[libcxx] P2278R4: cbegin and friends

Implements `basic_const_iterator` and the const accessors.

Does not implement `const_view`

Author: strega-nil

libcxx/docs/Status/Cxx23Papers.csv

@@ -61,7 +61,7 @@
 "`P1899R3 <https://wg21.link/P1899R3>`__","LWG","``stride_view``","July 2022","","","|ranges|"
 "`P2093R14 <https://wg21.link/P2093R14>`__","LWG","Formatted output","July 2022","|Complete|","18.0","|format|"
 "`P2165R4 <https://wg21.link/P2165R4>`__","LWG","Compatibility between ``tuple``, ``pair`` and ``tuple-like`` objects","July 2022","",""
-"`P2278R4 <https://wg21.link/P2278R4>`__","LWG","``cbegin`` should always return a constant iterator","July 2022","","","|ranges|"
+"`P2278R4 <https://wg21.link/P2278R4>`__","LWG","``cbegin`` should always return a constant iterator","July 2022","In Progress","","|ranges|"
 "`P2286R8 <https://wg21.link/P2286R8>`__","LWG","Formatting Ranges","July 2022","|Complete|","16.0","|format| |ranges|"
 "`P2291R3 <https://wg21.link/P2291R3>`__","LWG","Add Constexpr Modifiers to Functions ``to_chars`` and ``from_chars`` for Integral Types in ``<charconv>`` Header","July 2022","|Complete|","16.0"
 "`P2302R4 <https://wg21.link/P2302R4>`__","LWG","``std::ranges::contains``","July 2022","|Complete|","19.0","|ranges|"

libcxx/include/CMakeLists.txt

@@ -444,6 +444,7 @@ set(files
   __iterator/bounded_iter.h
   __iterator/common_iterator.h
   __iterator/concepts.h
+  __iterator/const_iterator.h
   __iterator/counted_iterator.h
   __iterator/cpp17_iterator_concepts.h
   __iterator/data.h
@@ -639,6 +640,7 @@ set(files
   __ranges/chunk_by_view.h
   __ranges/common_view.h
   __ranges/concepts.h
+  __ranges/const_access.h
   __ranges/container_compatible_range.h
   __ranges/counted.h
   __ranges/dangling.h

