asio::async_write and strand

Question

asio::async_write(m_socket, asio::buffer(buf, bytes),
                custom_alloc(m_strand.wrap(custom_alloc(_OnSend))));

Does this code guarantee that all asynchronous operation handlers(calls to async_write_some) inside async_write are called through strand? (or it's just for my_handler?)

Answer 1

With the following code:

asio::async_write(stream, ..., custom_alloc(m_strand.wrap(...)));

For this composed operation, all calls to stream.async_write_some() will be invoked within m_strand if all of the following conditions are true:

• The initiating async_write(...) call is running within m_strand():

  assert(m_strand.running_in_this_thread());
  asio::async_write(stream, ..., custom_alloc(m_strand.wrap(...)));
  

• The return type from custom_alloc is either:

  • the exact type returned from strand::wrap()

    template <typename Handler> 
    Handler custom_alloc(Handler) { ... }
    

  • a custom handler that appropriate chains invocations of asio_handler_invoke():

    template <class Handler>
    class custom_handler
    {
    public:
      custom_handler(Handler handler)
        : handler_(handler)
      {}
    
      template <class... Args>
      void operator()(Args&&... args)
      {
        handler_(std::forward<Args>(args)...);
      }
    
      template <typename Function>
      friend void asio_handler_invoke(
        Function intermediate_handler,
        custom_handler* my_handler)
      {
        // Support chaining custom strategies incase the wrapped handler
        // has a custom strategy of its own.
        using boost::asio::asio_handler_invoke;
        asio_handler_invoke(intermediate_handler, &my_handler->handler_);
      }
    
    private:
      Handler handler_;
    };
    
    template <typename Handler>
    custom_handler<Handler> custom_alloc(Handler handler)
    {
      return {handler};
    }
    

See this answer for more details on strands, and this answer for details on asio_handler_invoke.