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Subject: Re: [boost] [ptr_container] ptr_vector across DLL boundaries
From: Stewart, Robert (Robert.Stewart_at_[hidden])
Date: 2009-06-26 09:34:48

On Friday, June 26, 2009 6:35 AM
Thorsten Ottosen wrote:
> Christian Schladetsch skrev:
> >> Types in object hierarchies usually don't expose their
> >> copy-constructor to avoid slicing.
> >
> > But again, isn't this out of the scope? It's like the tail
> > wagging the dog - ptr_container shouldn't really be concerned
> > with object hierarchies or not. That's up to the user.
> >> Furthermore, and more importantly, you lose the concrete
> >> type of the objects by having a container of pointers to the
> >> base class. Virtual functions are the most elegant way to
> >> ask the object for a copy of the right type.
> >
> > Isn't this about ptr_container? We can't really have the
> > specific usage of OO hierarchies with containers of base
> > pointers dictate the implementation of an otherwise general
> > system.
> Please understand that pointer containers are written almost for the
> sole purpose of supporting OO programming better.

I know of use cases in which a similar container is used to hold
homogeneous collections of objects by pointer, so cloning is
irrelevant. Indeed, I recall that Rogue Wave had such containers
available. Here's one:

Can't cloning be handled using a smart pointer in a value-based
container? That is, rather than make the pointer containers try
to manage cloning, let them assume T is the most derived type
rather than what might be a base type, and then supply a smart
pointer for use with value-based containers to enable cloning.
The smart pointer can call T::clone() or defer to a policy class
to copy the T * it holds.

Rob Stewart robert.stewart_at_[hidden]
Software Engineer, Core Software using std::disclaimer;
Susquehanna International Group, LLP

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