[patch] No allocation for empty unordered containers

[François Dumont]

On 13/08/2014 11:50, Jonathan Wakely wrote:
>
> Yes you can, it's conforming to replace a (non-virtual) member function
> with default arguments by two or more member functions. We do it all
> the time.
>
> See 17.6.5.5 [member.functions] p2.
>
>
     You should have told it sooner ! But of course no-one is supposed 
to ignore the Standard :-).

     Then here is the patch to introduce default constructor with 
compiler computed noexcept qualification. Note that I also made 
allocator aware default constructor allocation free however noexcept 
qualification has to be manually written which I find quite a burden. Do 
you think we shall do so now ?

2014-08-14  François Dumont <fdumont@gcc.gnu.org>

     * include/bits/hashtable_policy.h (_Prime_rehash_policy): Qualify 
constructor
     noexcept.
     (_Hash_code_base<>): All specialization default constructible if
     possible.
     (_Hashtable_base<>): Likewise.
     * include/bits/hashtable.h (_Hashtable<>()): Implementation defaulted.
     * include/bits/unordered_map.h (unordered_map<>::unordered_map()): New,
     implementation defaulted.
     (unordered_multimap<>::unordered_multimap()): Likewise.
     * include/bits/unordered_set.h
     (unordered_set<>::unordered_set()): Likewise.
     (unordered_multiset<>::unordered_multiset()): Likewise.
     * include/debug/unordered_map: Likewise.
     * include/debug/unordered_set: Likewise.
     * testsuite/23_containers/unordered_map/allocator/noexcept.cc
     (test04()): New.
     * testsuite/23_containers/unordered_multimap/allocator/noexcept.cc
     (test04()): New.
     * testsuite/23_containers/unordered_set/allocator/noexcept.cc
     (test04()): New.
     * testsuite/23_containers/unordered_multiset/allocator/noexcept.cc
     (test04()): New.

I am preparing a patch for profile mode so I will submit modification 
for this mode with this big patch.

Tested under Linux x86_64.

Ok to commit ?

François
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