Does this work with multiple inheritance?

-----Ursprüngliche Nachricht-----
Von: Frederick Virchanza Gotham via Std-Proposals <>
Gesendet: Mo 26.02.2024 11:22
Betreff: [std-proposals] dynamic_cast<void*>(void*)
An: std-proposals <>;
CC: Frederick Virchanza Gotham <>;
With regard to dynamic_cast'ing to a void*, the Standard currently reads: Dynamic cast
   1. The result of the expression dynamic_cast<T>(v) is
      the result of converting the expression v to type T.
   5. Otherwise, v shall be a pointer to or a glvalue of a
      polymorphic type.
   7. If T is “pointer to cv void”, then the result is a pointer
      to the most derived object pointed to by v.

So let's consider the following function:

   void *Func(std::ostream *const v)
       return dynamic_cast<void*>(v);

On an x86_64 computer running Linux or macOS, using the GNU g++
compiler, this becomes:

   test   rdi,rdi
   je     the_pointer_is_null
   mov    rax,QWORD PTR [rdi]
   add    rdi,QWORD PTR [rax-0x10]
   mov    rax,rdi
   xor    eax,eax

If we remove the code that allows for 'v' to be a nullptr, it's
simplified further to:

   mov    rax,QWORD PTR [rdi]
   add    rdi,QWORD PTR [rax-0x10]
   mov    rax,rdi

This assembler can be converted back into C++ as follows:

   void *Func(void *const v)
       auto const p_vtable = *static_cast<uintptr_t**>(v);
       auto const top_offset = p_vtable[-2];
       return static_cast<char*>(v) + top_offset;

The implementation of this function is the same no matter what the
type of 'v' is -- so long as it's a polymorphic type. In fact, it can
be rewritten as follows:

   void *Func(void *const v)
       struct Dummy { virtual ~Dummy(void){} };
       return dynamic_cast<void*>( static_cast<Dummy*>(v) );

The point I want to make here is as follows: So long as 'v' points to
a polymorphic type, it is not necessary to know that that type is.
Therefore I think that the functionality of 'dynamic_cast' should be
expanded to allow 'v' to be a pointer to void -- with the note that
the behaviour is undefined if 'v' points to a non-polymorphic object.
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