30 #ifndef _GLIBCXX_RANGES_BASE_H
31 #define _GLIBCXX_RANGES_BASE_H 1
33 #pragma GCC system_header
35 #if __cplusplus > 201703L
40 #ifdef __cpp_lib_concepts
41 namespace std _GLIBCXX_VISIBILITY(default)
43 _GLIBCXX_BEGIN_NAMESPACE_VERSION
47 inline constexpr
bool disable_sized_range =
false;
49 template<
typename _Tp>
50 inline constexpr
bool enable_borrowed_range =
false;
54 constexpr __max_size_type
55 __to_unsigned_like(__max_size_type __t) noexcept
58 constexpr __max_size_type
59 __to_unsigned_like(__max_diff_type __t) noexcept
60 {
return __max_size_type(__t); }
62 template<
integral _Tp>
64 __to_unsigned_like(_Tp __t) noexcept
65 {
return static_cast<make_unsigned_t<_Tp>
>(__t); }
67 #if defined __STRICT_ANSI__ && defined __SIZEOF_INT128__
68 constexpr
unsigned __int128
69 __to_unsigned_like(__int128 __t) noexcept
72 constexpr
unsigned __int128
73 __to_unsigned_like(
unsigned __int128 __t) noexcept
77 template<
typename _Tp>
78 using __make_unsigned_like_t
79 = decltype(__detail::__to_unsigned_like(std::declval<_Tp>()));
82 template<
typename _Tp>
83 concept __maybe_borrowed_range
84 = is_lvalue_reference_v<_Tp>
85 || enable_borrowed_range<remove_cvref_t<_Tp>>;
89 namespace __cust_access
91 using std::ranges::__detail::__maybe_borrowed_range;
92 using std::__detail::__range_iter_t;
97 template<
typename _Tp>
101 if constexpr (is_array_v<remove_reference_t<_Tp>>)
103 else if constexpr (__member_begin<_Tp>)
110 template<__maybe_borrowed_range _Tp>
114 operator()[[nodiscard]](_Tp&& __t) const noexcept(_S_noexcept<_Tp&>())
116 if constexpr (is_array_v<remove_reference_t<_Tp>>)
118 static_assert(is_lvalue_reference_v<_Tp>);
121 else if constexpr (__member_begin<_Tp>)
128 template<
typename _Tp>
129 concept __member_end = requires(_Tp& __t)
131 { __decay_copy(__t.end()) } -> sentinel_for<__range_iter_t<_Tp>>;
135 void end(
auto&) =
delete;
136 void end(
const auto&) =
delete;
138 template<
typename _Tp>
139 concept __adl_end = __class_or_enum<remove_reference_t<_Tp>>
140 && requires(_Tp& __t)
142 { __decay_copy(
end(__t)) } -> sentinel_for<__range_iter_t<_Tp>>;
148 template<
typename _Tp>
149 static constexpr
bool
154 else if constexpr (__member_end<_Tp>)
161 template<__maybe_borrowed_range _Tp>
163 || __member_end<_Tp> || __adl_end<_Tp>
165 operator()[[nodiscard]](_Tp&& __t) const noexcept(_S_noexcept<_Tp&>())
169 static_assert(is_lvalue_reference_v<_Tp>);
170 return __t + extent_v<remove_reference_t<_Tp>>;
172 else if constexpr (__member_end<_Tp>)
180 template<
typename _To,
typename _Tp>
181 constexpr decltype(
auto)
182 __as_const(_Tp& __t) noexcept
184 static_assert(std::is_same_v<_To&, _Tp&>);
186 if constexpr (is_lvalue_reference_v<_To>)
187 return const_cast<const _Tp&
>(__t);
189 return static_cast<const _Tp&&
>(__t);
194 template<
typename _Tp>
197 operator()(_Tp&& __e)
const
198 noexcept(noexcept(_Begin{}(__cust_access::__as_const<_Tp>(__e))))
199 requires requires { _Begin{}(__cust_access::__as_const<_Tp>(__e)); }
201 return _Begin{}(__cust_access::__as_const<_Tp>(__e));
207 template<
typename _Tp>
210 operator()(_Tp&& __e)
const
211 noexcept(noexcept(_End{}(__cust_access::__as_const<_Tp>(__e))))
212 requires requires { _End{}(__cust_access::__as_const<_Tp>(__e)); }
214 return _End{}(__cust_access::__as_const<_Tp>(__e));
218 template<
typename _Tp>
219 concept __member_rbegin = requires(_Tp& __t)
221 { __decay_copy(__t.rbegin()) } -> input_or_output_iterator;
224 void rbegin(
auto&) =
delete;
225 void rbegin(
const auto&) =
delete;
227 template<
typename _Tp>
228 concept __adl_rbegin = __class_or_enum<remove_reference_t<_Tp>>
229 && requires(_Tp& __t)
231 { __decay_copy(
rbegin(__t)) } -> input_or_output_iterator;
234 template<
typename _Tp>
235 concept __reversable = requires(_Tp& __t)
237 { _Begin{}(__t) } -> bidirectional_iterator;
238 { _End{}(__t) } ->
same_as<decltype(_Begin{}(__t))>;
244 template<
typename _Tp>
245 static constexpr
bool
248 if constexpr (__member_rbegin<_Tp>)
249 return noexcept(__decay_copy(std::declval<_Tp&>().
