C++: Creating a shared object rather than a shared pointer to an object

Question

boost::shared_ptr really bothers me. Certainly, I understand the utility of such a thing, but I wish that I could use the shared_ptr<A> as an A*. Consider the following code

class A
{
public:
    A() {}
    A(int x) {mX = x;}
    virtual void setX(int x) {mX = x;}
    virtual int getX() const {return mX;}
private:
    int mX;
};


class HelpfulContainer
{
public:
    //Don't worry, I'll manager the memory from here.
    void eventHorizon(A*& a)
    {
        cout << "It's too late to save it now!" << endl;
        delete a;
        a = NULL;
    }
};


int main()
{
    HelpfulContainer helpfulContainer;

    A* a1 = new A(1);
    A* a2 = new A(*a1);
    cout << "*a1 = " << *a1 << endl;
    cout << "*a2 = " << *a2 << endl;
    a2->setX(2);
    cout << "*a1 = " << *a1 << endl;
    cout << "*a2 = " << *a2 << endl;
    cout << "Demonstrated here a2 is not connected to a1." << endl;

    //hey, I wonder what this event horizon function is.
    helpfulContainer.eventHorizon(a1);

    cout << "*a1 = " << *a1 << endl;//Bad things happen when running this line.
}

Whoever created the HelpfulContainer wasn't thinking about others wanting to retain pointers to A objects. We can't give HelpfulClass boost::shared_ptr objects. But one thing we could do is use the pimlp idiom to create a SharedA which itself is an A:

class SharedA : public A
{
public:
    SharedA(A* a) : mImpl(a){}
    virtual void setX(int x) {mImpl->setX(x);}
    virtual int getX() const {return mImpl->getX();}
private:
    boost::shared_ptr<A> mImpl;
};

And then the main function can look something like this:

int main()
{
    HelpfulContainer helpfulContainer;

    A* sa1 = new SharedA(new A(1));
    A* sa2 = new SharedA(sa1);
    cout << "*sa1 = " << *sa1 << endl;
    cout << "*sa2 = " << *sa2 << endl;
    sa2->setX(2);
    cout << "*sa1 = " << *sa1 << endl;
    cout << "*sa2 = " << *sa2 << endl;
    cout << "this demonstrates that sa2 is a shared version of sa1" << endl;

    helpfulContainer.eventHorizon(sa1);
    sa2->setX(3);
    //cout << "*sa1 = " << *sa1 << endl;//Bad things would happen here
    cout << "*sa2 = " << *sa2 << endl; 
    //but this line indicates that the originally created A is still safe and intact.
    //only when we call sa2 goes out of scope will the A be deleted.
}

So, my question is this: Is the above pattern a good pattern, or is there something I'm not considering yet. My current project inherited a HelpfulContainer class like above that's deleting the pointers that I need, but I still need the data structure present in the HelpfulContainer.

---

Update: This question is a follow-on question.

Answer 1

The whole point of shared_ptr is that it (and its copies) own the object that it points to. If you want to give an A to a container that manages its lifetime then you shouldn't be using a shared_ptr at all as it doesn't meet your needs; HelpfulContainer only knows how to be the sole owner of a dynamically created object so you need to give it a pointer to an object that isn't owned by anything else.

I think that it is usually poor design for an object to care about its own lifetime (there are exceptions). It is usually more useful if an object can do a job and something else manages its creation and descruction, choosing the simplest lifetime strategy possible (e.g. local/automatic variable).

If you absolutely have to share ownership between two things that don't co-operate (such as shared_ptr and HelpfulContainer) then you will have to use some sort of proxy technique.

In this case, though, it just looks like HelpfulContainer just isn't that helpful for your situation.