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Dim StrongRef As SomeObject Dim WeakRef As Long Set StrongRef = MyObj WeakRef = ObjPtr(MyObj) The first problem with a weak reference is that VB makes no provision for calling a method from a Long variable A weak reference is occasionally referred to as "freeze-dried," and the act of turning it into a strong reference is called "rehydration" As the more-colorful names imply, you can't consume a weak reference without first adding a little "water" to make it a strong reference A strong reference allows you to consume the object's methods and properties You can use the following function in straight VB code to achieve rehydration The CopyMemory API function is defined in the VBoostTypes type library Function ObjFromPtr(ByVal Ptr As Long) As Object Dim tmp As Object 'Copy the pointer into an object variable 'VB now thinks that tmp contains a proper strong 'reference; it will release the reference 'when tmp goes out of scope or is otherwise 'Set to Nothing CopyMemory tmp, Ptr, 4 'Use a normal Set statement to place a 'strong reference in the function name Set ObjFromPtr = tmp 'Clear the temporary variable The Set 'statement can't be used here because this 'would release a reference count that was 'never actually added CopyMemory tmp, 0&, 4 End Function ObjFromPtr does quite a bit of work considering that all you want is to assign a pointer value and AddRef it ObjFromPtr is not universal; you should use a different version of this function for each type on which you hold a weak reference The VBoost object makes this much more efficient by using the AssignAddRef function This function uses the As Any type to turn a long value into a reference for any object type without a QueryInterface call To use AssignAddRef to make a
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type-specific function, just wrap AssignAddRef within a typed function You can use this as a prototype for any type-specific conversion function Function MyTypeFromPtr(ByVal Ptr As Long) As MyType VBoostAssignAddRef MyTypeFromPtr, Ptr End Function You can now efficiently assign the return value of MyTypeFromPtr to a MyType variable or just use the function in a With statement With MyTypeFromPtr(WeakMyTypeRef) MyMethod End With Safety is also a concern with weak references If you get the ObjPtr of an object, then you don't hold a reference on the object, so you don't keep it alive VB uses the reference-counting mechanism to guarantee that you can't use an object that points to memory that has already been freed Weak references bypass this safety mechanism, so you must be 100 percent sure that the weak reference actually refers to a living object before you use AssignAddRef to retrieve a strong reference from the weak reference With weak references, you are responsible for ensuring that the weak references are valid The easiest way to do this is to let the target object set and clear the weak reference Since weak references are used in a circular-reference situation, the weak-referenced object (class A) already has a strong reference to the object that holds the corresponding weak reference (class B) You should use Friend functions to establish weak references Unlike Public functions, Friend functions allow you to maintain full internal control over your weak references 'Class A Private m_B As B Private Sub Class_Initialize() Set m_B = New B m_BSetAPtr ObjPtr(Me) End Sub Private Sub Class_Terminate() m_BSetAPtr 0 End Sub
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'Class B Private m_APtr As Long Friend Sub SetAPtr(ByVal pA As Long) m_APtr = pA End Sub Public Property Get A() As A 'AssignAddRef handles a 0 in m_APtr set the same way 'a Set statement handles the Nothing value: it checks 'the pointer value before calling AddRef VBoostAssignAddRef A, m_APtr End Property
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