[Boost-users] Avoiding a circular reference of boost::functions

From: Simon Pickles (sipickles_at_[hidden])
Date: 2008-07-12 07:14:12

Hello,

I am using boost::functions to provide a simple event system, but have 
come across a problem.

I have two classes that need to communicate with each other, CGUI and 
CEventHandler. Their instances are both shred_ptrs owned by a third 
class, CCore.

To establish comms, I am passing boost::functions which have been bound 
to members of the two classes:

//---------------------
typedef std::vector<boost::any> many

// CCore.cpp snippet

        boost::function<bool(many&)> cb = 
m_eventHandler->RegisterGUICallback( boost::bind(    
&IGUIInterface::HandleEventMessage, m_gui, _1  ) );

        m_gui->RegisterEventHandler( cb );

// CEventHandler.cpp snippet
// boost::function<bool(many&) m_guiTx declared in .h

boost::function<bool(many&)> 
CEventHandler::RegisterGUICallback(boost::function<bool(many&)> f)
{
    // MEM LEAK IS HERE - A circular reference?
    m_guiTx = f;
   
    return boost::bind(&CEventHandler::HandleGUIRx, this, _1);
}

// CGUI.cpp snippet
// boost::function<bool(many&) eventHandlerCallback declared in .h

void CGUI::RegisterEventHandler( boost::function<bool(many&)> cb )
{
    m_eventHandlerCallback = cb;
    m_log->GUI("EventHandler callback registered. Testing...\n");//, s );

    std::string message = "This was sent from CGUI::RegisterEventHandler\n";
    int id = 43;

    many msg;   
    boost::any _message = message;
    boost::any _id = id;
   
    msg.push_back(_id);
    msg.push_back(_message);

    m_eventHandlerCallback(msg);
}

//-----------------------------------------------------
As you can see, CCore binds a public member function of m_gui and passes 
it to the eventhandler, where it is stored and used to send events.

The event handler returns its own member function, bound to this, which 
CCore duly passes to m_gui.

Now both classes hold objects with refs to each other, and the program 
does a big memory dump when it is ended, on win32 with MSVC2005. I 
envisage this is because the shared_ptrs holding the instances of CGUI 
and CEvenHandler are not destructing at program close.

Commenting out the line:
   m_guiTx = f;
 
eliminates the issue.

What is a better way to achieve this?

Thanks

Simon