downcasting smart pointers

From: Eric Ford (eford_at_[hidden])
Date: 2001-03-18 03:54:31

I'm using the counted_ptr from http://ootips.org/yonat/4dev/ which is
similar to boost's shared_ptr.  However, I've run into a problem.  I'm
willing to switch to the boost smart pointers, if they offer a
solution
to my problem.  (But I'm guessing that a solution for one would work
for
the other.)  Any help with either would be appreciated...

I have an STL container of counted_ptr<A>'s.  At some point I need to
do a dynamic downcast of a counted_ptr<A> to a counted_ptr<B>.  How do
I do that?  

The code is something like the following...

class A 
{ 
public:
virtual int f() = 0;
...
};

class B : public A
{
public:
virtual int f() = 0;
virtual int g() { return -1; };
};

class C : public B
{
public:
virtual int f() { return 1; };
...
};

class D : public B
{
public:
virtual int f() { return 2; };
...
};

If I do...

counted_ptr<A> cpA = input from somewhere
A* pA = cpA.get();
B* pB = dynamic_cast<A*>(pA);
if(pB==NULL)  throw("Illegal down cast!");
counted_ptr<B> cpB = counted_ptr<B>(pB);

...then the counter will get messed up and my cpA could get deleted 
when the counter of cpB's gos to zero, even if the counter of cpA is
>0.

If I were using regular pointers (and non-STL contains), I could just
do...
A* pA = input from somewhere
B* pB = dynamic_cast<A*>(pA);
if(pB==NULL)  throw("Illegal down cast!");

If I were using counted_ptr's, but B wasn't virtual, then I could
do...

counted_ptr<A> cpA = input from somewhere
A* pA = cpA.get();
B* pB = dynamic_cast<A*>(pA);
if(pB==NULL)  throw("Illegal down cast!");
B* pBn = new B(*pB);
counted_ptr<B> cpB = counted_ptr<B>(pBn);

...but I can't allocate a new B object, since B's virtual.  I suppose
I could make a switch statement to test of each of C and D, although
in my real code, that would result in a very large switch statement
and make a pain for adding new classes.  But I'm hoping you have a
better suggestion (aside from avoiding dynamic downcasts).

Thanks,

Eric