text: //
  // async_tcp_client.cpp
  // ~~~~~~~~~~~~~~~~~~~~
  //
  // Copyright (c) 2003-2017 Christopher M. Kohlhoff (chris at kohlhoff dot com)
  //
  // Distributed under the Boost Software License, Version 1.0. (See accompanying
  // file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
  //
  
  #include <boost/asio/deadline_timer.hpp>
  #include <boost/asio/io_context.hpp>
  #include <boost/asio/ip/tcp.hpp>
  #include <boost/asio/read_until.hpp>
  #include <boost/asio/streambuf.hpp>
  #include <boost/asio/write.hpp>
  #include <boost/bind.hpp>
  #include <iostream>
  
  using boost::asio::deadline_timer;
  using boost::asio::ip::tcp;
  
  //
  // This class manages socket timeouts by applying the concept of a deadline.
  // Some asynchronous operations are given deadlines by which they must complete.
  // Deadlines are enforced by an "actor" that persists for the lifetime of the
  // client object:
  //
  //  +----------------+
  //  |                |
  //  | check_deadline |<---+
  //  |                |    |
  //  +----------------+    | async_wait()
  //              |         |
  //              +---------+
  //
  // If the deadline actor determines that the deadline has expired, the socket
  // is closed and any outstanding operations are consequently cancelled.
  //
  // Connection establishment involves trying each endpoint in turn until a
  // connection is successful, or the available endpoints are exhausted. If the
  // deadline actor closes the socket, the connect actor is woken up and moves to
  // the next endpoint.
  //
  //  +---------------+
  //  |               |
  //  | start_connect |<---+
  //  |               |    |
  //  +---------------+    |
  //           |           |
  //  async_-  |    +----------------+
  // connect() |    |                |
  //           +--->| handle_connect |
  //                |                |
  //                +----------------+
  //                          :
  // Once a connection is     :
  // made, the connect        :
  // actor forks in two -     :
  //                          :
  // an actor for reading     :       and an actor for
  // inbound messages:        :       sending heartbeats:
  //                          :
  //  +------------+          :          +-------------+
  //  |            |<- - - - -+- - - - ->|             |
  //  | start_read |                     | start_write |<---+
  //  |            |<---+                |             |    |
  //  +------------+    |                +-------------+    | async_wait()
  //          |         |                        |          |
  //  async_- |    +-------------+       async_- |    +--------------+
  //   read_- |    |             |       write() |    |              |
  //  until() +--->| handle_read |               +--->| handle_write |
  //               |             |                    |              |
  //               +-------------+                    +--------------+
  //
  // The input actor reads messages from the socket, where messages are delimited
  // by the newline character. The deadline for a complete message is 30 seconds.
  //
  // The heartbeat actor sends a heartbeat (a message that consists of a single
  // newline character) every 10 seconds. In this example, no deadline is applied
  // message sending.
  //
  class client
  {
  public:
    client(boost::asio::io_context& io_context)
      : stopped_(false),
        socket_(io_context),
        deadline_(io_context),
        heartbeat_timer_(io_context)
    {
    }
  
    // Called by the user of the client class to initiate the connection process.
    // The endpoints will have been obtained using a tcp::resolver.
    void start(tcp::resolver::results_type endpoints)
    {
      // Start the connect actor.
      endpoints_ = endpoints;
      start_connect(endpoints_.begin());
  
      // Start the deadline actor. You will note that we're not setting any
      // particular deadline here. Instead, the connect and input actors will
      // update the deadline prior to each asynchronous operation.
      deadline_.async_wait(boost::bind(&client::check_deadline, this));
    }
  
    // This function terminates all the actors to shut down the connection. It
    // may be called by the user of the client class, or by the class itself in
    // response to graceful termination or an unrecoverable error.
    void stop()
    {
      stopped_ = true;
      boost::system::error_code ignored_ec;
      socket_.close(ignored_ec);
      deadline_.cancel();
      heartbeat_timer_.cancel();
    }
  
  private:
    void start_connect(tcp::resolver::results_type::iterator endpoint_iter)
    {
      if (endpoint_iter != endpoints_.end())
      {
        std::cout << "Trying " << endpoint_iter->endpoint() << "...\n";
  
        // Set a deadline for the connect operation.
        deadline_.expires_from_now(boost::posix_time::seconds(60));
  
