text: //
  // Copyright (c) 2016-2019 Vinnie Falco (vinnie dot falco at gmail 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)
  //
  // Official repository: https://github.com/boostorg/beast
  //
  
  //------------------------------------------------------------------------------
  //
  // Example: HTTP SSL client, coroutine
  //
  //------------------------------------------------------------------------------
  
  #include "example/common/root_certificates.hpp"
  
  #include <boost/beast/core.hpp>
  #include <boost/beast/http.hpp>
  #include <boost/beast/ssl.hpp>
  #include <boost/beast/version.hpp>
  #include <boost/asio/spawn.hpp>
  #include <cstdlib>
  #include <functional>
  #include <iostream>
  #include <string>
  
  namespace beast = boost::beast;         // from <boost/beast.hpp>
  namespace http = beast::http;           // from <boost/beast/http.hpp>
  namespace net = boost::asio;            // from <boost/asio.hpp>
  namespace ssl = boost::asio::ssl;       // from <boost/asio/ssl.hpp>
  using tcp = boost::asio::ip::tcp;       // from <boost/asio/ip/tcp.hpp>
  
  //------------------------------------------------------------------------------
  
  // Report a failure
  void
  fail(beast::error_code ec, char const* what)
  {
      std::cerr << what << ": " << ec.message() << "\n";
  }
  
  // Performs an HTTP GET and prints the response
  void
  do_session(
      std::string const& host,
      std::string const& port,
      std::string const& target,
      int version,
      net::io_context& ioc,
      ssl::context& ctx,
      net::yield_context yield)
  {
      beast::error_code ec;
  
      // These objects perform our I/O
      tcp::resolver resolver(ioc);
      beast::ssl_stream<beast::tcp_stream> stream(ioc, ctx);
  
      // Set SNI Hostname (many hosts need this to handshake successfully)
      if(! SSL_set_tlsext_host_name(stream.native_handle(), host.c_str()))
      {
          ec.assign(static_cast<int>(::ERR_get_error()), net::error::get_ssl_category());
          std::cerr << ec.message() << "\n";
          return;
      }
  
      // Look up the domain name
      auto const results = resolver.async_resolve(host, port, yield[ec]);
      if(ec)
          return fail(ec, "resolve");
  
      // Set the timeout.
      beast::get_lowest_layer(stream).expires_after(std::chrono::seconds(30));
  
      // Make the connection on the IP address we get from a lookup
      get_lowest_layer(stream).async_connect(results, yield[ec]);
      if(ec)
          return fail(ec, "connect");
  
      // Set the timeout.
      beast::get_lowest_layer(stream).expires_after(std::chrono::seconds(30));
  
      // Perform the SSL handshake
      stream.async_handshake(ssl::stream_base::client, yield[ec]);
      if(ec)
          return fail(ec, "handshake");
  
      // Set up an HTTP GET request message
      http::request<http::string_body> req{http::verb::get, target, version};
      req.set(http::field::host, host);
      req.set(http::field::user_agent, BOOST_BEAST_VERSION_STRING);
  
      // Set the timeout.
      beast::get_lowest_layer(stream).expires_after(std::chrono::seconds(30));
  
      // Send the HTTP request to the remote host
      http::async_write(stream, req, yield[ec]);
      if(ec)
          return fail(ec, "write");
  
      // This buffer is used for reading and must be persisted
      beast::flat_buffer b;
  
      // Declare a container to hold the response
      http::response<http::dynamic_body> res;
  
      // Receive the HTTP response
      http::async_read(stream, b, res, yield[ec]);
      if(ec)
          return fail(ec, "read");
  
      // Write the message to standard out
      std::cout << res << std::endl;
  
      // Set the timeout.
      beast::get_lowest_layer(stream).expires_after(std::chrono::seconds(30));
  
      // Gracefully close the stream
      stream.async_shutdown(yield[ec]);
      if(ec == net::error::eof)
      {
          // Rationale:
          // http://stackoverflow.com/questions/25587403/boost-asio-ssl-async-shutdown-always-finishes-with-an-error
          ec = {};
      }
      if(ec)
          return fail(ec, "shutdown");
  
      // If we get here then the connection is closed gracefully
  }
  
  //------------------------------------------------------------------------------
  
  int main(int argc, char** argv)
  {
      // Check command line arguments.
      if(argc != 4 && argc != 5)
      {
          std::cerr <<
              "Usage: http-client-coro-ssl <host> <port> <target> [<HTTP version: 1.0 or 1.1(default)>]\n" <<
              "Example:\n" <<
              "    http-client-coro-ssl www.example.com 443 /\n" <<
              "    http-client-coro-ssl www.example.com 443 / 1.0\n";
          return EXIT_FAILURE;
      }
      auto const host = argv[1];
      auto const port = argv[2];
      auto const target = argv[3];
      int version = argc == 5 && !std::strcmp("1.0", argv[4]) ? 10 : 11;
  
      // The io_context is required for all I/O
      net::io_context ioc;
  
      // The SSL context is required, and holds certificates
      ssl::context ctx{ssl::context::sslv23_client};
  
      // This holds the root certificate used for verification
      load_root_certificates(ctx);
      
      // Verify the remote server's certificate
      ctx.set_verify_mode(ssl::verify_peer);
  
      // Launch the asynchronous operation
      net::spawn(ioc, std::bind(
          &do_session,
          std::string(host),
          std::string(port),
          std::string(target),
          version,
          std::ref(ioc),
          std::ref(ctx),
          std::placeholders::_1));
  
      // Run the I/O service. The call will return when
      // the get operation is complete.
      ioc.run();
  
      return EXIT_SUCCESS;
  }
  