    // Cleanup and yield the fetched data
    curl_easy_cleanup(curl);
    co_yield content;
}

In the above code snippet, we define the fetchData coroutine that uses libcurl to make an HTTP GET request to a website. We set the URL to fetch, set a callback function to handle the response, and perform the request. Finally, we clean up the curl handle and yield the fetched data using the co_yield keyword.

Processing HTML

Once we’ve fetched the HTML content, we can use a library like Gumbo or libxml2 to parse and extract the desired data from the HTML. In this example, let’s use Gumbo to extract all the text within <p> tags.

cppcoro::generator<std::string> fetchData() {
    // ...

    // Parse the fetched HTML content
    GumboOutput* output = gumbo_parse(content.c_str());

    // Extract text from <p> tags
    std::string data;
    extractParagraphText(output->root, data);
    gumbo_destroy_output(&kGumboDefaultOptions, output);
    co_yield data;
}

void extractParagraphText(GumboNode* node, std::string& data) {
    if (node->type == GUMBO_NODE_TEXT) {
        data += node->v.text.text;
    } else if (node->type == GUMBO_NODE_ELEMENT &&
        node->v.element.tag == GUMBO_TAG_P) {
        for (int i = 0; i < node->v.element.children.length; ++i) {
            extractParagraphText(static_cast<GumboNode*>(node->v.element.children.data[i]), data);
        }
    }
}

In this code snippet, we add the logic to parse the fetched HTML content using Gumbo. We define a helper function extractParagraphText to recursively traverse the HTML document and extract text from <p> tags. We then yield the extracted data using co_yield.

Putting it All Together

Our web scraper is now complete! We have defined a coroutine that fetches data from a website and extracts text from <p> tags. Here’s how we can use it:

int main() {
    for (const auto& data : fetchData()) {
        std::cout << data << std::endl;
    }

    return 0;
}

By iterating over the results of the fetchData function, we can process each chunk of data as it becomes available.

Conclusion

C++ coroutines provide a convenient way to build web scrapers by simplifying the handling of asynchronous operations. With coroutines, we can write modular and maintainable code that efficiently extracts data from websites. Give it a try and unleash the power of C++ coroutines in your web scraping projects!

#C++ #WebScraping