[libcxx-commits] [libcxx] [libc++] Implement P0429R9 `std::flat_map` (PR #98643)

Louis Dionne via libcxx-commits libcxx-commits at lists.llvm.org
Fri Oct 18 11:15:36 PDT 2024


================
@@ -0,0 +1,1340 @@
+// -*- 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___FLAT_MAP_FLAT_MAP_H
+#define _LIBCPP___FLAT_MAP_FLAT_MAP_H
+
+#include <__algorithm/lexicographical_compare_three_way.h>
+#include <__algorithm/ranges_adjacent_find.h>
+#include <__algorithm/ranges_equal.h>
+#include <__algorithm/ranges_inplace_merge.h>
+#include <__algorithm/ranges_lower_bound.h>
+#include <__algorithm/ranges_partition_point.h>
+#include <__algorithm/ranges_stable_sort.h>
+#include <__algorithm/ranges_unique.h>
+#include <__algorithm/ranges_upper_bound.h>
+#include <__compare/synth_three_way.h>
+#include <__concepts/convertible_to.h>
+#include <__concepts/swappable.h>
+#include <__config>
+#include <__flat_map/sorted_unique.h>
+#include <__functional/invoke.h>
+#include <__functional/is_transparent.h>
+#include <__functional/operations.h>
+#include <__iterator/concepts.h>
+#include <__iterator/distance.h>
+#include <__iterator/iterator_traits.h>
+#include <__iterator/ranges_iterator_traits.h>
+#include <__iterator/reverse_iterator.h>
+#include <__memory/allocator_traits.h>
+#include <__memory/uses_allocator.h>
+#include <__memory/uses_allocator_construction.h>
+#include <__ranges/concepts.h>
+#include <__ranges/container_compatible_range.h>
+#include <__ranges/drop_view.h>
+#include <__ranges/ref_view.h>
+#include <__ranges/subrange.h>
+#include <__ranges/zip_view.h>
+#include <__type_traits/conjunction.h>
+#include <__type_traits/container_traits.h>
+#include <__type_traits/invoke.h>
+#include <__type_traits/is_allocator.h>
+#include <__type_traits/is_nothrow_constructible.h>
+#include <__type_traits/is_same.h>
+#include <__type_traits/maybe_const.h>
+#include <__utility/pair.h>
+#include <initializer_list>
+#include <stdexcept>
+#include <string>
+#include <vector>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+_LIBCPP_PUSH_MACROS
+#include <__undef_macros>
+
+#if _LIBCPP_STD_VER >= 23
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+template <class _Key,
+          class _Tp,
+          class _Compare         = less<_Key>,
+          class _KeyContainer    = vector<_Key>,
+          class _MappedContainer = vector<_Tp>>
+class flat_map {
+  template <bool _Const>
+  struct __iterator;
+
+  template <class, class, class, class, class>
+  friend class flat_map;
+
+  static_assert(is_same_v<_Key, typename _KeyContainer::value_type>);
+  static_assert(is_same_v<_Tp, typename _MappedContainer::value_type>);
+  static_assert(!is_same_v<_KeyContainer, std::vector<bool>>, "vector<bool> is not a sequence container");
+  static_assert(!is_same_v<_MappedContainer, std::vector<bool>>, "vector<bool> is not a sequence container");
+
+public:
+  // types
+  using key_type        = _Key;
+  using mapped_type     = _Tp;
+  using value_type      = pair<key_type, mapped_type>;
+  using key_compare     = __type_identity_t<_Compare>;
+  using reference       = pair<const key_type&, mapped_type&>;
+  using const_reference = pair<const key_type&, const mapped_type&>;
+  using size_type              = size_t;
+  using difference_type        = ptrdiff_t;
+  using iterator               = __iterator<false>; // see [container.requirements]
+  using const_iterator         = __iterator<true>;  // see [container.requirements]
+  using reverse_iterator       = std::reverse_iterator<iterator>;
+  using const_reverse_iterator = std::reverse_iterator<const_iterator>;
+  using key_container_type     = _KeyContainer;
+  using mapped_container_type  = _MappedContainer;
+
+  class value_compare {
+  private:
+    key_compare __comp_;
+    value_compare(key_compare __c) : __comp_(__c) {}
+    friend flat_map;
+
+  public:
+    _LIBCPP_HIDE_FROM_ABI bool operator()(const_reference __x, const_reference __y) const {
+      return __comp_(__x.first, __y.first);
+    }
+  };
+
+  struct containers {
+    key_container_type keys;
+    mapped_container_type values;
+  };
+
+private:
+  template <class _Allocator>
+  _LIBCPP_HIDE_FROM_ABI static constexpr bool __allocator_ctor_constraint =
+      _And<uses_allocator<key_container_type, _Allocator>, uses_allocator<mapped_container_type, _Allocator>>::value;
+
+  _LIBCPP_HIDE_FROM_ABI static constexpr bool __is_compare_transparent = __is_transparent_v<_Compare, _Compare>;
+
+  template <bool _Const>
+  struct __iterator {
+  private:
+    using __key_iterator    = ranges::iterator_t<const key_container_type>;
+    using __mapped_iterator = ranges::iterator_t<__maybe_const<_Const, mapped_container_type>>;
+    using __reference       = pair<iter_reference_t<__key_iterator>, iter_reference_t<__mapped_iterator>>;
+
+    struct __arrow_proxy {
+      __reference __ref_;
+      _LIBCPP_HIDE_FROM_ABI __reference* operator->() { return std::addressof(__ref_); }
+    };
+
+    __key_iterator __key_iter_;
+    __mapped_iterator __mapped_iter_;
+
+    friend flat_map;
+
+  public:
+    using iterator_concept = random_access_iterator_tag;
+    // `flat_map::iterator` only satisfy "Cpp17InputIterator" named requirements, because
+    // its `reference` is not a reference type.
