Re: [boost] Proposal: Monotonic Containers

Subject: Re: [boost] Proposal: Monotonic Containers
From: joaquin_at_[hidden]
Date: 2009-06-09 10:03:12

Stewart, Robert escribió:
> 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.

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.

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.

Joaquín M López Muñoz
Telefónica, Investigación y Desarrollo