[Dune] allocators... dingleton vs. instance

[Markus Blatt]

Hi,

On Wed, Oct 03, 2012 at 08:44:31PM +0200, Carsten Gräser wrote:

> Am 02.10.2012 22:32, schrieb Christian Engwer:
> > Dear all,
> > 
> > I'm currently puzzled... and I couldn't find an answer in the
> > standard.
> > 
> > Assume the following:
> > 
> > AllocatorType<Foo> alloc1;
> > AllocatorType<Foo> alloc2;
> > Foo * f = alloc1.allocate(1);
> > alloc2.deallocate(f,1);
>
> Hello Christian,
> the standard (98) does discuss the issue, however in a very
> unsatisfying way:
> 
> Section 20.1.5 (Allocator requirements) clause 2 says that two
> allocators compare equal if storage allocated by one can be
> deallocated by the other. But it does _in general_ not require
> that two distinct instances compare equal.
> 
> (*) However, clause 4 says that all container classes defined in
> the stl are permitted to additionally require that two instances
> of the same type always compare equal. Then clause 5 imposes
> a very weak restriction to this exception:
> 
>     Implementors are encouraged to supply libraries that can accept
>     allocators  that  encapsulate  more general memory models and
>     that support non-equal  instances. [...]
> 
> In C++11 exception (*) was removed.
> 
> Conclusion:
> Standard conforming allocators are not required to support your
> use case. But an allocator that should work with any C++98 stl
> implementation must support it. For C++11 the later is no longer
> the case and so called 'stateful' allocators may become much more
> likely.

Thanks for citing this. I would interpret this even further:

If the two instances of the allocator compare equal, then it is
allowed to deallocate memory allocated from the other
allocator. Otherwise it is a bug in the code.

But that is probably even harder to test?

Cheers,

Markus

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