Weak references and breaking cycles

Weak references and breaking cycles (C++/CX)

 

In any type system that's based on reference-counting, references to types can form cycles—that is, one object refers to a second object, the second object refers to a third object, and so on until some final object refers back to the first object. In a cycle, objects can't be deleted correctly when one object's reference count becomes zero. To help you solve this problem, C++/CX provides the Platform::WeakReference Class class. A WeakReference object supports the Resolve method, which returns null if the object no longer exists, or throws an Platform::InvalidCastException if the object is alive but is not of type T.

One scenario in which WeakReference must be used is when the this pointer is captured in a lambda expression that's used to define an event handler. We recommend that you use named methods when you define event handlers, but if you want to use a lambda for your event handler—or if you have to break a reference counting cycle in some other situation—use WeakReference. Here's an example:


using namespace Platform::Details;
using namespace Windows::UI::Xaml;
using namespace Windows::UI::Xaml::Input;
using namespace Windows::UI::Xaml::Controls;

Class1::Class1()
{
    // Class1 has a reference to m_Page
    m_Page = ref new Page();

    // m_Page will have a reference to this Class1
    // so create a weak reference to this
    WeakReference wr(this);
    m_Page->DoubleTapped += ref new DoubleTappedEventHandler( 
        [wr](Object^ sender, DoubleTappedRoutedEventArgs^ args)
    {
       // Use the weak reference to get the object
       Class1^ c = wr.Resolve<Class1>();
       if (c != nullptr)
       {
           c->m_eventFired = true;
       }
       else
       {
           // Inform the event that this handler should be removed
           // from the subscriber list
           throw ref new DisconnectedException();
       }
    }); 
}

}

When an event handler throws DisconnectedException, it causes the event to remove the handler from the subscriber list.

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