From: Phil Bouchard<phil@fornux.com>
Newsgroups: comp.std.c++
Subject: Re: To standardize Boost.Pool
Date: Thu, 14 Apr 2011 13:10:41 CST
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On 4/12/2011 12:10 PM, Phil Bouchard wrote:
>
>  Boost.Pool provides an is_from() member function that does a simple range
>  checks of the heap memory pages and returns whether a pointer is part of
>  the
>  pool or not. This is a very useful function which I think should be part of
>  the standards because it could be used for the global pool as well; i.e.
>  the
>  one used by operator ::new and ::delete. The benefits for memory management
>  of knowing whether an object resides on the heap or stack outweigh the
>  costs
>  of having to pollute the global namespace.

is_from() actually invokes undefined behavior if its parameter is a
pointer referring to an element outside of the pool according to 5.7.6.
  If the pointer was to be casted to the largest integer type available
on the system then a range check would be valid for that system.  For
example:

bool is_from(const char* p)
{
   return reinterpret_cast<long>(buffer)<= reinterpret_cast<long>(p)&&
reinterpret_cast<long>(p)<  reinterpret_cast<long>(buffer + sizeof(buffer));
}

It's "implementation defined" but perhaps a reinterpret_cast of a
pointer to a long integer could be guaranteed to be valid by the
standards, long being the greatest integer available on a system.

Because otherwise the current implementation of is_from would then be
useless if a false statement invokes undefined behavior.  The current
implementation of is_from can be found here:
http://www.boost.org/doc/libs/1_46_1/boost/pool/pool.hpp

is_from and a heap pool are frequent utilities used by memory managers
such a garbage collectors and Shifted Pointer.  An overview of Shifted
Pointer can be found at:
http://www.fornux.com/personal/philippe/devel/shifted_ptr/libs/smart_ptr/doc/overview.html


Regards,
-Phil


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