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

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...