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# http/client/async_client.cpp

    #include <iostream>
    #include <istream>
    #include <ostream>
    #include <string>
    #include <boost/asio.hpp>
    #include <boost/bind.hpp>
    #include <boost/regex.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::asio::error& err,
          tcp::resolver::iterator endpoint_iterator)
      {
        if (!err)
        {
          // Attempt a connection to the first endpoint in the list. Each endpoint
          // will be tried until we successfully establish a connection.
          tcp::endpoint endpoint = *endpoint_iterator;
          socket_.async_connect(endpoint,
              boost::bind(&client::handle_connect, this,
                boost::asio::placeholders::error, ++endpoint_iterator));
        }
        else
        {
          std::cout << "Error: " << err << "\n";
        }
      }

      void handle_connect(const boost::asio::error& err,
          tcp::resolver::iterator endpoint_iterator)
      {
        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 if (endpoint_iterator != tcp::resolver::iterator())
        {
          // The connection failed. Try the next endpoint in the list.
          socket_.close();
          tcp::endpoint endpoint = *endpoint_iterator;
          socket_.async_connect(endpoint,
              boost::bind(&client::handle_connect, this,
                boost::asio::placeholders::error, ++endpoint_iterator));
        }
        else
        {
          std::cout << "Error: " << err << "\n";
        }
      }

      void handle_write_request(const boost::asio::error& err)
      {
        if (!err)
        {
          // Read the response status line.
          boost::asio::async_read_until(socket_, response_, boost::regex("\r\n"),
              boost::bind(&client::handle_read_status_line, this,
                boost::asio::placeholders::error));
        }
        else
        {
          std::cout << "Error: " << err << "\n";
        }
      }

      void handle_read_status_line(const boost::asio::error& 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_, boost::regex("\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::asio::error& 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::asio::error& 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;
    }

| Copyright © 2003 - 2006 Christopher M. Kohlhoff