Re: [boost] Proposal: Monotonic Containers

Subject: Re: [boost] Proposal: Monotonic Containers
From: Stewart, Robert (Robert.Stewart_at_[hidden])
Date: 2009-06-09 10:16:19

joaquin_at_[hidden] wrote
On Tuesday, June 09, 2009 10:03 AM
> Stewart, Robert escribió:
>
> > 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.
>
> Point conceded, but how would you do otherwise? Specifying a
> custom allocator is your only way to affect the container's
> allocation behavior.  Seems like you're implying that the
> container wrapping approach is in some way more powerful than
> directly specifying the custom allocator, which is not.

My intention was to point out the deficiency of the allocator approach, but I should also have highlighted the deficiency of simply extending the node based containers via derivation.  With knowledge of implementation details, that may be sufficient, but it would require potentially different details for each supported platform.  That, of course, implies the need to rewrite the node based containers to have complete control.

> Your only chance to solve the problem you mention is to make
> custom_allocator<node_type<T> > use the same memory pool.
> There's a way to do this.
>
> If you construct the container with
>
>   custom_allocator<T> a;
>   xxx<T,...,custom_allocator<T> > x(a);
>
> then xxx will use an object of type custom_allocator<node<T>
> for its allocation needs, which perforce will be an object
> differnt to the allocator a you've passed; but the container
> is required to construct this internal allocator as a copy of
> a:
>
>   // inside xxx implementation
>   custom_allocator<node<T> > internal_a(a);
>
> So you can use this knowledge to pass info around, for
> instance on the memory pool to be used.

Interesting.  That complicates the allocator, but it seems like it could work.  The remaining issue, of course, is how the memory pools are managed since sizeof(node<T>) is not likely sizeof(T).

_____
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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