Re: [boost] Proposal: Monotonic Containers

Subject: Re: [boost] Proposal: Monotonic Containers
From: Stewart, Robert (Robert.Stewart_at_[hidden])
Date: 2009-06-09 09:44:44

joaquin_at_[hidden] wrote:
On Tuesday, June 09, 2009 9:34 AM
> Stewart, Robert escribió:
> >
> > There's a problem with using an allocator for the node based
> > containers: they use the allocator to allocate the elements
> > but not the nodes.  That implies free store (de)allocations
> > for the nodes which is contrary to the intended purpose of
> > this proposal.
>
> Umm, I beg to differ. If we have a node-based container
>
>   xxx<T,...,custom_allocator<T> >
>
> and node<T> is the internal node type, the container is required
> to do the allocation using an allocator of type
>
>   custom_allocator<T>::rebind<node<T> >::other.
>
> The container is not allowed to do any kind of allocation using
> other means than the custom_allocator<X> family of allocators.

If custom_allocator<node<T> > is not specialized to do the same as custom_allocator<T>, or to use the same memory pool, then it doesn't fit the proposal.  It is possible that the primary specialization uses the free store and only specific specializations use the custom pool.  Since you can't know the node type of a standard container, you can't specialize for it.  Since the node is likely of a different size than the element type, the nodes cannot be put into the same pool as the elements, which greatly complicates using an allocator for the proposed purpose.

_____
Rob Stewart                           robert.stewart_at_[hidden]
Software Engineer, Core Software      using std::disclaimer;
Susquehanna International Group, LLP  http://www.sig.com

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