rbegin()));
250 else if constexpr (__adl_rbegin<_Tp>)
251 return noexcept(__decay_copy(
rbegin(std::declval<_Tp&>())));
254 if constexpr (noexcept(_End{}(std::declval<_Tp&>())))
256 using _It = decltype(_End{}(std::declval<_Tp&>()));
258 return is_nothrow_copy_constructible_v<_It>;
266 template<__maybe_borrowed_range _Tp>
267 requires __member_rbegin<_Tp> || __adl_rbegin<_Tp> || __reversable<_Tp>
269 operator()[[nodiscard]](_Tp&& __t)
const
270 noexcept(_S_noexcept<_Tp&>())
272 if constexpr (__member_rbegin<_Tp>)
274 else if constexpr (__adl_rbegin<_Tp>)
281 template<
typename _Tp>
282 concept __member_rend = requires(_Tp& __t)
284 { __decay_copy(__t.rend()) }
285 -> sentinel_for<decltype(_RBegin{}(std::forward<_Tp>(__t)))>;
288 void rend(
auto&) =
delete;
289 void rend(
const auto&) =
delete;
291 template<
typename _Tp>
292 concept __adl_rend = __class_or_enum<remove_reference_t<_Tp>>
293 && requires(_Tp& __t)
295 { __decay_copy(
rend(__t)) }
296 -> sentinel_for<decltype(_RBegin{}(std::forward<_Tp>(__t)))>;
302 template<
typename _Tp>
303 static constexpr
bool
306 if constexpr (__member_rend<_Tp>)
307 return noexcept(__decay_copy(std::declval<_Tp&>().
rend()));
308 else if constexpr (__adl_rend<_Tp>)
309 return noexcept(__decay_copy(
rend(std::declval<_Tp&>())));
312 if constexpr (noexcept(_Begin{}(std::declval<_Tp&>())))
314 using _It = decltype(_Begin{}(std::declval<_Tp&>()));
316 return is_nothrow_copy_constructible_v<_It>;
324 template<__maybe_borrowed_range _Tp>
325 requires __member_rend<_Tp> || __adl_rend<_Tp> || __reversable<_Tp>
327 operator()[[nodiscard]](_Tp&& __t)
const
328 noexcept(_S_noexcept<_Tp&>())
330 if constexpr (__member_rend<_Tp>)
332 else if constexpr (__adl_rend<_Tp>)
341 template<
typename _Tp>
344 operator()(_Tp&& __e)
const
345 noexcept(noexcept(_RBegin{}(__cust_access::__as_const<_Tp>(__e))))
346 requires requires { _RBegin{}(__cust_access::__as_const<_Tp>(__e)); }
348 return _RBegin{}(__cust_access::__as_const<_Tp>(__e));
354 template<
typename _Tp>
357 operator()(_Tp&& __e)
const
358 noexcept(noexcept(_REnd{}(__cust_access::__as_const<_Tp>(__e))))
359 requires requires { _REnd{}(__cust_access::__as_const<_Tp>(__e)); }
361 return _REnd{}(__cust_access::__as_const<_Tp>(__e));
365 template<
typename _Tp>
366 concept __member_size = !disable_sized_range<remove_cvref_t<_Tp>>
367 && requires(_Tp& __t)
369 { __decay_copy(__t.size()) } -> __detail::__is_integer_like;
372 void size(
auto&) =
delete;
373 void size(
const auto&) =
delete;
375 template<
typename _Tp>
376 concept __adl_size = __class_or_enum<remove_reference_t<_Tp>>
377 && !disable_sized_range<remove_cvref_t<_Tp>>
378 && requires(_Tp& __t)
380 { __decay_copy(
size(__t)) } -> __detail::__is_integer_like;
383 template<
typename _Tp>
384 concept __sentinel_size = requires(_Tp& __t)
386 { _Begin{}(__t) } -> forward_iterator;
388 { _End{}(__t) } -> sized_sentinel_for<decltype(_Begin{}(__t))>;
390 __detail::__to_unsigned_like(_End{}(__t) - _Begin{}(__t));