libcxx/include/__iterator/const_iterator.h

@@ -0,0 +1,328 @@
+// -*- C++ -*-
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef _LIBCPP___ITERATOR_CONST_ITERATOR_H
+#define _LIBCPP___ITERATOR_CONST_ITERATOR_H
+
+#include <__compare/three_way_comparable.h>
+#include <__concepts/common_with.h>
+#include <__concepts/different_from.h>
+#include <__iterator/concepts.h>
+#include <__iterator/iterator_traits.h>
+#include <__type_traits/common_reference.h>
+#include <__type_traits/common_type.h>
+#include <__type_traits/integral_constant.h>
+#include <__type_traits/is_specialization.h>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+_LIBCPP_PUSH_MACROS
+#include <__undef_macros>
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+#if _LIBCPP_STD_VER >= 23
+
+template <indirectly_readable _Iter>
+using iter_const_reference_t = common_reference_t<const iter_value_t<_Iter>&&, iter_reference_t<_Iter>>;
+
+template <class _Iter>
+concept __constant_iterator = input_iterator<_Iter> && same_as<iter_const_reference_t<_Iter>, iter_reference_t<_Iter>>;
+
+template <input_iterator _Iter>
+class basic_const_iterator;
+
+template <input_iterator _Iter>
+using const_iterator = conditional_t<__constant_iterator<_Iter>, _Iter, basic_const_iterator<_Iter>>;
+template <semiregular _Sent>
+using const_sentinel = conditional_t<input_iterator<_Sent>, const_iterator<_Sent>, _Sent>;
+
+template <class _Iter>
+concept __not_a_const_iterator = !__is_specialization_v<_Iter, basic_const_iterator>;
+
+template <indirectly_readable _Iter>
+using __iter_const_rvalue_reference_t = common_reference_t<const iter_value_t<_Iter>&&, iter_rvalue_reference_t<_Iter>>;
+
+template <class _Iter>
+struct __basic_const_iterator_concept {
+  // clang-format off
+  using iterator_concept =
+    conditional_t<contiguous_iterator<_Iter>,
+      contiguous_iterator_tag,
+    conditional_t<random_access_iterator<_Iter>,
+      random_access_iterator_tag,
+    conditional_t<bidirectional_iterator<_Iter>,
+      bidirectional_iterator_tag,
+    conditional_t<forward_iterator<_Iter>,
+      forward_iterator_tag,
+    // else
+      input_iterator_tag>>>>;
+  // clang-format on
+};
+
+template <class _Iter>
+struct __basic_const_iterator_category : __basic_const_iterator_concept<_Iter> {};
+template <forward_iterator _Iter>
+struct __basic_const_iterator_category<_Iter> : __basic_const_iterator_concept<_Iter> {
+  using iterator_category = __basic_const_iterator_concept<_Iter>::iterator_concept;
+};
+
+template <input_iterator _Iter>
+class _LIBCPP_TEMPLATE_VIS basic_const_iterator : __basic_const_iterator_category<_Iter> {
+  _Iter __current = _Iter();
+
+  using __reference        = iter_const_reference_t<_Iter>;
+  using __rvalue_reference = __iter_const_rvalue_reference_t<_Iter>;
+
+public:
+  using value_type      = iter_value_t<_Iter>;
+  using difference_type = iter_difference_t<_Iter>;
+
+  _LIBCPP_HIDE_FROM_ABI basic_const_iterator()
+    requires default_initializable<_Iter>
+  = default;
+
+  _LIBCPP_HIDE_FROM_ABI constexpr basic_const_iterator(_Iter __cur) : __current(std::move(__cur)) {}
+  template <convertible_to<_Iter> _Type>
+  _LIBCPP_HIDE_FROM_ABI constexpr basic_const_iterator(basic_const_iterator<_Type> __cur)
+      : __current(std::move(__cur.__current)) {}
+  template <__different_from<basic_const_iterator> _Type>
+    requires convertible_to<_Type, _Iter>
+  _LIBCPP_HIDE_FROM_ABI constexpr basic_const_iterator(_Type&& __cur) : __current(std::forward<_Type>(__cur)) {}
+
+  _LIBCPP_HIDE_FROM_ABI constexpr const _Iter& base() const& noexcept { return __current; }
+  _LIBCPP_HIDE_FROM_ABI constexpr _Iter base() && { return std::move(__current); }
+
+  constexpr __reference operator*() const { return static_cast<__reference>(*__current); }
+  _LIBCPP_HIDE_FROM_ABI constexpr const auto* operator->() const
+    requires is_lvalue_reference_v<iter_reference_t<_Iter>> &&
+             same_as<remove_cvref_t<iter_reference_t<_Iter>>, value_type>
+  {
+    if constexpr (contiguous_iterator<_Iter>) {
+      return std::to_address(__current);
