HTTP Client in C++: Making Requests the Right Way

Introduction: Why Write an HTTP Client in C++?

In modern development, interacting with remote servers is one of the key tasks. Whether it's fetching data from a REST API, sending a form to a server, or downloading a file, the HTTP protocol is used everywhere. Despite Python and JavaScript offering simple built-in solutions, C++ remains in demand for writing high-performance HTTP clients. This is necessary for embedded systems, game engines, high-load backends, or simply for learning low-level network mechanisms.

In this article, we will examine two popular approaches to implementing an HTTP client in C++: using the libcurl library (cURL) and a more modern asynchronous approach with Boost.Beast. You will learn how to properly form GET and POST requests, process responses, and avoid common mistakes.

1. Classic Approach: HTTP Client in C++ with libcurl

The libcurl library is a time-tested tool for transferring data over various protocols. It supports HTTP/1.1, HTTP/2, HTTPS, authentication, cookies, and much more. For C++, there is a convenient wrapper curl/curl.h that allows you to easily integrate cURL into your project.

Installing libcurl

On most Linux distributions, cURL is already installed. For development, you will need the developer package:

sudo apt-get install libcurl4-openssl-dev  # Ubuntu/Debianbrew install curl                          # macOS

Example: GET Request

Let's write a simple HTTP client in C++ that fetches data from a public API (e.g., JSONPlaceholder).

#include <iostream>#include <string>#include <curl/curl.h>

// Callback for writing data to std::stringstatic size_t WriteCallback(void *contents, size_t size, size_t nmemb, void *userp) {    ((std::string*)userp)->append((char*)contents, size * nmemb);    return size * nmemb;}

int main() {    CURL *curl;    CURLcode res;    std::string readBuffer;

    curl_global_init(CURL_GLOBAL_DEFAULT);    curl = curl_easy_init();

    if(curl) {        curl_easy_setopt(curl, CURLOPT_URL, "https://jsonplaceholder.typicode.com/posts/1");        curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, WriteCallback);        curl_easy_setopt(curl, CURLOPT_WRITEDATA, &readBuffer);                // Execute the request        res = curl_easy_perform(curl);                if(res != CURLE_OK) {            std::cerr << "curl_easy_perform() failed: " << curl_easy_strerror(res) << std::endl;        } else {            std::cout << "Server response:" << readBuffer << std::endl;        }                curl_easy_cleanup(curl);    }        curl_global_cleanup();    return 0;}

Example: POST Request with JSON

Now let's send data to the server. To do this, you need to specify the content type and the request body.

#include <iostream>#include <string>#include <curl/curl.h>

int main() {    CURL *curl;    CURLcode res;    std::string response;        // JSON data to send    std::string jsonData = R"({"title": "foo", "body": "bar", "userId": 1})";        curl_global_init(CURL_GLOBAL_DEFAULT);    curl = curl_easy_init();        if(curl) {        curl_easy_setopt(curl, CURLOPT_URL, "https://jsonplaceholder.typicode.com/posts");        curl_easy_setopt(curl, CURLOPT_POSTFIELDS, jsonData.c_str());                // Set headers        struct curl_slist *headers = NULL;        headers = curl_slist_append(headers, "Content-Type: application/json");        curl_easy_setopt(curl, CURLOPT_HTTPHEADER, headers);                // Callback for writing the response        curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, [](void *contents, size_t size, size_t nmemb, void *userp) -> siz