When I am trying to delete the derived object polymorphically (that is: base class has public virtual destructor) why derived class private destructor is still being called? Why the scope resolution private is not working here.
class Base
{
protected:
Base() { cout << "Base constructor.\n"; }
public:
virtual ~Base() { cout << "Base destructor.\n"; }
};
class Derived :public Base
{
public:
Derived() { cout << "Derived constructor.\n"; }
private:
~Derived() { cout << "Derived destructor.\n"; }
};
int main()
{
Base *p = new Derived();
delete p;
}
Output:
Base constructor.
Derived constructor.
Derived destructor.
Base destructor.
Because destructors are called in reversed order of constructors and virtual destructor will always be called.
private has nothing to do if a virtual function is going to be called.
As I pointed here out:
Why would a virtual function be private?
ISO C++ 1998 Standard onwards explicitly states:
§10.3 [...] Access control (clause 11) is not considered in determining overriding.
A bit philosophical offtopic:
Going further this is what STL does for iostreams: Definition of Non-Virtual Interface, i.e. all public functions (with exception of destructors) are non-virtual and all virtual functions are either protected or private. Public functions call virtual protected or private ones. This gives a very clear entry point into the entire hierarchy.