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POPlease explain this hardcore macro that does casting and typechecking
 

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    <p>The following code is from an existing application that has to be compiled in both C and C++. There's a macro:</p> <pre><code>/* Type-checking macro to provide arguments for CoCreateInstance() etc. * The pointer arithmetic is a compile-time pointer type check that 'obj' * really is a 'type **', but is intended to have no effect at runtime. */ #define COMPTR(type, obj) &amp;IID_##type, \ (void **)(void *)((obj) + (sizeof((obj)-(type **)(obj))) \ - (sizeof((obj)-(type **)(obj)))) </code></pre> <p>that is used as follows:</p> <pre><code>ISomeInterface *object; CoCreateInstance(&amp;CLSID_SomeInterfaceImpl, NULL, CLSCTX_INPROC_SERVER, COMPTR(ISomeInterface, &amp;object)))); </code></pre> <p>here the idea is that the last two parameters of <code>CoCreateInstance()</code> are <code>IID&amp;</code> and <code>void**</code> and that macro grabs <code>ISomeInterface**</code> and converts it to <code>IID&amp;</code> and <code>void**</code> at the same time enforcing a compile-time check that the address passed in place of <code>ISomeInterface**</code> is indeed an address of <code>ISomeInterface*</code> pointer variable.</p> <p>Okay, but what's the need for</p> <pre><code>((obj) + (sizeof((obj)-(type **)(obj))) \ - (sizeof((obj)-(type **)(obj))) </code></pre> <p>complex expression? I see that the type checking is enforced with <code>(obj)-(type**)(obj)</code> subexpression. What's the need for adding and then subtracting the <code>sizeof()</code>? And what's the need for casting to <code>void*</code> before casting to <code>void**</code>?</p> <p>I suppose the same can be done as follows:</p> <pre><code>#define COMPTR(type, obj) &amp;IID_##type, \ (void **)(sizeof((obj)-(type**)(obj)), obj) </code></pre> <p>here the first part of comma operator would contain a <code>sizeof()</code> that would enforce the typecheck and evaluate to a constant, the second part would just yield the same pointer and the pointer would be cast to <code>void**</code>.</p> <p>What can the original macro do what the one I suggest can't? What's the need for those complications?</p>
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    1. COAnd this is why this is bad code. Let me guess, you've been struggling with this all day?
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    2. CO@nightcracker: No, I don't maintain that code, I just found it and was curious how it works.
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    3. CO+1 for hardcore in the title, should be a tag!
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      1. USBjarke Freund-Hansen
 

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