🚀 go-pugleaf

reckdahl@leland.Stanford.EDU (Keith Reckdahl) wrote:
> 
> We've been having problems with the following pointer 
> typecast: We have 4 classes A,B,C, and D
>   B inherits from A
>   C inherits from A
>   D inherits from both B and C
> Note: We've have also tried to inherit D from B and C 
> using the virtual keyword so that only one instance 
> of A is instantiated, but this does not seem to affect 
> the problem.  We try to typecast a pointer from A to D 
> meaning, in the code, we have
>   A  *a1;
>   D  *d1;
>   d1 = (D *)a1;
> And this does not work.  We get an illegal typecast 
> compiler error.  We've tried this on a variety of 
> compilers, including CODE WARRIOR, BORLAND, and MPW.
> 
> We thought that typecasting pointers is a simple 
> matter because all pointers are simply addresses. 
> What are we doing wrong?

Compilers are right: it's illegal. That's because they cannot know if 
the pointer to A points to the B or C element of D:

         < A
    /  B <
    |    < B specific
    |
    |    < A
  D <  C <
    |    < C specific
    |
    \  D specific

But the following should work:

  d1 = (D *)(B *)a1;
  d2 = (D *)(C *)a1;

with a different result of course.

If A is declared virtual in B and C, the problem is different: it is 
illegal to cast a base virtual class to a derived class.

I don't have all my doc here, but may be dynamic_cast may work if you 
have RTTI and virtual functions in A.

 Fridiric LACHASSE (ECP 86)
 CompuServe: 100530,2005
 Internet: lachass@worldnet.fr



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