Re: Cast pointer to data member to integer type - legit?

"kanze" <>
15 May 2006 08:20:06 -0400
Martin Bonner wrote:

kanze wrote:

Vidar Hasfjord wrote:

Scott Lehman wrote:

I'm actually looking for the [...] (the offset),

See the "offsetof" macro defined in <cstddef>:

   size_t offsetof (structName, memberName);

Which only works for PODSs.

Note that its implementation is platform dependent.

By definition. That's why it's in the library -- there is
no way to implement it yourself.

On my compiler
(VC8) it is defined as:

#ifdef _WIN64
#define offsetof(s,m) (size_t)( (ptrdiff_t)&reinterpret_cast<const
volatile char&>((((s *)0)->m)) )
#define offsetof(s,m) (size_t)&reinterpret_cast<const volatile
char&>((((s *)0)->m))

That's weird (especially the difference -- I don't see what the
cast to ptrdiff_t adds). The historically traditional
definition in C was the much simpler:

     #define offsetof(s, m) (size_t)(&(((s *)0)->m))

and I don't see what the added complexity above buys.

The cast to const volatile char& is in case s is a class type
which defines operator &(). (The const volatile is in case s
is a const volatile type.)

I can buy that. Especially as, when looking at the standard, I
can see nothing which says that a class with a user defined
operator& is not a POD (which sort of surprised me -- I wonder
if it is intentional, or an oversight). Which in turn means
that the following code is legal:

     #include <stddef.h>

     struct Y { int x ; void operator&() {} } ;
     struct X { int a ; Y b ; void operator&() {} } ;

         return offsetof( X, b ) ;

And the traditional C definition won't work in C++. (And that
Sun CC has a bug.)

James Kanze GABI Software
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