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

Jonathan Wakely jwakely@redhat.com
Thu Aug 1 12:52:00 GMT 2019


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.




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