Saturday, March 22, 2008

protected virtual dtor

[[boost]] P 24 explains the technique of protected virtual destructor. Here's my take --

The A destructor is called by B's destructor. See http://www.cplusplus.com/forum/general/12344/. Remember you first clean up the outermost layer of the onion, and remember the B fields exist on the outer layer, outside A's real estate.

Now, if someone gets hold of an A pointer, where the pointee is B, she can't call delete on the pointer. Compile time (not run time) error. The special dtor is a protection.


class Base{
protected: virtual ~Base(){cout <<"base dtor" <<endl;}
};
class Derived : public Base {
public: virtual ~Derived(){cout <<"derived dtor" <<endl;}
};
int main(){
Base * p1 = new Derived;
Derived * p2 = new Derived;
delete p1; //won't compile
delete p2; // ok
}

If I swap the 2 words public and protected then I can delete Base ptr but not Derived ptr (compiler error). So it seems declared type must have a public dtor. In our examples, even though object is Derived, delete is allowed only on the ptr whose declared type has a public dtor.

Now, What if i have a private dtor? Probably less common, but it's possible to invoke this private virtual dtor --

class Base{
public:virtual ~Base(){cout <<"base dtor" <<endl;}
};
class Derived : public Base {
private: virtual ~Derived(){cout <<"derived dtor" <<endl;}
};
int main(){
Base * p1 = new Derived;
delete p1; // ok private destructor called!
}

In my experiments, qq(new) is mandatory as a stackVar of this class will trigger a compiler error, because compiler can't destroy the object automatically.

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