are member pointers fixed in size and reinterpret_cast?

I am trying to create a template class that contains a pointer to an arbitrary class instance and function as follows:

template<class C>
class A {
   typedef void (C::*FunctPtr)(); //e.g. void C::some_funct();

   FunctPtr functPtr_;
   C* instPtr_;
public:
   A(FunctPtr functPtr, C* instPtr)
      : functPtr_(functPtr)
      , instPtr_(instPtr) {}
};

However, I want to be able to create instances of this class without dynamic memory allocation using placement new. Does the C++ standard guarantee this template class is of fixed size for all classes C?

In Don Clugston's article on pointers I noticed a chart of the various sizes for member functions pointers on various compilers and a few compilers aren't always the same size. I thought I was hosed but is this standards compliant? From the C++ standard sec. 5.2.10 on Reinterpret cast:

— converting a prvalue of type “pointer to member function” to a different pointer to member function type and back to its original type yields the original pointer to member value.

Would that statement from the C++ standard indicate member function pointers are all of the same size?

If not I suppose I could still rewrite the code as follows to take advantage of that reinterpret_cast guarantee explicitly:

class GenericClass;

template<class C>
class A {

   typedef void (GenericClass::*GenFunctPtr)();
   typedef void (C::*SpecificFunctPtr)();

   GenFunctPtr functPtr_; //store any kind of function ptr in this fixed format
   GenericClass* instPtr_;

public:
   A(SpecificFunctPtr functPtr, C* instPtr)
      : functPtr_(reinterpret_cast<GenFunctPtr>(functPtr))
      , instPtr_(reinterpret_cast<GenericClass*>(instPtr)) {}

   void DoSomething()
   {
      //now convert pointers back to the original type to use...
      reinterpret_cast<SpecificFunctPtr>(functPtr_);
      reinterpret_cast<C*>(instPtr_);
   }
};

This now would seem to be required to be all the same size and yet be standards compliant, right? I'd prefer the first option however if I must the 2nd will also work. Thoughts?