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[Unknown make it ideal for use while implementing wrapper objects whose own IUnknown implementations defer to inner objects For example, the UnknownHook and ROTHook objects shown later use IUnknownUnrestricted to call the IUnknown functions on wrapped inner objects The AddRef and Release definitions look just like the native function definitions, except they return Long instead of unsigned long values QueryInterface, however, is modified to return a long instead of an HRESULT This means that VB doesn't raise an error automatically when the call fails The output parameter is also changed from a void** to a ByRef Long: VB can't do much with a void** Here are a couple of sample snippets that use IUnknownUnrestricted Once again, please don't interpret this as an endorsement of the frequent use of this interface 'Check the current refcount of an object by reading 'the return values from IUnknownRelease This is a 'handy debugging function, but not one you should 'rely on in production code Function RefCount(ByVal pUnkUR As IUnknownUnrestricted) As Long 'Balance the pending Release pUnkURAddRef 'Reduce the value returned by Release down 2 'because pUnkUR is itself holding a reference, and 'the implicit temporary variable in the calling 'function also holds a reference RefCount = pUnkURRelease - 2 End Function 'Calling code: output is 1 for a VB class module Dim cls1 As New Class1 DebugPrint RefCount(clsl) 'Call QueryInterface to get an IDispatch object This is 'equivalent to setting an As Object variable to a type that is 'not derived from IDispatch Function GetDispatch(ByVal pUnkUR As IUnknownUnrestricted) As Object Dim IID_IDispatch As VBGUID Dim pvObj As Long Dim hr As Long
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IID_IDispatch = _ IIDFromString("{00020400-0000-0000-C000-000000000046}") hr = pUnkURQueryInterface(IID_IDispatch, pvObj) If hr Then ErrRaise hr VBoostAssign GetDispatch, pvObj End Function The moral of the GetDispatch function: If you know the IID at compile time which you almost always do just use the Set statement
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Once you've used the New, CreateObject, or GetObject keywords to create an object, you invariably want to call a method or read a property on the object After all, an object isn't much good if you can't do anything with it once you have it In order to call a member function on an object, VB must perform an operation known as binding I'll define binding as "the process of determining the location of a specific function based on the pointer to the object"
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All types of binding use static compile-time data together with the dynamic runtime value of the object's pointer to find the function to call All binding falls into two categories In the first category, generally referred to as "vtable binding," the compiler generates code that applies a simple formula that locates the function based on the pointer to the object and a static function offset In the second category, called "IDispatch" or "late binding," there is not enough compile-time information to locate the function directly, so the compiler turns control over to a helper function on the object The helper function, called "Invoke," then locates and delegates to the real function The formula for locating a vtable-bound function relies on the running objects virtual function table (which is what "vtable" stands for) The vtable is an ordered array of function pointers By convention, the vtable pointer is stored in
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BINDING FUNCTIONS TO OBJECTS
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the object's first item As you'll see later in the discussion on lightweight objects, this leading vtable member is in fact the only difference between a COM object and a normal structure Here is the function for determining a function pointer given a function offset The function offset is the zero-based position of the function in the vtable array multiplied by the size of a function pointer 4 in Win32 Function VTableEntry( _ ByVal pObj As Long, ByVal Offset As Long) As Long With VBoost Assign VTableEntry, ByVal UAdd(Deref(pObj), Offset) End With End Function 'Calling code 24 is the 7th function, which is 'the standard IDispatch::Invoke entry for any 'object that supports IDispatch DebugPrint VTableEntry(ObjPtr(SomeObj), 24) The compiler retrieves a function offset from the type library that describes the type The offset is then compiled into the code along with information for providing parameters to the function and reading its return value If you reduce the VTableEntry formula to assembly code with a constant offset, you can see why vtable binding is very fast, especially when compared to the IDispatch binding I look at next
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