P2278R4: cbegin should always return a constant iterator ("Ranges" …

…and "Span" sections) (#3187) Co-authored-by: Stephan T. Lavavej <stl@nuwen.net>
microsoft · Nov 11, 2022 · 0b13eb0 · 0b13eb0
1 parent 0743f64
commit 0b13eb0
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Showing 15 changed files with 856 additions and 88 deletions.
diff --git a/stl/inc/ranges b/stl/inc/ranges
@@ -33,6 +33,17 @@ namespace ranges {
     template <class _Ty>
     inline constexpr bool _Is_initializer_list = _Is_specialization_v<remove_cvref_t<_Ty>, initializer_list>;
 
+#if _HAS_CXX23
+    _EXPORT_STD template <range _Rng>
+    using const_iterator_t = const_iterator<iterator_t<_Rng>>;
+
+    _EXPORT_STD template <range _Rng>
+    using const_sentinel_t = const_sentinel<sentinel_t<_Rng>>;
+
+    _EXPORT_STD template <range _Rng>
+    using range_const_reference_t = iter_const_reference_t<iterator_t<_Rng>>;
+#endif // _HAS_CXX23
+
     // clang-format off
     _EXPORT_STD template <class _Rng>
     concept viewable_range = range<_Rng>
@@ -45,20 +56,19 @@ namespace ranges {
     concept _Simple_view = view<_Rng> && range<const _Rng>
         && same_as<iterator_t<_Rng>, iterator_t<const _Rng>>
         && same_as<sentinel_t<_Rng>, sentinel_t<const _Rng>>;
+    // clang-format on
 
     template <class _Ty>
     concept _Valid_movable_box_object =
 #if _HAS_CXX23
-    move_constructible<_Ty>
+        move_constructible<_Ty>
 #else // ^^^ C++23 / C++20 vvv
-    copy_constructible<_Ty>
+        copy_constructible<_Ty>
 #endif // C++20
-    && _Destructible_object<_Ty>;
+        && _Destructible_object<_Ty>;
 
     template <class _It>
-    concept _Has_arrow = input_iterator<_It>
-        && (is_pointer_v<_It> || _Has_member_arrow<_It&>);
-    // clang-format on
+    concept _Has_arrow = input_iterator<_It> && (is_pointer_v<_It> || _Has_member_arrow<_It&>);
 
     template <bool _IsConst, class _Ty>
     using _Maybe_const = conditional_t<_IsConst, const _Ty, _Ty>;

diff --git a/stl/inc/span b/stl/inc/span
@@ -17,7 +17,6 @@ _EMIT_STL_WARNING(STL4038, "The contents of <span> are available only with C++20
 #include <type_traits>
 #include <xutility>
 
-
 #pragma pack(push, _CRT_PACKING)
 #pragma warning(push, _STL_WARNING_LEVEL)
 #pragma warning(disable : _STL_DISABLED_WARNINGS)
@@ -312,16 +311,20 @@ private:
     using _Mybase::_Mysize;
 
 public:
-    using element_type     = _Ty;
-    using value_type       = remove_cv_t<_Ty>;
-    using size_type        = size_t;
-    using difference_type  = ptrdiff_t;
-    using pointer          = _Ty*;
-    using const_pointer    = const _Ty*;
-    using reference        = _Ty&;
-    using const_reference  = const _Ty&;
-    using iterator         = _Span_iterator<_Ty>;
-    using reverse_iterator = _STD reverse_iterator<iterator>;
+    using element_type           = _Ty;
+    using value_type             = remove_cv_t<_Ty>;
+    using size_type              = size_t;
+    using difference_type        = ptrdiff_t;
+    using pointer                = _Ty*;
+    using const_pointer          = const _Ty*;
+    using reference              = _Ty&;
+    using const_reference        = const _Ty&;
+    using iterator               = _Span_iterator<_Ty>;
+    using reverse_iterator       = _STD reverse_iterator<iterator>;
+#if _HAS_CXX23 && defined(__cpp_lib_concepts)
+    using const_iterator         = _STD const_iterator<iterator>;
+    using const_reverse_iterator = _STD const_iterator<reverse_iterator>;
+#endif // _HAS_CXX23 && defined(__cpp_lib_concepts)
 
     static constexpr size_type extent = _Extent;
 
@@ -617,6 +620,16 @@ public:
 #endif // _ITERATOR_DEBUG_LEVEL
     }
 
+#if _HAS_CXX23 && defined(__cpp_lib_concepts)
+    _NODISCARD constexpr const_iterator cbegin() const noexcept {
+        return begin();
+    }
+
+    _NODISCARD constexpr const_iterator cend() const noexcept {
+        return end();
+    }
+#endif // _HAS_CXX23 && defined(__cpp_lib_concepts)
+
     _NODISCARD constexpr reverse_iterator rbegin() const noexcept {
         return reverse_iterator{end()};
     }
@@ -625,6 +638,16 @@ public:
         return reverse_iterator{begin()};
     }
 
+#if _HAS_CXX23 && defined(__cpp_lib_concepts)
+    _NODISCARD constexpr const_reverse_iterator crbegin() const noexcept {
+        return rbegin();
+    }
+
+    _NODISCARD constexpr const_reverse_iterator crend() const noexcept {
+        return rend();
+    }
+#endif // _HAS_CXX23 && defined(__cpp_lib_concepts)
+
     _NODISCARD constexpr pointer _Unchecked_begin() const noexcept {
         return _Mydata;
     }