Re: [boost] Performance comparison between ptr_vectorandvector<cow_ptr<Shape> >, vector<copy_ptr<Shape> >

From: David Maisonave (dmaisonave_at_[hidden])
Date: 2006-01-06 13:39:52

"Thorsten Ottosen" <tottosen_at_[hidden]> wrote in message
news:<dpmc2d$f75$1_at_[hidden]>...
> David Maisonave wrote:
> > "Thorsten Ottosen" <tottosen_at_[hidden]> wrote in message
> 
> >>I'm not sure I fully understand how the detection mechanism would 
> >>work in your cow_ptr.
> > 
> > 
> > Example code:
> > template<typename T_obj>
> > 	cow_ptr(T_obj* type): m_ref_count(new ref_count(type,
> > get_alloc_func(type)))
> > {
> > 	assert(typeid(*type) == typeid(T_obj));
> > }
> 
> how can that assertion ever fail when T_obj is deduced?

The only way that cow_ptr and copy_ptr can get slice is if the wrong
type is pass to the constructor.
Example code:

Derivedtype* p = new DerivedDerivedType;

cow_ptr<BaseType> pBase(p); //This will cause an assertion since p type
does not match the pointee

> 
> > Since the cow_ptr works by capturing the type on the constructor, it

> > can easily determine if the right type is being passed in, and if it

> > matches the pointee type. That can't be done with the boost pointer 
> > containers because they don't catch the type on the constructor, nor

> > is it part of the required logic for using a clone function method.
> > 
> > 
> >>Nevertheless, as soon as the object is outside
> >>a cow_ptr might be sliced. That's reason enough not to allow it.
> > 
> > 
> > I'm not sure what you're referring to here.  Can you post an 
> > example?
> 
> *ptr = *ptr2;

Can you please give a more complete example?
Which is the smart pointer, here, and exactly what type of pointers?