        // Start the asynchronous connect operation.
        socket_.async_connect(endpoint_iter->endpoint(),
            boost::bind(&client::handle_connect,
              this, _1, endpoint_iter));
      }
      else
      {
        // There are no more endpoints to try. Shut down the client.
        stop();
      }
    }
  
    void handle_connect(const boost::system::error_code& ec,
        tcp::resolver::results_type::iterator endpoint_iter)
    {
      if (stopped_)
        return;
  
      // The async_connect() function automatically opens the socket at the start
      // of the asynchronous operation. If the socket is closed at this time then
      // the timeout handler must have run first.
      if (!socket_.is_open())
      {
        std::cout << "Connect timed out\n";
  
        // Try the next available endpoint.
        start_connect(++endpoint_iter);
      }
  
      // Check if the connect operation failed before the deadline expired.
      else if (ec)
      {
        std::cout << "Connect error: " << ec.message() << "\n";
  
        // We need to close the socket used in the previous connection attempt
        // before starting a new one.
        socket_.close();
  
        // Try the next available endpoint.
        start_connect(++endpoint_iter);
      }
  
      // Otherwise we have successfully established a connection.
      else
      {
        std::cout << "Connected to " << endpoint_iter->endpoint() << "\n";
  
        // Start the input actor.
        start_read();
  
        // Start the heartbeat actor.
        start_write();
      }
    }
  
    void start_read()
    {
      // Set a deadline for the read operation.
      deadline_.expires_from_now(boost::posix_time::seconds(30));
  
      // Start an asynchronous operation to read a newline-delimited message.
      boost::asio::async_read_until(socket_, input_buffer_, '\n',
          boost::bind(&client::handle_read, this, _1));
    }
  
    void handle_read(const boost::system::error_code& ec)
    {
      if (stopped_)
        return;
  
      if (!ec)
      {
        // Extract the newline-delimited message from the buffer.
        std::string line;
        std::istream is(&input_buffer_);
        std::getline(is, line);
  
        // Empty messages are heartbeats and so ignored.
        if (!line.empty())
        {
          std::cout << "Received: " << line << "\n";
        }
  
        start_read();
      }
      else
      {
        std::cout << "Error on receive: " << ec.message() << "\n";
  
        stop();
      }
    }
  
    void start_write()
    {
      if (stopped_)
        return;
  
      // Start an asynchronous operation to send a heartbeat message.
      boost::asio::async_write(socket_, boost::asio::buffer("\n", 1),
          boost::bind(&client::handle_write, this, _1));
    }
  
    void handle_write(const boost::system::error_code& ec)
    {
      if (stopped_)
        return;
  
      if (!ec)
      {
        // Wait 10 seconds before sending the next heartbeat.
        heartbeat_timer_.expires_from_now(boost::posix_time::seconds(10));
        heartbeat_timer_.async_wait(boost::bind(&client::start_write, this));
      }
      else
      {
        std::cout << "Error on heartbeat: " << ec.message() << "\n";
  
        stop();
      }
    }
  
    void check_deadline()
    {
      if (stopped_)
        return;
  
      // Check whether the deadline has passed. We compare the deadline against
      // the current time since a new asynchronous operation may have moved the
      // deadline before this actor had a chance to run.
      if (deadline_.expires_at() <= deadline_timer::traits_type::now())
      {
        // The deadline has passed. The socket is closed so that any outstanding
        // asynchronous operations are cancelled.
        socket_.close();
  
        // There is no longer an active deadline. The expiry is set to positive
        // infinity so that the actor takes no action until a new deadline is set.
        deadline_.expires_at(boost::posix_time::pos_infin);
      }
  
      // Put the actor back to sleep.
      deadline_.async_wait(boost::bind(&client::check_deadline, this));
    }
  
  private:
    bool stopped_;
    tcp::resolver::results_type endpoints_;
    tcp::socket socket_;
    boost::asio::streambuf input_buffer_;
    deadline_timer deadline_;
    deadline_timer heartbeat_timer_;
  };
  
  int main(int argc, char* argv[])
  {
    try
    {
      if (argc != 3)
      {
        std::cerr << "Usage: client <host> <port>\n";
        return 1;
      }
  
      boost::asio::io_context io_context;
      tcp::resolver r(io_context);
      client c(io_context);
  
      c.start(r.resolve(argv[1], argv[2]));
  
      io_context.run();
    }
    catch (std::exception& e)
    {
      std::cerr << "Exception: " << e.what() << "\n";
    }
  
    return 0;
  }
  