Re: [boost] Propose adding Clone Smart Pointer (clone_ptr) to the boost library

From: Jonathan Wakely (cow_at_[hidden])
Date: 2005-08-16 09:39:18

David Abrahams wrote:

> Martin Bonner <martin.bonner_at_[hidden]> writes:
> 
> > That looked a very clever trick.  I couldn't see how clone_ptr was going to
> > copy the dynamic type of the argument to the constructor without any
> > information about that dynamic type.
> >
> > Having looked at the code, I don't think it does.  
> 
> Right, I went through the same thing.  The documentation claims a bit
> too much for this library.  I expected to find some magic in the
> library but instead I found what I already knew to be the practical
> limitation of C++: the copying logic is captured based on the static
> type of the pointer with which the original smart pointer was
> initialized, not on the dynamic type of the object referred to.

Which is why ptr_container requires a clone() member to ensure the
dynamic type is available when copying. Clone_ptr has no way to access
the dynamic type.

> > It would work if the
> > constructor was declared:
> > 	foo(DerivedClass*p): m_MyAbstractClassPtr(p) {}
> > or even
> > 	template <class Derived>
> > 	foo(Derived *p):m_MyAbstractClassPtr(p) {}
> >
> > but even that will fail if the constructor is called via:
> > 	Derived *p = new DerivedDerived;
> > 	foo foo_obj(p);
> >
> > My basic concern is that while clone_ptr could be useful, it only works if
> > people stick rigidly to certain rules.  Those rules need to be made VERY
> > clear, otherwise it provides a plentiful supply of rope.
> 
> It's basically the same rules as shared_ptr, so I don't think it's too

Except that a virtual dtor will ensure the correct destructor is run
according to the dynamic type.  Shared_ptr is concerned with destruction,
and gets it right.  Clone_ptr is about creation, and doesn't always get it
right.

> bad.  However, we're reviewing Dave and Andrei's policy_ptr soon so it
> might be better to use that framework for something like this.

I think ptr_container provides a safer and more generic solution to the
problems described in the rationale for clone_ptr.

jon