C++ socket::get_executor方法代码示例

  void operator()(CompletionHandler&& completion_handler,
      tcp::socket& socket, const char* message) const
  {
    // The async_write operation has a completion handler signature of:
    //
    //   void(boost::system::error_code error, std::size n)
    //
    // This differs from our operation's signature in that it is also passed
    // the number of bytes transferred as an argument of type std::size_t. We
    // will adapt our completion handler to async_write's completion handler
    // signature by using std::bind, which drops the additional argument.
    //
    // However, it is essential to the correctness of our composed operation
    // that we preserve the executor of the user-supplied completion handler.
    // The std::bind function will not do this for us, so we must do this by
    // first obtaining the completion handler's associated executor (defaulting
    // to the I/O executor - in this case the executor of the socket - if the
    // completion handler does not have its own) ...
    auto executor = boost::asio::get_associated_executor(
        completion_handler, socket.get_executor());

    // ... and then binding this executor to our adapted completion handler
    // using the boost::asio::bind_executor function.
    boost::asio::async_write(socket,
        boost::asio::buffer(message, std::strlen(message)),
        boost::asio::bind_executor(executor,
          std::bind(std::forward<CompletionHandler>(
            completion_handler), std::placeholders::_1)));
  }

auto async_write_messages(tcp::socket& socket,
    const T& message, std::size_t repeat_count,
    CompletionToken&& token)
  // The return type of the initiating function is deduced from the combination
  // of CompletionToken type and the completion handler's signature. When the
  // completion token is a simple callback, the return type is always void.
  // In this example, when the completion token is boost::asio::yield_context
  // (used for stackful coroutines) the return type would be also be void, as
  // there is no non-error argument to the completion handler. When the
  // completion token is boost::asio::use_future it would be std::future<void>.
  //
  // In C++14 we can omit the return type as it is automatically deduced from
  // the return type of boost::asio::async_initiate.
{
  // Encode the message and copy it into an allocated buffer. The buffer will
  // be maintained for the lifetime of the composed asynchronous operation.
  std::ostringstream os;
  os << message;
  std::unique_ptr<std::string> encoded_message(new std::string(os.str()));

  // Create a steady_timer to be used for the delay between messages.
  std::unique_ptr<boost::asio::steady_timer> delay_timer(
      new boost::asio::steady_timer(socket.get_executor()));

  // To manage the cycle between the multiple underlying asychronous
  // operations, our implementation is a state machine.
  enum { starting, waiting, writing };

  // The boost::asio::async_compose function takes:
  //
  // - our asynchronous operation implementation,
  // - the completion token,
  // - the completion handler signature, and
  // - any I/O objects (or executors) used by the operation
  //
  // It then wraps our implementation, which is implemented here as a state
  // machine in a lambda, in an intermediate completion handler that meets the
  // requirements of a conforming asynchronous operation. This includes
  // tracking outstanding work against the I/O executors associated with the
  // operation (in this example, this is the socket's executor).
  //
  // The first argument to our lambda is a reference to the enclosing
  // intermediate completion handler. This intermediate completion handler is
  // provided for us by the boost::asio::async_compose function, and takes care
  // of all the details required to implement a conforming asynchronous
  // operation. When calling an underlying asynchronous operation, we pass it
  // this enclosing intermediate completion handler as the completion token.
  //
  // All arguments to our lambda after the first must be defaulted to allow the
  // state machine to be started, as well as to allow the completion handler to
  // match the completion signature of both the async_write and
  // steady_timer::async_wait operations.
  return boost::asio::async_compose<
    CompletionToken, void(boost::system::error_code)>(
      [
        // The implementation holds a reference to the socket as it is used for
        // multiple async_write operations.
        &socket,

        // The allocated buffer for the encoded message. The std::unique_ptr
        // smart pointer is move-only, and as a consequence our lambda
        // implementation is also move-only.
        encoded_message = std::move(encoded_message),

        // The repeat count remaining.
        repeat_count,

        // A steady timer used for introducing a delay.
        delay_timer = std::move(delay_timer),

