Qsharedpointer class

Question

I am trying to use a smart pointer class in the following way

class A 
{

 friend  class B;
virtual methods ();

protected:
virtual ~classA();

}

class B:public QSharedPointer<class A>
{
  class B();
~ class B();

}

I plan to replace occurrences of Class A* with class B. Is this approach correct?

Why do you inherit from QSharedPointer? That doesn't make any sense to me. There are no virtual methods to reimplement (which makes inheritance pointless), and no virtual dtor (which makes it dangerous). Why not just use QSharedPointer? If all you want is a nicer name, use a typedef. — Frank Osterfeld, Mar 21 '11 at 22:14
I want to ensure that the class B can access the protected destructor of class A and delete when the reference count goes to zero — Vij, Mar 22 '11 at 15:00
But B doesn't inherit from A, but from QSharedPointer, so you can't access protected members of A. Just make the dtor public. The refcount is nothing you have to implement, that does QSharedPointer for you. — Frank Osterfeld, Mar 23 '11 at 07:37

score 3 · Answer 1 · answered Mar 30 '11 at 02:21

No this is not really the way to do this. It looks like your design goal here is to make it impossible for someone to allocate an object of type A without putting it in a smart pointer. The normal way to do this is not to inherit from the smart pointer, but to make your type have

A private constructor
A private destructor
A public static factory method returning in this case QSharedPointer
A private deleter class that is a friend of class A

Here is an example using boost::shared_ptr (I do not have a QT installation right now, but you should be able to just replace all instances of boost::shared_ptr with QSharedPointer)

#include <boost/shared_ptr.hpp>

class A {
private:
    A() {}
    ~A() {}


    struct deleter {
            void operator()(A* val) {delete val;}
    };
    friend class deleter;
public:
    static boost::shared_ptr<A> create() {
            return boost::shared_ptr<A>(new A(), A::deleter());
    }

};

int main()
{
    //A a1;                //compile error
    //A *a2 = new A();     //compile error
    boost::shared_ptr<A> a3 = A::create();

    return 0;
}

Qsharedpointer class

1 Answers1