C++中的异步网络编程是什么？

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

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

class session : public std::enable_shared_from_this<session> {
public:
    session(tcp::socket socket) : socket_(std::move(socket)) {}

    void start() {
        do_read();
    }

private:
    void do_read() {
        auto self(shared_from_this());
        socket_.async_read_some(boost::asio::buffer(data_, max_length),
            [this, self](boost::system::error_code ec, std::size_t length) {
                if (!ec) {
                    do_write(length);
                }
            });
    }

    void do_write(std::size_t length) {
        auto self(shared_from_this());
        boost::asio::async_write(socket_, boost::asio::buffer(data_, length),
            [this, self](boost::system::error_code ec, std::size_t /*length*/) {
                if (!ec) {
                    do_read();
                }
            });
    }

    tcp::socket socket_;
    enum { max_length = 1024 };
    char data_[max_length];
};

class server {
public:
    server(boost::asio::io_context& io_context, short port)
        : acceptor_(io_context, tcp::endpoint(tcp::v4(), port)) {
        do_accept();
    }

private:
    void do_accept() {
        acceptor_.async_accept(
            [this](boost::system::error_code ec, tcp::socket socket) {
                if (!ec) {
                    std::make_shared<session>(std::move(socket))->start();
                }

                do_accept();
            });
    }

    tcp::acceptor acceptor_;
};

int main() {
    try {
        boost::asio::io_context io_context;
        server s(io_context, 8080);
        io_context.run();
    }
    catch (std::exception& e) {
        std::cerr << "Exception: " << e.what() << "\n";
    }

    return 0;
}

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

using boost::asio::ip::tcp;
using boost::asio::awaitable;
using boost::asio::co_spawn;
using boost::asio::detached;
using boost::asio::use_awaitable;

awaitable<void> echo(tcp::socket socket) {
    char data[1024];
    for (;;) {
        std::size_t n = co_await socket.async_read_some(boost::asio::buffer(data), use_awaitable);
        co_await boost::asio::async_write(socket, boost::asio::buffer(data, n), use_awaitable);
    }
}

awaitable<void> listener() {
    auto executor = co_await boost::asio::this_coro::executor;
    tcp::acceptor acceptor(executor, {tcp::v4(), 8080});
    for (;;) {
        tcp::socket socket = co_await acceptor.async_accept(use_awaitable);
        co_spawn(executor, echo(std::move(socket)), detached);
    }
}

int main() {
    try {
        boost::asio::io_context io_context(1);
        co_spawn(io_context, listener(), detached);
        io_context.run();
    }
    catch (std::exception& e) {
        std::cerr << "Exception: " << e.what() << "\n";
    }
    return 0;
}