sock.connect(ip::tcp::endpoint(ip::address::from_string("127.0.0.1"),6688));	// client

tcp::acceptor acptr(io, tcp::endpoint(tcp::v4(), 6688));	//server

int client_end_point() {
    // Step 1. Assume that the client application has already 
    // obtained the IP-address and the protocol port number.
    std::string raw_ip_address = "127.0.0.1";
    unsigned short port_num = 3333;
    // Used to store information about error that happens
    // while parsing the raw IP-address.
    boost::system::error_code ec;
    // Step 2. Using IP protocol version independent address
    // representation.
    asio::ip::address ip_address =
        asio::ip::address::from_string(raw_ip_address, ec);
    if (ec.value() != 0) {
        // Provided IP address is invalid. Breaking execution.
        std::cout
            << "Failed to parse the IP address. Error code = "
            << ec.value() << ". Message: " << ec.message();
        return ec.value();
    }
    // Step 3.
    asio::ip::tcp::endpoint ep(ip_address, port_num);
    // Step 4. The endpoint is ready and can be used to specify a 
    // particular server in the network the client wants to 
    // communicate with.
    return 0;
}

int server_end_point(){
    // Step 1. Here we assume that the server application has
    //already obtained the protocol port number.
    unsigned short port_num = 3333;
    // Step 2. Create special object of asio::ip::address class
    // that specifies all IP-addresses available on the host. Note
    // that here we assume that server works over IPv4 protocol.
    asio::ip::address ip_address = asio::ip::address_v4::any(); 	//asio::ip::address_v6::any()也可
    // Step 3.
    asio::ip::tcp::endpoint ep(ip_address, port_num);
    // Step 4. The endpoint is created and can be used to 
    // specify the IP addresses and a port number on which 
    // the server application wants to listen for incoming 
    // connections.
    return 0;
}

int  connect_to_end() {
    // Step 1. Assume that the client application has already
            // obtained the IP address and protocol port number of the
            // target server.
    std::string raw_ip_address = "127.0.0.1";
    unsigned short port_num = 3333;
    try {
        // Step 2. Creating an endpoint designating 
        // a target server application.
        asio::ip::tcp::endpoint
            ep(asio::ip::address::from_string(raw_ip_address),
                port_num);
        asio::io_context ios;
        
        // Step 3. Creating and opening a socket.
        asio::ip::tcp::socket sock(ios, ep.protocol());
        
        // Step 4. Connecting a socket.
        sock.connect(ep);
    }
    // Overloads of asio::ip::address::from_string() and 
    // asio::ip::tcp::socket::connect() used here throw
    // exceptions in case of error condition.
    catch (system::system_error& e) {
        std::cout << "Error occured! Error code = " << e.code()
            << ". Message: " << e.what();
        return e.code().value();
    }
}

int accept_new_connection(){
    // The size of the queue containing the pending connection
            // requests.
    const int BACKLOG_SIZE = 30;		// tcp缓存的连接数
    // Step 1. Here we assume that the server application has
    // already obtained the protocol port number.
    unsigned short port_num = 3333;
    // Step 2. Creating a server endpoint.
    asio::ip::tcp::endpoint ep(asio::ip::address_v4::any(),
        port_num);
    asio::io_context  ios;
    try {
        // Step 3. Instantiating and opening an acceptor socket.
        asio::ip::tcp::acceptor acceptor(ios, ep.protocol());	// 创建的同时open了，相当于
        														// asio::ip::tcp::acceptor acceptor(ios);
        														// acceptor.open(ep.protocol(), ec);
        
        // Step 4. Binding the acceptor socket to the 
        // server endpint.
        acceptor.bind(ep);
        
        // Step 5. Starting to listen for incoming connection
        // requests.
        acceptor.listen(BACKLOG_SIZE);
        
        // Step 6. Creating an active socket.
        asio::ip::tcp::socket sock(ios);
        
        // Step 7. Processing the next connection request and 
        // connecting the active socket to the client.
        acceptor.accept(sock);
    }
    catch (system::system_error& e) {
        std::cout << "Error occured! Error code = " << e.code()
            << ". Message: " << e.what();
        return e.code().value();
    }
}

int dns_connect_to_end()
{
    std::string host = "example.com";
    std::string port_num = 3333;
    asio::io_context ioc;
    asio::ip::tcp::resolver::query rq(host, port_num, asio::ip::tcp::resolver::query::numeric_service);
    asio::ip::tcp::resolver r(ioc);
    
    try
    {
        asio::ip::tcp::resolver::iterator it = r.resolve(rq);
        asio::ip::tcp::socket s(ioc);
        asio::connect(s, it);
    }
    catch(system::system_error& e)
    {
        std::cout << "Error occured!Error Code=" << e.code() << ".Message:" << e.what();
        return e.code().value();
    }
}

template<typename ConstBufferSequence>
std::size_t send(const ConstBufferSequence & buffers);

void use_const_buffer() {
    std::string buf = "hello world!";
    asio::const_buffer  asio_buf(buf.c_str(), buf.length());
    std::vector<asio::const_buffer> buffers_sequence;
    buffers_sequence.push_back(asio_buf);
}

void use_buffer_str() {
    asio::const_buffers_1 output_buf = asio::buffer("hello world");
}

void use_buffer_array(){
    const size_t  BUF_SIZE_BYTES = 20;
    std::unique_ptr<char[] > buf(new char[BUF_SIZE_BYTES]);
    auto input_buf = asio::buffer(static_cast<void*>(buf.get()), BUF_SIZE_BYTES);
}

void use_stream_buffer() {
    asio::streambuf buf;
    std::ostream output(&buf);
    // Writing the message to the stream-based buffer.
    output << "Message1\nMessage2";
    
    // Now we want to read all data from a streambuf
    // until '\n' delimiter.
    // Instantiate an input stream which uses our 
    // stream buffer.
    std::istream input(&buf);
    
    // We'll read data into this string.
    std::string message1;
    std::getline(input, message1);
    // Now message1 string contains 'Message1'.
}