Re: [boost] downcasting smart pointers

From: Greg Colvin (gcolvin_at_[hidden])
Date: 2001-03-18 11:18:12

From: Eric Ford <eford_at_[hidden]>
> 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?  

Right now you don't.  It has been on our chronic list of
missing features for a long time but no one has been
motivated enough to contibute a solution.

> 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
> 
> 
> 
> 
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