396 template<
typename _Tp>
397 static constexpr
bool
402 else if constexpr (__member_size<_Tp>)
404 else if constexpr (__adl_size<_Tp>)
406 else if constexpr (__sentinel_size<_Tp>)
407 return noexcept(_End{}(std::declval<_Tp&>())
408 - _Begin{}(std::declval<_Tp&>()));
412 template<
typename _Tp>
413 requires is_bounded_array_v<remove_reference_t<_Tp>>
414 || __member_size<_Tp> || __adl_size<_Tp> || __sentinel_size<_Tp>
416 operator()[[nodiscard]](_Tp&& __t)
const noexcept(_S_noexcept<_Tp&>())
420 else if constexpr (__member_size<_Tp>)
422 else if constexpr (__adl_size<_Tp>)
424 else if constexpr (__sentinel_size<_Tp>)
425 return __detail::__to_unsigned_like(_End{}(__t) - _Begin{}(__t));
433 template<
typename _Tp>
434 requires requires (_Tp& __t) { _Size{}(__t); }
436 operator()[[nodiscard]](_Tp&& __t)
const noexcept(noexcept(_Size{}(__t)))
438 auto __size = _Size{}(__t);
439 using __size_type = decltype(__size);
441 if constexpr (integral<__size_type>)
444 if constexpr (__int_traits<__size_type>::__digits
445 < __int_traits<ptrdiff_t>::__digits)
446 return static_cast<ptrdiff_t
>(__size);
448 return static_cast<make_signed_t<__size_type>
>(__size);
450 #if defined __STRICT_ANSI__ && defined __SIZEOF_INT128__
452 else if constexpr (__detail::__is_int128<__size_type>)
453 return static_cast<__int128
>(__size);
456 return __detail::__max_diff_type(__size);
460 template<
typename _Tp>
461 concept __member_empty = requires(_Tp& __t) { bool(__t.empty()); };
463 template<
typename _Tp>
464 concept __size0_empty = requires(_Tp& __t) { _Size{}(__t) == 0; };
466 template<
typename _Tp>
467 concept __eq_iter_empty = requires(_Tp& __t)
469 { _Begin{}(__t) } -> forward_iterator;
471 bool(_Begin{}(__t) == _End{}(__t));
477 template<
typename _Tp>
478 static constexpr
bool
481 if constexpr (__member_empty<_Tp>)
482 return noexcept(
bool(std::declval<_Tp&>().
empty()));
483 else if constexpr (__size0_empty<_Tp>)
484 return noexcept(_Size{}(std::declval<_Tp&>()) == 0);
486 return noexcept(
bool(_Begin{}(std::declval<_Tp&>())
487 == _End{}(std::declval<_Tp&>())));
491 template<
typename _Tp>
492 requires __member_empty<_Tp> || __size0_empty<_Tp>
493 || __eq_iter_empty<_Tp>
495 operator()[[nodiscard]](_Tp&& __t)
const noexcept(_S_noexcept<_Tp&>())
497 if constexpr (__member_empty<_Tp>)
498 return bool(__t.empty());
499 else if constexpr (__size0_empty<_Tp>)
500 return _Size{}(__t) == 0;
502 return bool(_Begin{}(__t) == _End{}(__t));
506 template<
typename _Tp>
507 concept __pointer_to_object = is_pointer_v<_Tp>
508 && is_object_v<remove_pointer_t<_Tp>>;
510 template<
typename _Tp>
511 concept __member_data = requires(_Tp& __t)
513 { __decay_copy(__t.data()) } -> __pointer_to_object;
516 template<
typename _Tp>
517 concept __begin_data = contiguous_iterator<__range_iter_t<_Tp>>;
522 template<
typename _Tp>
523 static constexpr
bool
526 if constexpr (__member_data<_Tp>)
527 return noexcept(__decay_copy(std::declval<_Tp&>().
