Re: PR 90409 Deque fiil/copy/move/copy_backward/move_backward/equal overloads

[Daniel Krügler <daniel dot kruegler at gmail dot com>]

Am Do., 1. Aug. 2019 um 11:57 Uhr schrieb Jonathan Wakely <jwakely@redhat.com>:
>
> More comments inline below ...
[..]
>
> >François
> >
> >On 6/19/19 7:32 PM, François Dumont wrote:
> >>I wanted to implement Debug overloads for those already existing
> >>overloads but then realized that those algos could be generalized.
> >>This way we will benefit from the memmove replacement when operating
> >>with C array or std::array or std::vector iterators.
> >>
> >>I might do the same for lexicographical_compare one day.
> >>
> >>The ChangeLog below is quite huge so I attached it. I wonder if I
> >>could use deque::iterator and deque::const_iterator in place of the
> >>_Deque_iterator<> to reduce it ?
> >>
> >>Tested under Linux x86_64 normal and debug modes, ok to commit ?
> >>
> >>François
> >>
> >
>
> >diff --git a/libstdc++-v3/include/bits/deque.tcc b/libstdc++-v3/include/bits/deque.tcc
> >index 3f77b4f079c..9db869fb666 100644
> >--- a/libstdc++-v3/include/bits/deque.tcc
> >+++ b/libstdc++-v3/include/bits/deque.tcc
> >@@ -967,155 +967,507 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
> >       this->_M_impl._M_finish._M_set_node(__new_nstart + __old_num_nodes - 1);
> >     }
> >
[..]
>
> And anyway, isn't _Deque_iterator<T, T&, T*>::_Self just the same type as
> _Deque_iterator<T, T&, T*> ? It should be something like:
>
>       typedef typename _GLIBCXX_STD_C::_Deque_iterator<_Tp, _Tp&, _Tp*> _Iter;
>
> >+  template<typename _II, typename _Tp>
> >+    typename enable_if<
> >+      is_same<typename std::iterator_traits<_II>::iterator_category,
> >+            std::random_access_iterator_tag>::value,
>
> Use is_base_of<random_access_iterator_tag, ...::iterator_category> so
> it works for types derived from random_access_iterator_tag too.

Interesting. Traditional type tag dispatching approaches (as function
parameters) do have more in a manner that would be equivalent to an
implicit conversion (Being used as "by-value-parameters"), so I'm
wondering whether this should not instead refer to is_convertible? I
also found examples where this trait is currently used in <stl_algo.h>
such as

      static_assert(
      __or_<is_convertible<__pop_cat, forward_iterator_tag>,
        is_convertible<__samp_cat, random_access_iterator_tag>>::value,
      "output range must use a RandomAccessIterator when input range"
      " does not meet the ForwardIterator requirements");

Should possibly this trait be preferred?

- Daniel