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always passed nevertheless To get the memory address of the object (which you need only when it is time to free the memory), simply call VarPtr(This) or the equivalent VarPtr(ThispVTable) In the CreateLightEmpty function, a LightEmpty structure is filled and then copied into an allocated memory location The point of transfer seems pretty complicated Your first attempt might involve just coding the copy with a simple CopyMemory call CopyMemory ByVal ThisPtr, Struct, LenB(Struct) There are two problems with this code when you apply it to an arbitrary structure These problems don't exist with the LightEmpty structure because it isn't complex enough, but you must watch out for them in general The first problem occurs if Struct contains string parameters If you use a CopyMemory API call declared in VB, any string parameters undergo UNICODE/ANSI translation during the API call You can prevent string translation by replacing Struct with ByVal VarPtr(Struct) In the case of a lightweight object, however, the first element of the structure is a Long, which doesn't undergo translation during an API call Combine the lack of translation on a Long with the fact that VarPtr(Struct) and VarPtr(Struct<FirstMember>) are always equivalent, and you see that ByVal VarPtr(Struct) is equivalent to ByRef StructpVTable Of course, if the CopyMemory is typelib declared, such as the VBoostTypes-declared CopyMemory function, you won't have this potential problem, but the code shown in the listing is always safe The second problem with the single CopyMemory call is that VB thinks the data in Struct needs to be freed when the function goes out of scope VB has no way of knowing that ownership of the structure has moved elsewhere If the structure contains object or variable-size String or array types, VB will kindly free them for you when the object goes out of scope But you are still using the structure, so this is an unwanted favor To prevent VB from freeing referenced memory in the stack object, simply ZeroMemory the structure When you apply the CopyMemory call's ANSI/UNICODE precautions to ZeroMemory, you get the transfer code seen in the listing There is an additional memory problem to watch out for In this situation, you are responsible for explicitly freeing pointer-type members in the structure
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LIGHTWEIGHT COM OBJECTS
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before you delete its memory The CopyMemory call transfers ownership to the heap allocation, so you take ownership of the object's scope This makes you responsible for freeing contained pointer references before the structure goes out of scope at the CoTaskMemFree call There are two ways to do this The first is to use brute force in-line code This code assigns all strings to vbNullString (not ""), sets all objects to Nothing, and erases all arrays This is a hassle for a complex object and a potential source of memory leak bugs The alternative is to write a helper sub that makes VB clean the structure for you This procedure is used in place of the CoTaskMemFree call in the Release function Private Sub DeleteThis(This As AnyStruct) Dim tmp As AnyStruct Dim pThis As Long pThis = VarPtr(This) CopyMemory ByVal VarPtr(tmp), ByVal pThis, LenB(This) CoTaskMemFree pThis End Sub DeleteThis uses the temporary structure variable to do the work you prevented VB from doing way back in the creation function DeleteThis transfers ownership of the This structure back to a VB local variable, making VB free the structure members when the procedure exits Note that a ZeroMemory call after the CopyMemory is redundant because the This memory is about to be freed Of course, if you like to clean your garbage before throwing it out, you're welcome to make the ZeroMemory call Stack-Allocated Lightweight Objects The memory-allocation and data-transfer work forms the majority of the code in the heap-based LightEmpty example This code is greatly simplified when the lightweight object gets stack or embedded memory on which to build the object The nonheap lightweight case actually splits into two cases In the first case, the COM object needs to run code when the object is destroyed In the second case, no termination code is required In either case, the memory on which the COM object is built is cleaned and freed automatically when the structure it is built on goes out of scope Let's look at LightEmpty code for these two cases
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