data()));
529 return noexcept(_Begin{}(std::declval<_Tp&>()));
533 template<__maybe_borrowed_range _Tp>
534 requires __member_data<_Tp> || __begin_data<_Tp>
536 operator()[[nodiscard]](_Tp&& __t)
const noexcept(_S_noexcept<_Tp>())
538 if constexpr (__member_data<_Tp>)
547 template<
typename _Tp>
550 operator()(_Tp&& __e)
const
551 noexcept(noexcept(_Data{}(__cust_access::__as_const<_Tp>(__e))))
552 requires requires { _Data{}(__cust_access::__as_const<_Tp>(__e)); }
554 return _Data{}(__cust_access::__as_const<_Tp>(__e));
560 inline namespace __cust
562 inline constexpr __cust_access::_Begin
begin{};
563 inline constexpr __cust_access::_End
end{};
564 inline constexpr __cust_access::_CBegin
cbegin{};
565 inline constexpr __cust_access::_CEnd
cend{};
566 inline constexpr __cust_access::_RBegin
rbegin{};
567 inline constexpr __cust_access::_REnd
rend{};
568 inline constexpr __cust_access::_CRBegin
crbegin{};
569 inline constexpr __cust_access::_CREnd
crend{};
570 inline constexpr __cust_access::_Size
size{};
571 inline constexpr __cust_access::_SSize ssize{};
572 inline constexpr __cust_access::_Empty
empty{};
573 inline constexpr __cust_access::_Data
data{};
574 inline constexpr __cust_access::_CData cdata{};
578 template<
typename _Tp>
586 template<
typename _Tp>
588 = range<_Tp> && __detail::__maybe_borrowed_range<_Tp>;
590 template<
typename _Tp>
591 using iterator_t = std::__detail::__range_iter_t<_Tp>;
593 template<range _Range>
594 using sentinel_t = decltype(ranges::end(std::declval<_Range&>()));
596 template<range _Range>
597 using range_difference_t = iter_difference_t<iterator_t<_Range>>;
599 template<range _Range>
600 using range_value_t = iter_value_t<iterator_t<_Range>>;
602 template<range _Range>
603 using range_reference_t = iter_reference_t<iterator_t<_Range>>;
605 template<range _Range>
606 using range_rvalue_reference_t
607 = iter_rvalue_reference_t<iterator_t<_Range>>;
610 template<
typename _Tp>
612 && requires(_Tp& __t) { ranges::size(__t); };
614 template<sized_range _Range>
615 using range_size_t = decltype(ranges::size(std::declval<_Range&>()));
617 template<
typename _Derived>
618 requires is_class_v<_Derived> && same_as<_Derived, remove_cv_t<_Derived>>
619 class view_interface;
623 template<
typename _Tp,
typename _Up>
624 requires (!
