Class async_write_message_initiation

example/cpp11/operations/composed_2.cpp:46–86

In addition to determining the mechanism by which an asynchronous operation delivers its result, a completion token also determines the time when the operation commences. For example, when the completion token is a simple callback the operation commences before the initiating function returns. However, if the completion token's delivery mechanism uses a future, we might instead want to defer initi

// To enable this, when implementing an asynchronous operation we must package
// the initiation step as a function object.
struct async_write_message_initiation
{
  // The initiation function object's call operator is passed the concrete
  // completion handler produced by the completion token. This completion
  // handler matches the asynchronous operation's completion handler signature,
  // which in this example is:
  //
  //   void(boost::system::error_code error, std::size_t)
  //
  // The initiation function object also receives any additional arguments
  // required to start the operation. (Note: We could have instead passed these
  // arguments as members in the initiaton function object. However, we should
  // prefer to propagate them as function call arguments as this allows the
  // completion token to optimise how they are passed. For example, a lazy
  // future which defers initiation would need to make a decay-copy of the
  // arguments, but when using a simple callback the arguments can be trivially
  // forwarded straight through.)
  template <typename CompletionHandler>
  void operator()(CompletionHandler&& completion_handler, tcp::socket& socket,
      const char* message, bool allow_partial_write) const
  {
    if (allow_partial_write)
    {
      // When delegating to an underlying operation we must take care to
      // perfectly forward the completion handler. This ensures that our
      // operation works correctly with move-only function objects as
      // callbacks.
      return socket.async_write_some(
          boost::asio::buffer(message, std::strlen(message)),
          std::forward<CompletionHandler>(completion_handler));
    }
    else
    {
      // As above, we must perfectly forward the completion handler when calling
      // the alternate underlying operation.
      return boost::asio::async_write(socket,
          boost::asio::buffer(message, std::strlen(message)),
          std::forward<CompletionHandler>(completion_handler));
    }
  }
};

template <typename CompletionToken>
auto async_write_message(tcp::socket& socket,