How to make a C++ boost::signal be caught from an object which encapsulates the object which emits it?

问题

I have a TcpDevice class which encapsulates a TCP connection, which has an onRemoteDisconnect method which gets called whenever the remote end hangs up. Then, there's a SessionManager object which creates TcpSession objects which take a TcpDevice as a communication channel and inserts them in an internal pointer container for the application to use. In case any of the managed TcpSessions should end, I would like the SessionManager instance to be notified about it and then remove the corresponding session from the container, freeing up the resources associated with it.

I found my problem to be very similar to this question:

Object delete itself from container

but since he has a thread for checking the connections state, it gets a little different from mine and the way I intended to solve it using boost::signals, so I decided to go for a new question geared towards it - I apologize if it's the wrong way to do it... I'm still getting the feel on how to properly use S.O. :)

Since I'm kind of familiar with QT signals/slots, I found boost::signals offers a similar mechanism (I'm already using boost::asio and have no QT in this project), so I decided to implement a remoteDeviceDisconnected signal to be emitted by TcpDevice's onRemoteDisconnect, and for which I would have a slot in SessionManager, which would then delete the disconnected session and device from the container.

To initially try it out I declared the signal as a public member of TcpDevice in tcpdevice.hpp:

class TcpDevice
{
             (...)
  public:
    boost::signal <void ()> remoteDeviceDisconnected;
             (...)
}

Then I emitted it from TcpDevice's onRemoteDisconnect method like this:

remoteDeviceDisconnected();

Now, is there any way to connect this signal to my SessionManager slot from inside session manager? I tried this:

unsigned int SessionManager::createSession(TcpDevice* device)
{
  unsigned int session_id = session_counter++;
  boost::mutex::scoped_lock lock(sessions_mutex);
  sessions.push_back(new TcpSession(device, session_id));
  device->remoteDeviceDisconnected.connect(boost::bind(&SessionManager::removeDeadSessionSlot, this));
  return session_id;
}

It compiles fine but at link time it complains of multiple definitions of remoteDeviceDisconnected in several object code files:

tcpsession.cpp.o:(.bss+0x0): multiple definition of `remoteDeviceDisconnected'
tcpdevice.cpp.o: (.bss+0x0): first defined here
sessionmanager.cpp.o:(.bss+0x0): multiple definition of `remoteDeviceDisconnected'
tcpdevice.cpp.o: (.bss+0x0): first defined here

I found this strange, since I didn't redefine the signal anywhere, but just used it at the createSession method above.

Any tips would be greatly appreciated! Thank you!

回答1:

My bad! Like we all should expect, the linker was right... there was indeed a second definition, I just couldn't spot it right away because it wasn't defined by any of my classes, but just "floating" around one of my .cpp files, like those found on boost::signals examples.

Just for the record, the initial idea worked like a charm: when a given TcpDevice gets disconnected from the remote end, it emits the remoteDeviceDisconnected signal, which is then caught by the SessionManager object which holds the TcpSession instance that points to that TcpDevice. Once notified, SessionManager's method removeDeadSessionSlot gets executed, iterating through the sessions ptr_list container and removing the one which was disconnected:

void SessionManager::removeDeadSessionSlot()
{
  boost::mutex::scoped_lock lock(sessions_mutex);
  TcpSession_ptr_list_it it = sessions.begin();
  while (it != sessions.end()) {
    if (!(*it).device->isConnected())
      it = sessions.erase(it);
    else
      ++it;
  }
}

Hope that may serve as a reference to somebody!

来源：https://stackoverflow.com/questions/10149391/how-to-make-a-c-boostsignal-be-caught-from-an-object-which-encapsulates-the