same_as<_Tp, view_interface<_Up>>)
625 void __is_derived_from_view_interface_fn(const _Tp&,
626 const view_interface<_Up>&);
630 template<typename _Tp>
631 concept __is_derived_from_view_interface
632 = requires (_Tp __t) { __is_derived_from_view_interface_fn(__t, __t); };
639 template<
typename _Tp>
641 || __detail::__is_derived_from_view_interface<_Tp>;
644 template<
typename _Tp>
646 = range<_Tp> && movable<_Tp> && enable_view<_Tp>;
651 template<
typename _Range,
typename _Tp>
653 = range<_Range> && output_iterator<iterator_t<_Range>, _Tp>;
656 template<
typename _Tp>
657 concept
input_range = range<_Tp> && input_iterator<iterator_t<_Tp>>;
660 template<
typename _Tp>
662 = input_range<_Tp> && forward_iterator<iterator_t<_Tp>>;
665 template<
typename _Tp>
667 = forward_range<_Tp> && bidirectional_iterator<iterator_t<_Tp>>;
670 template<
typename _Tp>
672 = bidirectional_range<_Tp> && random_access_iterator<iterator_t<_Tp>>;
675 template<
typename _Tp>
677 = random_access_range<_Tp> && contiguous_iterator<iterator_t<_Tp>>
678 && requires(_Tp& __t)
680 { ranges::data(__t) } -> same_as<add_pointer_t<range_reference_t<_Tp>>>;
684 template<
typename _Tp>
686 = range<_Tp> && same_as<iterator_t<_Tp>, sentinel_t<_Tp>>;
689 template<
typename _Tp>
691 && ((view<remove_cvref_t<_Tp>> && constructible_from<remove_cvref_t<_Tp>, _Tp>)
696 struct __advance_fn final
698 template<input_or_output_iterator _It>
700 operator()(_It& __it, iter_difference_t<_It> __n)
const
702 if constexpr (random_access_iterator<_It>)
704 else if constexpr (bidirectional_iterator<_It>)
726 __glibcxx_assert(__n >= 0);
732 template<input_or_output_iterator _It, sentinel_for<_It> _Sent>
734 operator()(_It& __it, _Sent __bound)
const
736 if constexpr (assignable_from<_It&, _Sent>)
738 else if constexpr (sized_sentinel_for<_Sent, _It>)
739 (*this)(__it, __bound - __it);
742 while (__it != __bound)
747 template<input_or_output_iterator _It, sentinel_for<_It> _Sent>
748 constexpr iter_difference_t<_It>
749 operator()(_It& __it, iter_difference_t<_It> __n, _Sent __bound)
const
751 if constexpr (sized_sentinel_for<_Sent, _It>)
753 const auto __diff = __bound - __it;
756 __glibcxx_assert(__n == 0 || __diff == 0 || (__n < 0 == __diff < 0));
757 const auto __absdiff = __diff < 0 ? -__diff : __diff;
758 const auto __absn = __n < 0 ? -__n : __n;;
759 if (__absn >= __absdiff)
761 (*this)(__it, __bound);
770 else if (__it == __bound || __n == 0)
774 iter_difference_t<_It> __m = 0;
780 while (__m != __n && __it != __bound);
783 else if constexpr (bidirectional_iterator<_It> && same_as<_It, _Sent>)
785 iter_difference_t<_It> __m = 0;
791 while (__m != __n && __it != __bound);
797 __glibcxx_assert(__n >= 0);
805 inline constexpr __advance_fn
advance{};
807 struct __distance_fn final
809 template<input_or_output_iterator _It, sentinel_for<_It> _Sent>
810 requires (!sized_sentinel_for<_Sent, _It>)
811 constexpr iter_difference_t<_It>
812 operator()[[nodiscard]](_It __first, _Sent __last)
const
814 iter_difference_t<_It> __n = 0;
815 while (__first != __last)
823 template<input_or_output_iterator _It, sized_sentinel_for<_It> _Sent>
825 constexpr iter_difference_t<_It>
826 operator()(
const _It& __first,
const _Sent& __last)
const
828 return __last - __first;
831 template<range _Range>
833 constexpr range_difference_t<_Range>
834 operator()(_Range&& __r)
const
836 if constexpr (sized_range<_Range>)
837 return static_cast<range_difference_t<_Range>
>(ranges::size(__r));
839 return (*
this)(ranges::begin(__r), ranges::end(__r));
845 inline constexpr __distance_fn
distance{};
847 struct __next_fn final
849 template<input_or_output_iterator _It>
852 operator()(_It __x)
const
858 template<input_or_output_iterator _It>
861 operator()(_It __x, iter_difference_t<_It> __n)
const
863 ranges::advance(__x, __n);
867 template<input_or_output_iterator _It, sentinel_for<_It> _Sent>
870 operator()(_It __x, _Sent __bound)
const
872 ranges::advance(__x, __bound);
876 template<input_or_output_iterator _It, sentinel_for<_It> _Sent>
879 operator()(_It __x, iter_difference_t<_It> __n, _Sent __bound)
const
881 ranges::advance(__x, __n, __bound);
888 inline constexpr __next_fn next{};
890 struct __prev_fn final
892 template<b
idirectional_iterator _It>
895 operator()(_It __x)
const
901 template<b
idirectional_iterator _It>
904 operator()(_It __x, iter_difference_t<_It> __n)
const
906 ranges::advance(__x, -__n);
910 template<b
idirectional_iterator _It>
913 operator()(_It __x, iter_difference_t<_It> __n, _It __bound)
const
915 ranges::advance(__x, -__n, __bound);
922 inline constexpr __prev_fn prev{};
927 constexpr
dangling() noexcept =
default;
928 template<
typename... _Args>
929 constexpr
dangling(_Args&&...) noexcept { }
932 template<range _Range>
933 using borrowed_iterator_t = __conditional_t<borrowed_range<_Range>,
938 _GLIBCXX_END_NAMESPACE_VERSION
concept bidirectional_range
A range for which ranges::begin returns a bidirectional iterator.
