Value type of map need not be default copyable

[François Dumont]

On 08/08/2012 09:34 AM, Marc Glisse wrote:
> On Tue, 7 Aug 2012, Richard Smith wrote:
>
>>> I've attached a patch for unordered_map which solves the rvalue
>>> reference problem.  For efficiency, I've created a new
>>> _M_emplace_bucket method rather than call emplace directly.
>>>
>>> I've verified all libstdc++ tests pass (sorry for the previous
>>> oversight) and am running the full GCC test suite now.  However, I'd
>>> appreciate any feedback on whether this is a reasonable approach.  STL
>>> hacking is way outside my comfort zone.  ;-)
>>>
>>> If this looks good, I'll take a stab at std::map.
>>
>> I think you should remove the mapped_type() argument from the call to
>> _M_emplace_bucket. In C++11, the mapped_type is not required to be 
>> copyable
>> at all, just to be DefaultInsertable.
>
> Indeed. The reason I was talking about emplace is that you want an 
> object to be created only at the time the node is created. That might 
> mean passing piecewise_construct_t and an empty tuple to emplace 
> (otherwise it is too similar to insert). Or for unordered_map where 
> the node functions are "exposed", you could just create the node 
> directly without passing through emplace.
>
This is what I try to do in the attached patch. I replace 
_M_insert_bucket with _M_insert_node and use it for operator[] 
implementation. I have also introduce a special std::pair constructor 
for container usage so that we do not have to include the whole tuple 
stuff just for associative container implementations.

However one test is failing:
/home/fdt/dev/gcc/libstdc++-v3/testsuite/23_containers/unordered_map/insert/array_syntax_move.cc:39:18: 
required from here
/home/fdt/dev/gcc-build/x86_64-unknown-linux-gnu/libstdc++-v3/include/bits/stl_pair.h:175:42: 
error: use of deleted function '__gnu_test::rvalstruct::rvalstruct(const 
__gnu_test::rvalstruct&)'
   : first(std::forward<_T1>(__x)), second() { }

I don't understand why it doesn't use the move constructor. I can't see 
any std::forward call missing. Anyone ?

François

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