text: //
  // async_client.cpp
  // ~~~~~~~~~~~~~~~~
  //
  // Copyright (c) 2003-2015 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 <iostream>
  #include <istream>
  #include <ostream>
  #include <string>
  #include <boost/asio.hpp>
  #include <boost/bind.hpp>
  
  using boost::asio::ip::tcp;
  
  class client
  {
  public:
    client(boost::asio::io_service& io_service,
        const std::string& server, const std::string& path)
      : resolver_(io_service),
        socket_(io_service)
    {
      // Form the request. We specify the "Connection: close" header so that the
      // server will close the socket after transmitting the response. This will
      // allow us to treat all data up until the EOF as the content.
      std::ostream request_stream(&request_);
      request_stream << "GET " << path << " HTTP/1.0\r\n";
      request_stream << "Host: " << server << "\r\n";
      request_stream << "Accept: */*\r\n";
      request_stream << "Connection: close\r\n\r\n";
  
      // Start an asynchronous resolve to translate the server and service names
      // into a list of endpoints.
      tcp::resolver::query query(server, "http");
      resolver_.async_resolve(query,
          boost::bind(&client::handle_resolve, this,
            boost::asio::placeholders::error,
            boost::asio::placeholders::iterator));
    }
  
  private:
    void handle_resolve(const boost::system::error_code& err,
        tcp::resolver::iterator endpoint_iterator)
    {
      if (!err)
      {
        // Attempt a connection to each endpoint in the list until we
        // successfully establish a connection.
        boost::asio::async_connect(socket_, endpoint_iterator,
            boost::bind(&client::handle_connect, this,
              boost::asio::placeholders::error));
      }
      else
      {
        std::cout << "Error: " << err.message() << "\n";
      }
    }
  
    void handle_connect(const boost::system::error_code& err)
    {
      if (!err)
      {
        // The connection was successful. Send the request.
        boost::asio::async_write(socket_, request_,
            boost::bind(&client::handle_write_request, this,
              boost::asio::placeholders::error));
      }
      else
      {
        std::cout << "Error: " << err.message() << "\n";
      }
    }
  
    void handle_write_request(const boost::system::error_code& err)
    {
      if (!err)
      {
        // Read the response status line. The response_ streambuf will
        // automatically grow to accommodate the entire line. The growth may be
        // limited by passing a maximum size to the streambuf constructor.
        boost::asio::async_read_until(socket_, response_, "\r\n",
            boost::bind(&client::handle_read_status_line, this,
              boost::asio::placeholders::error));
      }
      else
      {
        std::cout << "Error: " << err.message() << "\n";
      }
    }
  
    void handle_read_status_line(const boost::system::error_code& err)
    {
      if (!err)
      {
        // Check that response is OK.
        std::istream response_stream(&response_);
        std::string http_version;
        response_stream >> http_version;
        unsigned int status_code;
        response_stream >> status_code;
        std::string status_message;
        std::getline(response_stream, status_message);
        if (!response_stream || http_version.substr(0, 5) != "HTTP/")
        {
          std::cout << "Invalid response\n";
          return;
        }
        if (status_code != 200)
        {
          std::cout << "Response returned with status code ";
          std::cout << status_code << "\n";
          return;
        }
  
        // Read the response headers, which are terminated by a blank line.
        boost::asio::async_read_until(socket_, response_, "\r\n\r\n",
            boost::bind(&client::handle_read_headers, this,
              boost::asio::placeholders::error));
      }
      else
      {
        std::cout << "Error: " << err << "\n";
      }
    }
  
    void handle_read_headers(const boost::system::error_code& err)
    {
      if (!err)
      {
        // Process the response headers.
        std::istream response_stream(&response_);
        std::string header;
        while (std::getline(response_stream, header) && header != "\r")
          std::cout << header << "\n";
        std::cout << "\n";
  
        // Write whatever content we already have to output.
        if (response_.size() > 0)
          std::cout << &response_;
  
        // Start reading remaining data until EOF.
        boost::asio::async_read(socket_, response_,
            boost::asio::transfer_at_least(1),
            boost::bind(&client::handle_read_content, this,
              boost::asio::placeholders::error));
      }
      else
      {
        std::cout << "Error: " << err << "\n";
      }
    }
  
    void handle_read_content(const boost::system::error_code& err)
    {
      if (!err)
      {
        // Write all of the data that has been read so far.
        std::cout << &response_;
  
        // Continue reading remaining data until EOF.
        boost::asio::async_read(socket_, response_,
            boost::asio::transfer_at_least(1),
            boost::bind(&client::handle_read_content, this,
              boost::asio::placeholders::error));
      }
      else if (err != boost::asio::error::eof)
      {
        std::cout << "Error: " << err << "\n";
      }
    }
  
    tcp::resolver resolver_;
    tcp::socket socket_;
    boost::asio::streambuf request_;
    boost::asio::streambuf response_;
  };
  
  int main(int argc, char* argv[])
  {
    try
    {
      if (argc != 3)
      {
        std::cout << "Usage: async_client <server> <path>\n";
        std::cout << "Example:\n";
        std::cout << "  async_client www.boost.org /LICENSE_1_0.txt\n";
        return 1;
      }
  
      boost::asio::io_service io_service;
      client c(io_service, argv[1], argv[2]);
      io_service.run();
    }
    catch (std::exception& e)
    {
      std::cout << "Exception: " << e.what() << "\n";
    }
  
    return 0;
  }
  