Question

I am trying to figure out whether it’s safe to delete an object created in module A from module B using a virtual destructor.

When using MS C++ compiler, deleting an object having a virtual destructor results in just a single vtable call and the underlying free() call is performed inside it (i.e. in the same module that created the object, i.e. safe).

The questions is: is it just the current MS implementation, or is it guaranteed in some standard or document and can be safely relied on?

Here’s the example of a disassembled virtual destructor call in VS2010:

int _tmain(int argc, _TCHAR* argv[])
{
    test *p = new test();
008A1030  push        4  
008A1032  call        dword ptr [__imp_operator new (8A20A0h)]  
008A1038  add         esp,4  
008A103B  test        eax,eax  
008A103D  je          wmain+19h (8A1049h)  
008A103F  mov         dword ptr [eax],offset test::`vftable' (8A2100h)  
008A1045  mov         ecx,eax  
008A1047  jmp         wmain+1Bh (8A104Bh)  
008A1049  xor         ecx,ecx  
    __asm int 3;
008A104B  int         3  
    delete p;
008A104C  test        ecx,ecx  
008A104E  je          wmain+28h (8A1058h)  
008A1050  mov         eax,dword ptr [ecx]  
008A1052  mov         edx,dword ptr [eax]  
008A1054  push        1  
008A1056  call        edx  
    __asm int 3;
008A1058  int         3  
    return 0;
008A1059  xor         eax,eax  
}

Answer 1

Well, the caller isn’t going to know the type of the object? And without knowing that type, how could the caller call Derived::operator delete ? Only Derived::~Derived can know whether Derived::operator delete must be called instead of the generic ::operator delete.

Hence, we can deduce that the caller cannot call ::operator delete.

_{It’s unspecified whether operator delete later calls free(), but that’s a detail anyway.}