What's this extra parameter passed into virtual destructor?

Question

I have this code:

class Class {
public:
    virtual ~Class() {}
};

int main()
{
    Class* object = new Class();
    delete object;
}

which I compile with Visual C++ 10 and get this disassembly for delete object statement:

delete object;
test        eax,eax  
je          wmain+23h (401041h)  
mov         edx,dword ptr [eax]  
push        1  
mov         ecx,eax  
call        dword ptr [edx]

and this for the actual destructor:

Class::`scalar deleting destructor':
test        byte ptr [esp+4],1  
push        esi  
mov         esi,ecx  
mov         dword ptr [esi],offset Class::`vftable' (402100h)  
je          Class::`scalar deleting destructor'+18h (401018h)  
push        esi  
call        dword ptr [__imp_operator delete (4020A8h)]  
pop         ecx  
mov         eax,esi  
pop         esi  
ret         4

What is that push 1 doing at the call site and why is the test at the destructor entry point checking for that value and conditionally bypass call to operator delete()?

Answer 1

The argument is used by the destructor to know if it should call delete at the end.

3 cases where you don't want to call it :

• The destructor is called by a derived class destructor
• The object is allocated on the stack, thus not created with new.
• The object is a field of another object, thus not created by new

EDIT: Add a third case