Boost Async_* Functions and Shared_Ptr'S

Boost async_* functions and shared_ptr's

In short, boost::bind creates a copy of the boost::shared_ptr<Connection> that is returned from shared_from_this(), and boost::asio may create a copy of the handler. The copy of the handler will remain alive until one of the following occurs:

• The handler has been called by a thread from which the service's run(), run_one(), poll() or poll_one() member function has been invoked.
• The io_service is destroyed.
• The io_service::service that owns the handler is shutdown via shutdown_service().

Here are the relevant excerpts from the documentation:

• boost::bind documentation:

  The arguments that bind takes are copied and held internally by the returned function object.

• boost::asio io_service::post:

  The io_service guarantees that the handler will only be called in a thread in which the run(), run_one(), poll() or poll_one() member functions is currently being invoked. [...] The io_service will make a copy of the handler object as required.

• boost::asio io_service::~io_service:

  Uninvoked handler objects that were scheduled for deferred invocation on the io_service, or any associated strand, are destroyed.
  
  
  
  Where an object's lifetime is tied to the lifetime of a connection (or some other sequence of asynchronous operations), a shared_ptr to the object would be bound into the handlers for all asynchronous operations associated with it. [...] When a single connection ends, all associated asynchronous operations complete. The corresponding handler objects are destroyed, and all shared_ptr references to the objects are destroyed.

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While dated (2007), the Networking Library Proposal for TR2 (Revision 1) was derived from Boost.Asio. Section 5.3.2.7. Requirements on asynchronous operations provides some details for the arguments to async_ functions:

In this clause, an asynchronous operation is initiated by a function that is named with the prefix async_. These functions shall be known as initiating functions. [...] The library implementation may make copies of the handler argument, and
the original handler argument and all copies are interchangeable.

The lifetime of arguments to initiating functions shall be treated as follows:

• If the parameter is declared as a const reference or by-value [...] the implementation may make copies of the argument, and all copies shall be destroyed no later than immediately after invocation of the handler.

[...] Any calls made by the library implementation to functions associated with the initiating function's arguments will be performed such that calls occur in a sequence call₁ to call_n, where for all i, 1 ≤ i < n, call_i precedes call _i+1.

Thus:

• The implementation may create a copy of the handler. In the example, the copied handler will create a copy of the shared_ptr<Connection>, increasing the reference count of the Connection instance while the copies of handler remain alive.
• The implementation may destroy the handler prior to invoking handler. This occurs if the async operation is outstanding when io_serive::service is shutdown or the io_service is destroyed. In the example, the copies of handler will be destroyed, decreasing the reference count of Connection, and potentially causing the Connection instance to be destroyed.
• If handler is invoked, then all copies of handler will immediately be destroyed once execution returns from the handler. Again, the copies of handler will be destroyed, decreasing the reference count of Connection, and potentially causing it to be destroyed.
• The functions associated with the asnyc_'s arguments, will be executed sequentially, and not concurrent. This includes io_handler_deallocate and io_handler_invoke. This guarantees that the handler will not be deallocated while the handler is being invoked. In most areas of the boost::asio implementation, the handler is copied or moved to stack variables, allowing the destruction to occur once execution exits the block in which it was declared. In the example, this ensures that the reference count for Connection will be at least one during the invocation of the handler.

C++ boost::asio how to properly use std::shared_ptr on async functions

@Mendez I have several introductory samples when people ran into the requirement for this "ASIO pattern".

You could look at them, because I do explain the pattern and why it's introduced:

• boost asio deadline_timer async_wait(N seconds) twice within N seconds cause operation canceled

• C++: Boost.Asio: Start SSL Server session in a new thread

  This one was live-coded, so you could watch the recorded sessions

Member std::future preventing boost::shared_ptr from going out of scope

Your bind is holding onto a shared_ptr longer than it needs to. You can change it to explicitly release the captured pointer when it is done.

f_read = std::async(std::launch::async, [=, self = shared_from_this()]() mutable { self->handle_add_data(bytes_transferred); self = nullptr; });

Prior to C++14, you need to do the initialisation as a local that gets captured

auto self = shared_from_this();
f_read = std::async(std::launch::async, [=]() mutable { self->handle_add_data(bytes_transferred); self = nullptr; });

Does an instance of boost::bind retain a shared_ptr for it's lifetime?

Yes, boost::bind (as well as std::bind) creates functor that holds copies of the parameters, but one can't count on the number of copies made. So, all you can assume is that at the point (B) the number of references is greater than it was at point (A). Certainly, when the functor gets detroyed, all the shared_ptr's that it held are released.

Reserving memory for asynchronous send buffers (boost asio sockets)

What I usually do is to wrap it in a class that inherits from std::enable_shared_from_this<> something along the following lines:

class Sender : public std::enable_shared_from_this<Sender> {
 public:
  using CompletionHandler =
      std::function<void(const boost::system::error_code& ec,
                         size_t bytes_transferred,
                         std::shared_ptr<Sender> sender)>;

  ~Sender() = default;

  template<typename... Args>
  static std::shared_ptr<Sender> Create(Args&&... args) {
    return std::shared_ptr<Sender>(new Sender(std::forward<Args>(args)...));
  }

  void AsyncSendTo(const char* buffer, size_t buffer_size,
                   CompletionHandler completion_handler) {
    data_.append(buffer, buffer_size);
    socket.async_send_to(
        boost::asio::buffer(data_), endpoint_,
        [self = shared_from_this(),
         completion_handler = std::move(completion_handler)]
        (const boost::system::error_code& ec,
         size_t bytes_transferred) mutable {
          completion_handler(ec, bytes_transferred, std::move(self));
        });
  }

 private:
  Sender() = default;
  Sender(const Sender&) = delete;
  Sender(Sender&&) = delete;
  Sender& operator=(const Sender&) = delete;
  Sender& operator=(Sender&&) = delete;

  SocketType socket_;
  EndpointType endpoint_;
  std::string data_;
}

Obviously, you have to guarantee the completion_handler's lifetime. But other than that, the completion handler is gonna come back with a valid std::shared_ptr<Sender> whenever it's done and you can do whatever you need with the data Sender carries.

In the example you posted, buf would leave scope and get destroyed on send_to return, unless you first captured it in bind.

Footnote1: Those std::move()s might need to be removed depending on whether your compiler is C++14 compatible when it comes to lambdas.

Footnote2: Stay away from bind unless you absolutely need to exploit its dynamic nature.

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