        // To manage the cycle between the multiple underlying asychronous
        // operations, our implementation is a state machine.
        state = starting
      ]
      (
        auto& self,
        const boost::system::error_code& error = {},
        std::size_t = 0
      ) mutable
      {
        if (!error)
        {
          switch (state)
          {
          case starting:
          case writing:
            if (repeat_count > 0)
            {
              --repeat_count;
              state = waiting;
              delay_timer->expires_after(std::chrono::seconds(1));
              delay_timer->async_wait(std::move(self));
              return; // Composed operation not yet complete.
            }
            break; // Composed operation complete, continue below.
          case waiting:
            state = writing;
            boost::asio::async_write(socket,
                boost::asio::buffer(*encoded_message), std::move(self));
            return; // Composed operation not yet complete.
//.........这里部分代码省略.........

  void operator()(CompletionHandler&& completion_handler, tcp::socket& socket,
      std::unique_ptr<std::string> encoded_message, std::size_t repeat_count,
      std::unique_ptr<boost::asio::steady_timer> delay_timer) const
  {
    // In this example, the composed operation's intermediate completion
    // handler is implemented as a hand-crafted function object.
    struct intermediate_completion_handler
    {
      // The intermediate completion handler holds a reference to the socket as
      // it is used for multiple async_write operations, as well as for
      // obtaining the I/O executor (see get_executor below).
      tcp::socket& socket_;

      // The allocated buffer for the encoded message. The std::unique_ptr
      // smart pointer is move-only, and as a consequence our intermediate
      // completion handler is also move-only.
      std::unique_ptr<std::string> encoded_message_;

      // The repeat count remaining.
      std::size_t repeat_count_;

      // A steady timer used for introducing a delay.
      std::unique_ptr<boost::asio::steady_timer> delay_timer_;

      // To manage the cycle between the multiple underlying asychronous
      // operations, our intermediate completion handler is implemented as a
      // state machine.
      enum { starting, waiting, writing } state_;

      // As our composed operation performs multiple underlying I/O operations,
      // we should maintain a work object against the I/O executor. This tells
      // the I/O executor that there is still more work to come in the future.
      boost::asio::executor_work_guard<tcp::socket::executor_type> io_work_;

      // The user-supplied completion handler, called once only on completion
      // of the entire composed operation.
      typename std::decay<CompletionHandler>::type handler_;

      // By having a default value for the second argument, this function call
      // operator matches the completion signature of both the async_write and
      // steady_timer::async_wait operations.
      void operator()(const boost::system::error_code& error, std::size_t = 0)
      {
        if (!error)
        {
          switch (state_)
          {
          case starting:
          case writing:
            if (repeat_count_ > 0)
            {
              --repeat_count_;
              state_ = waiting;
              delay_timer_->expires_after(std::chrono::seconds(1));
              delay_timer_->async_wait(std::move(*this));
              return; // Composed operation not yet complete.
            }
            break; // Composed operation complete, continue below.
          case waiting:
            state_ = writing;
            boost::asio::async_write(socket_,
                boost::asio::buffer(*encoded_message_), std::move(*this));
            return; // Composed operation not yet complete.
          }
        }

        // This point is reached only on completion of the entire composed
        // operation.

        // We no longer have any future work coming for the I/O executor.
        io_work_.reset();

        // Deallocate the encoded message before calling the user-supplied
        // completion handler.
        encoded_message_.reset();

        // Call the user-supplied handler with the result of the operation.
        handler_(error);
      }

      // It is essential to the correctness of our composed operation that we
      // preserve the executor of the user-supplied completion handler. With a
      // hand-crafted function object we can do this by defining a nested type
      // executor_type and member function get_executor. These obtain the
      // completion handler's associated executor, and default to the I/O
      // executor - in this case the executor of the socket - if the completion
      // handler does not have its own.
      using executor_type = boost::asio::associated_executor_t<
          typename std::decay<CompletionHandler>::type,
          tcp::socket::executor_type>;

      executor_type get_executor() const noexcept
      {
        return boost::asio::get_associated_executor(
            handler_, socket_.get_executor());
      }

      // Although not necessary for correctness, we may also preserve the
      // allocator of the user-supplied completion handler. This is achieved by
      // defining a nested type allocator_type and member function
//.........这里部分代码省略.........