+    } else {
+      return std::addressof(*__current);
+    }
+  }
+
+  _LIBCPP_HIDE_FROM_ABI constexpr basic_const_iterator& operator++() {
+    ++__current;
+    return *this;
+  }
+  constexpr void operator++(int) { ++__current; }
+  _LIBCPP_HIDE_FROM_ABI constexpr basic_const_iterator operator++(int)
+    requires forward_iterator<_Iter>
+  {
+    auto __tmp = *this;
+    ++__current;
+    return __tmp;
+  }
+
+  _LIBCPP_HIDE_FROM_ABI constexpr basic_const_iterator& operator--()
+    requires bidirectional_iterator<_Iter>
+  {
+    --__current;
+    return *this;
+  }
+  _LIBCPP_HIDE_FROM_ABI constexpr basic_const_iterator operator--(int)
+    requires bidirectional_iterator<_Iter>
+  {
+    auto __tmp = *this;
+    --__current;
+    return __tmp;
+  }
+
+  _LIBCPP_HIDE_FROM_ABI constexpr basic_const_iterator& operator+=(difference_type __n)
+    requires random_access_iterator<_Iter>
+  {
+    __current += __n;
+    return *this;
+  }
+  _LIBCPP_HIDE_FROM_ABI constexpr basic_const_iterator& operator-=(difference_type __n)
+    requires random_access_iterator<_Iter>
+  {
+    __current -= __n;
+    return *this;
+  }
+
+  _LIBCPP_HIDE_FROM_ABI constexpr __reference operator[](difference_type __n) const
+    requires random_access_iterator<_Iter>
+  {
+    return static_cast<__reference>(__current[__n]);
+  }
+
+  template <sentinel_for<_Iter> _Sent>
+  _LIBCPP_HIDE_FROM_ABI constexpr bool operator==(const _Sent& __sent) const {
+    return __current == __sent;
+  }
+
+  template <__not_a_const_iterator _ConstIt>
+    requires __constant_iterator<_ConstIt> && convertible_to<_Iter const&, _ConstIt>
+  _LIBCPP_HIDE_FROM_ABI constexpr operator _ConstIt() const& {
+    return __current;
+  }
+  template <__not_a_const_iterator _ConstIt>
+    requires __constant_iterator<_ConstIt> && convertible_to<_Iter, _ConstIt>
+  _LIBCPP_HIDE_FROM_ABI constexpr operator _ConstIt() && {
+    return std::move(__current);
+  }
+
+  _LIBCPP_HIDE_FROM_ABI constexpr bool operator<(const basic_const_iterator& __rhs) const
+    requires random_access_iterator<_Iter>
+  {
+    return __current < __rhs.__current;
+  }
+  _LIBCPP_HIDE_FROM_ABI constexpr bool operator>(const basic_const_iterator& __rhs) const
+    requires random_access_iterator<_Iter>
+  {
+    return __current > __rhs.__current;
+  }
+  _LIBCPP_HIDE_FROM_ABI constexpr bool operator<=(const basic_const_iterator& __rhs) const
+    requires random_access_iterator<_Iter>
+  {
+    return __current <= __rhs.__current;
+  }
+  _LIBCPP_HIDE_FROM_ABI constexpr bool operator>=(const basic_const_iterator& __rhs) const
+    requires random_access_iterator<_Iter>
+  {
+    return __current >= __rhs.__current;
+  }
+  _LIBCPP_HIDE_FROM_ABI constexpr auto operator<=>(const basic_const_iterator& __rhs) const
+    requires random_access_iterator<_Iter> && three_way_comparable<_Iter>
+  {
+    return __current <=> __rhs.__current;
+  }
+
+  template <__different_from<basic_const_iterator> _Iter2>
+  _LIBCPP_HIDE_FROM_ABI constexpr bool operator<(const _Iter2& __rhs) const
+    requires random_access_iterator<_Iter> && totally_ordered_with<_Iter, _Iter2>
+  {
+    return __current < __rhs.__current;
+  }
+  template <__different_from<basic_const_iterator> _Iter2>
+  _LIBCPP_HIDE_FROM_ABI constexpr bool operator>(const _Iter2& __rhs) const
+    requires random_access_iterator<_Iter> && totally_ordered_with<_Iter, _Iter2>
+  {
+    return __current > __rhs.__current;
+  }
+  template <__different_from<basic_const_iterator> _Iter2>
+  _LIBCPP_HIDE_FROM_ABI constexpr bool operator<=(const _Iter2& __rhs) const
+    requires random_access_iterator<_Iter> && totally_ordered_with<_Iter, _Iter2>
+  {
+    return __current <= __rhs.__current;
+  }
+  template <__different_from<basic_const_iterator> _Iter2>
+  _LIBCPP_HIDE_FROM_ABI constexpr bool operator>=(const _Iter2& __rhs) const
+    requires random_access_iterator<_Iter> && totally_ordered_with<_Iter, _Iter2>
+  {
+    return __current >= __rhs.__current;
+  }
+  template <__different_from<basic_const_iterator> _Iter2>
