Re: [Boost-users] custom allocators Re:pool_alloc

Subject: Re: [Boost-users] custom allocators Re:pool_alloc
From: Eric Whitcombe (ericwsf_at_[hidden])
Date: 2010-03-19 16:03:51

Yeah, I see what I am missing - Steve pointed it out that _node-based_ 
conatiners use the same allocator with rebind.

Exponential growth - there oughtta be a law. ;-)

----- Original Message ----- 
From: "Scott McMurray" <me22.ca+boost_at_[hidden]>
To: <boost-users_at_[hidden]>
Sent: Friday, March 19, 2010 3:50 PM
Subject: Re: [Boost-users] custom allocators Re:pool_alloc

> On 19 March 2010 15:12, Eric Whitcombe <ericwsf_at_[hidden]> wrote:
>>
>> I don't know what you mean when you say "vector is the only container 
>> that
>> actually allocates from passed-in allocator." All containers allocate 
>> memory
>> for a copy of the element you add. That is why one of the requirements 
>> for
>> user-defined types to be stored in STL containers is that they have to be
>> copy-constructible.
>>
>
> I mean that all the other containers need to rebind the allocator, so
> for consistency, it seems like vector should too.  But then once
> everything is rebinding the allocator, it no longer makes sense to
> parametrise the passed-in allocator by the type, since it's just going
> to rebind it.
>
> Basically, the most basic way would just be to do:
>
> template <typename T = void>
> class allocator {
>   ... as currently ...
> };
>
> template <>
> class allocator<void> {
>    template <typename T>
>    struct rebind { typedef allocator<T> type; };
> };
>
> Then you would just pass std::allocator<> to all the containers, and
> everything could easily be made to work.
>
>>
>> I'd say that has more to do with your compiler and the general hazards of
>> recursion. There is no infinite capacity for recursion whether it be your
>> compiler's symbol table capacty (or it's inability to simplify the symbol
>> table) or the limits on stack space at run time.
>>
>
> The point is that (using A for std::allocator and V for std::vector), a
>
>    V<V<V<T>>>
>
> is actually a
>
>    V<V<V<T,A<T>>, A<V<T,A<T>>>>,A<V<V<T,A<T>>, A<V<T,A<T>>>>>>
>
> because the names are exponential in depth, rather than linear in
> depth as they ought to be.
>
> With a non-parametrised allocator, it'd just be
>
>    V<V<V<T,A>,A>,A>
>
> which is much easier to deal with.
> _______________________________________________
> Boost-users mailing list
> Boost-users_at_[hidden]
> http://lists.boost.org/mailman/listinfo.cgi/boost-users