Question

Using boost::asio i use async_accept to accept connections. This works good, but there is one issue and i need a suggestion how to deal with it. Using typical async_accept:

  Listener::Listener(int port)
        : acceptor(io, ip::tcp::endpoint(ip::tcp::v4(), port))
        , socket(io) {
          start_accept();
  }

  void Listener::start_accept() {
      Request *r = new Request(io);
      acceptor.async_accept(r->socket(), 
        boost::bind(&Listener::handle_accept, this, r, placeholders::error));
  }

Works fine but there is a issue: Request object is created with plain new so it can memory “leak”. Not really a leak, it leaks only at program stop, but i want to make valgrind happy.

Sure there is an option: i can replace it with shared_ptr, and pass it to every event handler. This will work until program stop, when asio io_service is stopping, all objects will be destroyed and Request will be free’d. But this way i always must have an active asio event for Request, or it will be destroyed! I think its direct way to crash so i dont like this variant, too.

UPD Third variant: Listener holds list of shared_ptr to active connections. Looks great and i prefer to use this unless some better way will be found. The drawback is: since this schema allows to do “garbage collection” on idle connects, its not safe: removing connection pointer from Listener will immediately destroy it, what can lead to segfault when some of connection’s handler is active in other thread. Using mutex cant fix this cus in this case we must lock nearly anything.

Is there a way to make acceptor work with connection management some beautiful and safe way? I will be glad to hear any suggestions.

Answer 1

The typical recipe for avoiding memory leaks when using this library is using a shared_ptr, the io_service documentation specifically mentions this

Remarks

The destruction sequence described above permits programs to simplify
their resource management by using shared_ptr<>. 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. This works as follows:

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. To shut down the
whole program, the io_service function stop() is called to terminate
any run() calls as soon as possible. The io_service destructor defined
above destroys all handlers, causing all shared_ptr references to all
connection objects to be destroyed.

For your scenario, change your Listener::handle_accept() method to take a boost::shared_ptr<Request> parameter. Your second concern

removing connection pointer from Listener will immediately destroy it,
what can lead to segfault when some of connection’s handler is active
in other thread. Using mutex cant fix this cus in this case we must
lock nearly anything.

is mitigated by inheriting from the boost::enable_shared_from_this template in your classes:

class Listener : public boost::enable_shared_from_this<Listener>
{
   ...
};

then when you dispatch handlers, use shared_from_this() instead of this when binding to member functions of Listener.