Re: iter_swap on move_iterator

[François Dumont <frs dot dumont at gmail dot 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 ?
> > Why is the comipiler talking about a call to swap on
> > move_iterator<greedy_ops::X*>::value_type. It should be on the return type
> > of move_iterator operator* which is move_iterator<>::reference, no ?
> Yes, but move_iterator<>::reference is value_type&&.
Agree but the compiler message do not show the '&&', this is especially 
obvious in:

/home/fdt/dev/gcc-build/x86_64-unknown-linux-gnu/libstdc++-v3/include/bits/move.h:166:5: 
note:   no known conversion for argument 1 from 
'std::move_iterator<greedy_ops::X*>::value_type {aka greedy_ops::X}' to 
'greedy_ops::X&'

The compiler is perfectly able to take a lvalue reference to a 
value_type so this message makes no sens. If it was saying 
"...::reference {aka greedy_ops::X&&}' to ..." it would be clearer.

François