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copied!<p>Maybe the original author wasn't aware of function templates. This macro is begging to be replaced by a function template.</p> <p>Apparently the fourth argument to <code>CoCreateInstance</code> is a pointer to some global object of type IID that pertains to the <code>type</code> (type argument to COMPTR) at hand. The fifth and final argument to <code>CoCreateInstance</code> is supposed to be a <code>type**</code> pointer.</p> <p>Instead, the function <code>CoCreateInstance</code> takes a <code>void**</code> (yech!) pointer as its last argument, obtained by casting the supposed <code>type**</code> pointer. The cast goes through <code>void*</code> as an intermediary because any pointer can be cast to/from a void* pointer.</p> <p>Sans the protection in that COMPTR macro, one could pass a <code>double*</code> pointer, or even a <code>long long</code> (not a pointer!) as the fifth argument to <code>CoCreateInstance</code>. Of course, this whole mess would have been avoided had the original author used C++ what its very good at, type safety. Instead he/she decided to go the void* pointer route and put the protection in the macro.</p> <p>What the goofiness is doing: The argument to <code>sizeof</code> is the pointer difference expression <code>(obj)-(type**)(obj)</code>. If <code>obj</code> is a <code>type**</code> pointer, this is 0 (as type ptrdiff_t). If <code>obj</code> is something else, this pointer difference expression is ill-formed. So, two cases, <code>obj</code> is a <code>type**</code> pointer, or it isn't.</p> <p>Case 1, <code>obj</code> is a <code>type**</code> pointer: The pointer difference expression is valid, so the last argument to <code>CoCreateInstance</code> expands to <code>(void**)(void*)(obj+8-8)</code>, assuming a 64 bit machine. (The +8-8 becomes +4-4 on a 32 bit machine.) Regardless of machine size, the offset is added and subtracted, leaving the original pointer.</p> <p>Case 2, <code>obj</code> is not a <code>type**</code> pointer: The pointer difference expression is ill-formed, so the code doesn't compile.</p>

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