写一个C++异步回调案例

在C++中，我们可以使用各种异步编程模型，例如Boost.Asio、C++17的std::async和std::future等。这里以Boost.Asio库为例，展示一个简单的异步网络读取回调案例：

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

using boost::asio::ip::tcp;

void on_read(const boost::system::error_code& error, size_t bytes_transferred,
             std::string data) {
    if (!error) {
        std::cout << "Received " << bytes_transferred << " bytes of data: " << data.substr(0, bytes_transferred) << '\n';
    } else {
        std::cerr << "Error reading from the socket: " << error.message() << '\n';
    }
}

int main() {
    try {
        boost::asio::io_context io_context;

        tcp::resolver resolver(io_context);
        tcp::resolver::results_type endpoints = resolver.resolve("www.example.com", "http");

        tcp::socket socket(io_context);
        for (auto const& endpoint : endpoints) {
            boost::asio::connect(socket, endpoint);
            break; // Assume only one endpoint will be used.
        }

        boost::asio::streambuf receive_buffer;
        boost::asio::read(socket, receive_buffer, boost::asio::transfer_all());

        auto read_result = boost::asio::buffer_cast<const char*>(receive_buffer.data());
        size_t length = receive_buffer.size();
        std::string received_data(read_result, read_result + length);

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

        socket.async_read_some(boost::asio::dynamic_buffer(receive_buffer),
                              on_read);  // 异步读取数据

        io_context.run();  // 主循环，等待所有操作完成
    }
    catch (const std::exception& e) {
        std::cerr << "Exception caught: " << e.what() << '\n';
    }

    return 0;
}

这个程序首先解析域名到IP地址，连接到服务器，然后发送一个HTTP GET请求。当接收到响应的一部分时，它会调用`on_read`回调函数，将接收到的数据传递给它。`main`函数通过`io_context.run()`保持运行，直到所有的I/O操作都完成。