Re: Why virtual destructor CAN'T be private
On Jun 27, 5:15 pm, Chris Uzdavinis <cuz...@gmail.com> wrote:
The error message is correct and unambiguous. Derived class
destructors automatically invoke their base destructors. However, if
the base destructor is private then the derived class destructor won't
have access to make that call, and so it's an error. Note: it is ok
to have a private destructor in a class has no descendants, but then
making it virtual is just teasing. (But the usability of such a class
will be limited.)
Yes, but a virtual private destructor still has nontrivial uses (even
without friends.) Take my example below. It can be used to control
what can be derived from a class while still allowing polymorphism
within the allowed derived classes. The base class need not be
abstract or declare friends.
#include <iostream>
class mine
{
public:
template <class>
class another_mine;
virtual void display()
{ std::cout << "class 'mine'\n"; }
static void delete_mine(mine *mMine)
{ delete mMine; }
private:
virtual ~mine()
{ std::cout << "delete 'mine'\n"; }
} /* cannot_instantiate */;
template <class Type>
struct mine::another_mine : virtual public mine
{
void display()
{
std::cout << "class 'mine::another_mine <"
<< typeid(Type).name() << ">'\n";
}
~another_mine()
{
std::cout << "delete 'mine::another_mine <"
<< typeid(Type).name() << ">'\n";
}
};
template <>
struct mine::another_mine <void> : virtual public mine { };
//struct cannot_derive1 : public mine {} not_valid1;
//struct cannot_derive2 : public mine::another_mine <void> {}
not_valid2;
int main()
{
mine *test_object1 = NULL;
test_object1 = new mine::another_mine <void>;
test_object1->display();
mine::delete_mine(test_object1);
test_object1 = NULL;
test_object1 = new mine::another_mine <int>;
test_object1->display();
mine::delete_mine(test_object1);
test_object1 = NULL;
mine::another_mine <char> test_object2;
test_object2.display();
}
Obviously the template defeats the purpose, but the idea should be
clear.
Kevin P. Barry
--
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
[ comp.lang.c++.moderated. First time posters: Do this! ]