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

From:
=?ISO-8859-1?Q?Falk_Tannh=E4user?= <clcppm-poster@this.is.invalid>
Newsgroups:
comp.lang.c++.moderated
Date:
11 May 2006 19:14:44 -0400
Message-ID:
<e3v9a5$c2$1@s1.news.oleane.net>
Scott Lehman wrote:

When I first researched this, I read that pointers to data members were
just a byte offset which made sense, and I wanted to store that offset.
Using GCC 3.3 at the time, casting a pointer to a data member to a
offset_t or ptrdiff_t posed no issues and things worked great.


As it was already stated, the offsetof() macro from <cstddef> or <stddef.h>,
which is part of the Standard Library both in C++ and in C, should be used
for this. Note that in C++ it is only guaranteed to work with POD types.

However, if you really need to convert member pointers to offsets,
you can try the following function, for which one can imagine 2
different ways to implement it (I put them in a conditional
compilation directive):

   template<typename ClassT, typename MemberT> inline
   std::ptrdiff_t offset_off(MemberT ClassT::*memptr)
   {
   #if 0 // *** [1] ***
     ClassT* pc = 0;
     return reinterpret_cast<char*>(&(pc->*memptr)) - reinterpret_cast<char*>(pc);
   #else // *** [2] ***
     ClassT c;
     return reinterpret_cast<char*>(&(c.*memptr)) - reinterpret_cast<char*>(&c);
   #endif
   }

Keep in mind that the way [1] relies on Undefined Behaviour,
thus it may work on some implementations and break on others
- especially in presence of multiple or virtual inheritance.

Way [2], while not suffering from this problem, has the
disadvantage of instantiating the class in question -
in particular it requires the presence of an accessible
default constructor.

So there is no 100% satisfying solution to your problem,
short of proposing an appropriate new feature to the Standard...

Falk

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