[boost] [interprocess] container requirements for B.IP allocators

From: Joaquín Mª López Muñoz (joaquin_at_[hidden])
Date: 2007-06-04 06:44:48

In the section entitled "Container requirements for Boost.Interprocess
allocators"
from Boost.Interprocess documentation (http://tinyurl.com/2bruhb ), it
is
stated that containers compatible with B.IP allocators must honor the
following
conditions:

  1. STL containers may not assume that memory allocated with an
allocator can
  be deallocated with other allocators of the same type [...]
  2. Containers' internal pointers should be of the type
allocator::pointer and
  containers may not assume allocator::pointer is a raw pointer.
  3. All objects must be constructed-destroyed via allocator::construct
and
  allocator::destroy functions.

I see two problems with requirement 3:

  a. Chances are that allocator member functions construct, destroy and
  address be deprecated in C++0x, if the N2257 paper by Matt Austern
  (http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2007/n2257.html )
  is taken into consideration.
  b. I've casually inspected the code of B.IP and seems like the
allocator
  construct/destroy interface is *not* actually used, see for instance
list_alloc
  in boost/interprocess/containers/list.hpp

So, if condition 3 is not used, I wonder what the actual requirements
are with
respect to allocator::pointer. I guess the main assumption B.IP
containers work
on is that allocator::pointer is implicitly convertible to/from T*.

Also, what the requirements are when handling  (nonraw) pointers to
types
other than T? Consider the following:

  struct T
  {
    X x;
  };

If I've got an allocator::pointer pt to an object t of type T and want
to store a
nonraw pointer to x, how should I do? I guess something like:

  allocator::rebind<X>::other::pointer px=pt->x;

but this is just a guess from my part, as it's not documented. Some
clarifications
on these issues, and maybe a rewriting of the relevant doc sections,
would be
welcome for those container writers willing to become B.IP allocator
compatible.

Thank you!

Joaquín M López Muñoz
Telefónica, Investigación y Desarrollo