Re: iter_swap on move_iterator

[Jonathan Wakely <jwakely dot gcc at gmail dot com>]

On 18 May 2012 19:12, Daniel Krügler wrote:
> 2012/5/18 François Dumont <frs.dumont@gmail.com>:
>> On 05/17/2012 08:47 PM, Daniel Krügler wrote:
>>>
>>> 2012/5/17 François Dumont<frs.dumont@gmail.com>:
>>>>
>>>> Hi
>>>>
>>>> ? ?While working on removal of code duplications in algos implementation
>>>> I
>>>> challenge them with move_iterator. I started with stable_sort and it
>>>> doesn't
>>>> compile. I summarize the problem to the fact that iter_swap can't be used
>>>> on
>>>> move_iterator. As, in C++11, iter_swap simply forward to swap(*__a, *__b)
>>>> it
>>>> is normal that it can't accept rvalue but shouldn't iter_swap be adapted
>>>> to
>>>> accept move_iterator ?
>>>
>>> Yes, when the referenced type (or the iterator's reference) provides a
>>> swap overload
>>> that accepts rvalues. Remember that move_iterator will produce an rvalue
>>> on dereference, this rvalue is provided to the swap overload that is found
>>> by ADL. Given your description I would expect that the test type does not
>>> provide a swap overload accepting rvalues, thus the std::swap is selected,
>>> but cannot accept the arguments.
>>
>> Ok for how to have iter_swap work with move_iterator, users must grant the
>> necessary swap overload. But what about the other algos like stable_sort I
>> have tested at first ? Shouldn't they be adapted to work with move_iterator
>> ?
>
> I don't think so. The library requirements on the iterator type is
> ValueSwappable, which
> means that "for any dereferenceable object x of type X, *x is
> swappable". So, unless
> there exists active rvalue-swap support provided by user-provided value type of
> the wrapped iterator of std::move_iterator (or by the reference type
> of the wrapped
> iterator, once LWG 2106 becomes accepted), the library implementation should
> not interfere here.

I agree, and will just add that support for swapping rvalues of
standard library classes was added to a C++0x draft but then
intentionally removed again as undesirable and IIRC in some cases
unsafe.