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

Answer 2

Basically, the virtual destructor also implements calling operator delete. The parameter is there to decide whether or not call it.

See this answer that shows the meaning of such hidden destructor parameter.