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

[Jonathan Wakely <jwakely@redhat.com>]

On 01/08/19 13:52 +0100, Jonathan Wakely wrote:
>On 01/08/19 13:31 +0200, Daniel Krügler wrote:
>>Am Do., 1. Aug. 2019 um 13:01 Uhr schrieb Jonathan Wakely <jwakely@redhat.com>:
>>>
>>>On 01/08/19 12:36 +0200, Daniel Krügler wrote:
>>>>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?
>>>
>>>Hmm, I don't know why I did it that way in sample.
>>>
>>>The standard requires derivation in a couple of places today, see
>>>[reverse.iterator] bullet 2.1 and [move.iterator] bullet 1.1 which use
>>>DerivedFrom<random_access_iterator_tag> to check whether the base
>>>iterator is random access or not.
>>
>>If you want to mimic DerivedFrom you also need to include
>>is_convertible in some way, because is_base_of does not care about
>>access.
>
>Ah yes, that's probably why I used is_convertible :-)
>
>>Maybe introduce __is_derived_from?
>
>Whatever we do, we should make it work for C++98 too, as that's needed
>for François's patch. I wonder if it's good enough to just check if
>iterator_traits<I>::iterator_category* converts to
>random_access_iterator_tag*.
>
>So rather than a generic is_derived_from, just a check for
>is_random_access, as that's all we need here.

I already added a C++11-and-later version of that to <numeric>, but
forgot to check that the base is public and unambiguous:

  template<typename _It, typename _Traits = iterator_traits<_It>,
           typename _Cat = typename _Traits::iterator_category>
    using __is_random_access_iter
      = is_base_of<random_access_iterator_tag, _Cat>;