Re: instantiating an allocator with void
On 8 Apr., 00:31, Joaqu?n M L?pez Mu?oz <joaq...@tid.es> wrote:
[I would have posted this to comp.std.c++, where it fits better, but
looks like that
group has been down for several weeks.]
Yes, I think that this is worth to mention that - like "Ceterum censeo
Carthaginem esse delendam" ;-)
[See
http://en.wikipedia.org/wiki/Carthago_delenda_est
and I hasten to add that I only want to emphasize the repetitive
part,
not the actual content part, you never know who reads this...]
The standard default allocator std::allocator has the following
explicit specialization
for void to cope with the peculiarities of this type (vg. there are no
references to
void and most of the general allocator interface makes no sense for
void):
template <> class allocator<void> {
public:
typedef void* pointer;
typedef const void* const_pointer;
typedef void value_type;
template <class U> struct rebind { typedef allocator<U> other; };
};
However, this special provision for void is not mentioned in the
general allocator
requirements in section 20.1.2 of the standard. Is this an overlook in
the standard
text or shouldn't we really relying on allocators being instantiable
with void?
Just my personal two Euro cent: I don't think that this is an
oversight
of the standard. First, the allocators requirements do not require
an allocator to be a template, second, it is impossible to provide
a conforming allocator for void. Yes, the requirements on rebind
somehow imply a template-based mechanism, but it does not
require that every allocator shall be a class template where the
first template parameter specifies the object type T, for which this
allocators allocates and deallocates.
So, my impression is, that the provision of an std::allocator<void>
specialization - actually only a "fake" implementation, which
allows an instantiation, but not a reasonable full-fledged usage of
this
"allocator" - is just a convenience of the standard to ease template
program in generic context.
Greetings from Bremen,
Daniel Kr?gler
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