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

19 Mar 2010

      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" 
To: 
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  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>
is actually a
V, A>>,A, A>>>>
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,A>,A>
which is much easier to deal with.
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