何时在asio中使用异步操作

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

using namespace boost::asio;

void handle_read(const boost::system::error_code& err, std::size_t bytes_transferred) {
    if (!err) {
        std::cout << "Received "<< bytes_transferred << " bytes"<< std::endl;
    } else {
        std::cout << "Error: "<< err.message()<< std::endl;
    }
}

int main() {
    io_service ios;
    ip::tcp::socket sock(ios);
    ip::tcp::endpoint ep(ip::address::from_string("127.0.0.1"), 8080);

    sock.async_connect(ep, boost::bind(handle_read, boost::asio::placeholders::error, boost::asio::placeholders::bytes_transferred));

    ios.run();

    return 0;
}