C++ - downcasting a diamond shape inherited object without RTTI/dynamic_cast

Question

I\'m currently working on integrating a third-party package that uses lots of RTTI stuff on a non-RTTI platform (Android). Basically, I did my own RTTI implementation but I\

Answer 1

the code:

template 
E& force_exact(const T& ref)
 {
   static const E* exact_obj;
   static const T& exact_obj_ref = *exact_obj;
   static const ptrdiff_t exact_offset = ...

doesn't work very well for me as static const E* exact_obj is zero, so static const T& exact_obj_ref = *exact_obj derefs zero, too, and thus static const ptrdiff_t exact_offset becomes also zero.

It seems to me that the derived class needs to be instantiated (which may be a problem for abstract classes...). So my code is:

template 
D & Cast2Derived(B & b)
{ static D d;
  static D * pD = & d;
  static B * pB = pD;
  static ptrdiff_t off = (char *) pB - (char *) pD;

  return * (D *) ((char *) & b - off);
} 

Tested under MSVC 2008, WinXP 32b.

Any comments / better solution(s) are welcome.

LuP