Macro question

[Riskbreaker]

I'm in the middle of dissecting a program, and I've run across a weird macro... I can't seem to figure out what it does:

MACRO (z,w) ( (int) ( &(((z*)0)->w) )

I think w is a member variable of a class or structure.
That means the ((z*)0) has to be a pointer to a class or structure... but how's it work? That's not a dereference, since things are dereferenced with the '*' before the variable name. And why the '0'? This has got to be the weirdest piece of C code I've ever seen.
I know it has to return an int, since that's the final cast. Also, I know it's an address location, because of the &. But what does the ((z*)0) do? Ohh, I'm so confused. C masters, please help!

My guess is that z would be some class type, and that the "&(((z*)0)->w)" is the address of the declaration of w in memory, if that's possible. That really doesn't seem right though!

[Alex F]

It looks like a perfect way to get NULL-pointer exception, but I am
not sure. Check out /P compiler switch - it allows to see the code
after preprocessor. This helps to understand crazy macros.

[SeventhStar]

MACRO (z,w) ( (int) ( &(((z*)0)->w) )

z is a structure(class) name and w is a member

struct z {
//...
type w;
};

and the MACRO returns which byte from the strusture w thakes

if you have en element
z myz;

&myz + MACRO(z, w) will return the address of w in myz
or &(myz.w)

[Graham]

Actually, to be insufferably pedantic for a moment, it does nothing of the sort. There's a space between the word MACRO and the opening parenthesis, so it won't be parsed as a macro with arguments, but as a no-argument macro with the replacement text: (z,w) ( (int) ( &(((z*)0)->w) ), which is probably going to result in a syntax error.

[Riskbreaker]

SeventhStar, you're dead on. The space between the macro and the arguments was my own sloppy fault. Thank you for clearing up that weird piece of code... you've made me a little bit smarter!
-Riskbreaker

[Richard.J]

this macro is usually known as "offsetof"