Fix stable_sort to work on iterators returning rvalue

[Paolo Carlini]

Hi,

On 05/28/2012 12:14 PM, Chris Jefferson wrote:
> (Resending due to broken mail client).
>
> On 25/05/12 23:10, Paolo Carlini wrote:
>> On 05/25/2012 10:42 PM, François Dumont wrote:
>>> Hi
>>>
>>>     The issues I had when trying to use stable_sort with 
>>> move_iterator was in fact not all coming from a missing swap 
>>> overload. In fact stable_sort do not accept iterators with 
>>> dereference operator returning pure rvalue. I use 
>>> std::vector<bool>::iterator to illustrate this problem. The problem 
>>> was in fact coming from __uninitialized_construct_buf that was 
>>> taking a lvalue reference. I simply modify the function and the 
>>> underlying helper struct to dereference the iterator as late as 
>>> possible that is to say only when we need to pass it to the 
>>> std::move function.
>>> ...
>>> Ok to commit ?
>> Patch looks pretty good to me. It would be great if the usual friends 
>> of libstdc++ could also have a look, in particular Chris (if I 
>> remember correctly he invented this stuff)
>
> My main concern is one I have mentioned previously. I'm unsure all our 
> code works with things like move_iterator, even when it correctly 
> compiles. We previously took out internal uses of move_iterator, 
> because while the code compiled it did not work correctly. Look at bug 
> : http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48038 for any example.
>
> With this change, code which uses move_iterator, and operator< which 
> pass by value, will cause values to be 'eaten' during the sort. Now, 
> the standard in my opinion doesn't say this would be a bug, but it 
> certainly seems like it would be unpleasant.
>
> How to fix this? The simplest way might be to wrap all predicates / 
> operator< in a wrapper which just takes lvalue references. Not sure if 
> it is worth going to that much pain just to fix this.
Hug. Thus, if I understand correctly the general idea, and considering 
that nobody complained so far wrt the released versions of the library, 
isn't this something that Francois could do at once with his work in 
mainline about removing the duplicate algorithms? Or I misunderstand? I 
would like to see either something neatly belonging to that work (and 
not making it even more complex) or something maybe a bit ugly but safe 
and restricted to stable_sort. Is it possible?

Thanks!
Paolo.