Does dynamic_cast work inside overloaded operator delete?

Question

I came across this:

struct Base {
  void* operator new (size_t);
  void operator delete (void*);
  virtual ~Base () {}  // <--- polymorphic
};
struct Derived : Base {};

void Base::operator delete (void *p)
{
  Base *pB = static_cast<Base*>(p);
  if(dynamic_cast<Derived*>(pB) != 0)
  { /* ... NOT reaching here ? ... */ }
  free(p);
}

Now if we do,

Base *p = new Derived;
delete p;

Surprisingly, the condition inside the Base::delete is not satisfied Am I doing anything wrong ? Or casting from void* looses the information of Derived* ?

Answer 1

Function operator delete is a raw-memory deallocation function. It is invoked when the actual object (that used to reside in that memory) has already been destructed. I.e. by the time you get into operator delete you object has already been wiped out. The memory the pointer points to is essentially "raw", it no longer contains an object. Trying to use any polymorphic functionality on this raw memory is useless - it will not work.

In more formal terms, according to the language standard the lifetime of an object with non-trivial destructor ends once its destructor starts. In your case all destructors have already done their work. The lifetime of the object is already over, while dynamic_cast requires a "live" object.

P.S. Formally, it is permissible to use dynamic_cast in a destructor as long as some conditions are met (see 12.7/5), but when all destructors are finished (as in your case), dynamic_cast is no longer usable.