concept forward_range
A range for which ranges::begin returns a forward iterator.
constexpr bool enable_view
[range.view] The ranges::enable_view boolean.
concept viewable_range
A range which can be safely converted to a view.
concept contiguous_range
A range for which ranges::begin returns a contiguous iterator.
concept view
[range.view] The ranges::view concept.
concept input_range
A range for which ranges::begin returns an input iterator.
concept output_range
A range for which ranges::begin returns an output iterator.
concept range
[range.range] The range concept.
concept random_access_range
A range for which ranges::begin returns a random access iterator.
concept sized_range
[range.sized] The sized_range concept.
concept common_range
A range for which ranges::begin and ranges::end return the same type.
concept borrowed_range
[range.range] The borrowed_range concept.
constexpr _Tp * to_address(_Tp *__ptr) noexcept
Obtain address referenced by a pointer to an object.
typename remove_reference< _Tp >::type remove_reference_t
Alias template for remove_reference.
typename remove_cvref< _Tp >::type remove_cvref_t
constexpr bool is_bounded_array_v
auto declval() noexcept -> decltype(__declval< _Tp >(0))
constexpr std::remove_reference< _Tp >::type && move(_Tp &&__t) noexcept
Convert a value to an rvalue.
_Tp * end(valarray< _Tp > &__va) noexcept
Return an iterator pointing to one past the last element of the valarray.
_Tp * begin(valarray< _Tp > &__va) noexcept
Return an iterator pointing to the first element of the valarray.
constexpr reverse_iterator< _Iterator > make_reverse_iterator(_Iterator __i)
Generator function for reverse_iterator.
ISO C++ entities toplevel namespace is std.
constexpr iterator_traits< _InputIterator >::difference_type distance(_InputIterator __first, _InputIterator __last)
A generalization of pointer arithmetic.
concept same_as
[concept.same], concept same_as
constexpr auto crend(const _Container &__cont) -> decltype(std::rend(__cont))
Return a reverse iterator pointing one past the first element of the const container.
constexpr auto rend(_Container &__cont) -> decltype(__cont.rend())
Return a reverse iterator pointing one past the first element of the container.
constexpr auto cend(const _Container &__cont) noexcept(noexcept(std::end(__cont))) -> decltype(std::end(__cont))
Return an iterator pointing to one past the last element of the const container.
constexpr auto empty(const _Container &__cont) noexcept(noexcept(__cont.empty())) -> decltype(__cont.empty())
Return whether a container is empty.
bitset< _Nb > operator&(const bitset< _Nb > &__x, const bitset< _Nb > &__y) noexcept
Global bitwise operations on bitsets.
constexpr auto size(const _Container &__cont) noexcept(noexcept(__cont.size())) -> decltype(__cont.size())
Return the size of a container.
constexpr void advance(_InputIterator &__i, _Distance __n)
A generalization of pointer arithmetic.
constexpr auto rbegin(_Container &__cont) -> decltype(__cont.rbegin())
Return a reverse iterator pointing to the last element of the container.
constexpr auto crbegin(const _Container &__cont) -> decltype(std::rbegin(__cont))
Return a reverse iterator pointing to the last element of the const container.
constexpr auto data(_Container &__cont) noexcept(noexcept(__cont.data())) -> decltype(__cont.data())
Return the data pointer of a container.
constexpr auto cbegin(const _Container &__cont) noexcept(noexcept(std::begin(__cont))) -> decltype(std::begin(__cont))
Return an iterator pointing to the first element of the const container.
__numeric_traits_integer< _Tp > __int_traits
Convenience alias for __numeric_traits<integer-type>.
[range.view] The ranges::view_base type.
Type returned by algorithms instead of a dangling iterator or subrange.