+  _LIBCPP_HIDE_FROM_ABI constexpr auto operator<=>(const _Iter2& __rhs) const
+    requires random_access_iterator<_Iter> && totally_ordered_with<_Iter, _Iter2> &&
+             three_way_comparable_with<_Iter, _Iter2>
+  {
+    return __current <=> __rhs.__current;
+  }
+
+  template <__not_a_const_iterator _Iter2>
+  friend _LIBCPP_HIDE_FROM_ABI constexpr bool operator<(const _Iter2& __lhs, const basic_const_iterator& __rhs)
+    requires random_access_iterator<_Iter> && totally_ordered_with<_Iter, _Iter2>
+  {
+    return __lhs < __rhs.__current;
+  }
+  template <__not_a_const_iterator _Iter2>
+  friend _LIBCPP_HIDE_FROM_ABI constexpr bool operator>(const _Iter2& __lhs, const basic_const_iterator& __rhs)
+    requires random_access_iterator<_Iter> && totally_ordered_with<_Iter, _Iter2>
+  {
+    return __lhs > __rhs.__current;
+  }
+  template <__not_a_const_iterator _Iter2>
+  friend _LIBCPP_HIDE_FROM_ABI constexpr bool operator<=(const _Iter2& __lhs, const basic_const_iterator& __rhs)
+    requires random_access_iterator<_Iter> && totally_ordered_with<_Iter, _Iter2>
+  {
+    return __lhs <= __rhs.__current;
+  }
+  template <__not_a_const_iterator _Iter2>
+  friend _LIBCPP_HIDE_FROM_ABI constexpr bool operator>=(const _Iter2& __lhs, const basic_const_iterator& __rhs)
+    requires random_access_iterator<_Iter> && totally_ordered_with<_Iter, _Iter2>
+  {
+    return __lhs >= __rhs.__current;
+  }
+
+  friend _LIBCPP_HIDE_FROM_ABI constexpr basic_const_iterator
+  operator+(const basic_const_iterator& __it, difference_type __n)
+    requires random_access_iterator<_Iter>
+  {
+    return basic_const_iterator(__it.__current + __n);
+  }
+  friend _LIBCPP_HIDE_FROM_ABI constexpr basic_const_iterator
+  operator+(difference_type __n, const basic_const_iterator& __it)
+    requires random_access_iterator<_Iter>
+  {
+    return basic_const_iterator(__it.__current + __n);
+  }
+
+  friend _LIBCPP_HIDE_FROM_ABI constexpr basic_const_iterator
+  operator-(const basic_const_iterator& __it, difference_type __n)
+    requires random_access_iterator<_Iter>
+  {
+    return basic_const_iterator(__it.__current - __n);
+  }
+  template <sized_sentinel_for<_Iter> _Sent>
+  _LIBCPP_HIDE_FROM_ABI constexpr difference_type operator-(const _Sent& __rhs) const {
+    return __current - __rhs;
+  }
+  template <__not_a_const_iterator _Sent>
+    requires sized_sentinel_for<_Sent, _Iter>
+  friend _LIBCPP_HIDE_FROM_ABI constexpr difference_type
+  operator-(const _Sent& __lhs, const basic_const_iterator& __rhs) {
+    return __lhs - __rhs;
+  }
+
+  friend _LIBCPP_HIDE_FROM_ABI constexpr __rvalue_reference iter_move(const basic_const_iterator& __it) noexcept(
+      noexcept(static_cast<__rvalue_reference>(ranges::iter_move(__it.current_)))) {
+    return static_cast<__rvalue_reference>(ranges::iter_move(__it.current_));
+  }
+};
+
+template <class _Type1, common_with<_Type1> _Type2>
+  requires input_iterator<common_type_t<_Type1, _Type2>>
+struct common_type<basic_const_iterator<_Type1>, _Type2> {
+  using type = basic_const_iterator<common_type_t<_Type1, _Type2>>;
+};
+template <class _Type1, common_with<_Type1> _Type2>
+  requires input_iterator<common_type_t<_Type1, _Type2>>
+struct common_type<_Type2, basic_const_iterator<_Type1>> {
+  using type = basic_const_iterator<common_type_t<_Type1, _Type2>>;
+};
+template <class _Type1, common_with<_Type1> _Type2>
+  requires input_iterator<common_type_t<_Type1, _Type2>>
+struct common_type<basic_const_iterator<_Type1>, basic_const_iterator<_Type2>> {
+  using type = basic_const_iterator<common_type_t<_Type1, _Type2>>;
+};
+
+template <input_iterator _Iter>
+_LIBCPP_HIDE_FROM_ABI constexpr const_iterator<_Iter> make_const_iterator(_Iter __it) {
+  return __it;
+}
+template <semiregular _Sent>
+_LIBCPP_HIDE_FROM_ABI constexpr const_sentinel<_Sent> make_const_sentinel(_Sent __sent) {
+  return __sent;
+}
+
+#endif // _LIBCPP_STD_VER >= 23
+
+_LIBCPP_END_NAMESPACE_STD
+
+_LIBCPP_POP_MACROS
+
+#endif // _LIBCPP___ITERATOR_CONST_ITERATOR_H