+    // However, to avoid surprising runtime behaviour when it is used with the
+    // Cpp17 algorithms or operations, iterator_category is set to random_access_iterator_tag.
+    using iterator_category = random_access_iterator_tag;
+    using value_type        = flat_map::value_type;
+    using difference_type   = flat_map::difference_type;
+
+    _LIBCPP_HIDE_FROM_ABI __iterator() = default;
+
+    _LIBCPP_HIDE_FROM_ABI __iterator(__iterator<!_Const> __i)
+      requires _Const && convertible_to<ranges::iterator_t<key_container_type>, __key_iterator> &&
+                   convertible_to<ranges::iterator_t<mapped_container_type>, __mapped_iterator>
+        : __key_iter_(std::move(__i.__key_iter_)), __mapped_iter_(std::move(__i.__mapped_iter_)) {}
+
+    _LIBCPP_HIDE_FROM_ABI __iterator(__key_iterator __key_iter, __mapped_iterator __mapped_iter)
+        : __key_iter_(std::move(__key_iter)), __mapped_iter_(std::move(__mapped_iter)) {}
+
+    _LIBCPP_HIDE_FROM_ABI __reference operator*() const { return __reference(*__key_iter_, *__mapped_iter_); }
+    _LIBCPP_HIDE_FROM_ABI __arrow_proxy operator->() const { return __arrow_proxy(**this); }
+
+    _LIBCPP_HIDE_FROM_ABI __iterator& operator++() {
+      ++__key_iter_;
+      ++__mapped_iter_;
+      return *this;
+    }
+
+    _LIBCPP_HIDE_FROM_ABI __iterator operator++(int) {
+      __iterator __tmp(*this);
+      ++*this;
+      return __tmp;
+    }
+
+    _LIBCPP_HIDE_FROM_ABI __iterator& operator--() {
+      --__key_iter_;
+      --__mapped_iter_;
+      return *this;
+    }
+
+    _LIBCPP_HIDE_FROM_ABI __iterator operator--(int) {
+      __iterator __tmp(*this);
+      --*this;
+      return __tmp;
+    }
+
+    _LIBCPP_HIDE_FROM_ABI __iterator& operator+=(difference_type __x) {
+      __key_iter_ += __x;
+      __mapped_iter_ += __x;
+      return *this;
+    }
+
+    _LIBCPP_HIDE_FROM_ABI __iterator& operator-=(difference_type __x) {
+      __key_iter_ -= __x;
+      __mapped_iter_ -= __x;
+      return *this;
+    }
+
+    _LIBCPP_HIDE_FROM_ABI __reference operator[](difference_type __n) const { return *(*this + __n); }
+
+    _LIBCPP_HIDE_FROM_ABI friend constexpr bool operator==(const __iterator& __x, const __iterator& __y) {
+      return __x.__key_iter_ == __y.__key_iter_;
+    }
+
+    _LIBCPP_HIDE_FROM_ABI friend bool operator<(const __iterator& __x, const __iterator& __y) {
+      return __x.__key_iter_ < __y.__key_iter_;
+    }
+
+    _LIBCPP_HIDE_FROM_ABI friend bool operator>(const __iterator& __x, const __iterator& __y) { return __y < __x; }
+
+    _LIBCPP_HIDE_FROM_ABI friend bool operator<=(const __iterator& __x, const __iterator& __y) { return !(__y < __x); }
+
+    _LIBCPP_HIDE_FROM_ABI friend bool operator>=(const __iterator& __x, const __iterator& __y) { return !(__x < __y); }
+
+    _LIBCPP_HIDE_FROM_ABI friend auto operator<=>(const __iterator& __x, const __iterator& __y)
+      requires three_way_comparable<__key_iterator>
+    {
+      return __x.__key_iter_ <=> __y.__key_iter_;
+    }
+
+    _LIBCPP_HIDE_FROM_ABI friend __iterator operator+(const __iterator& __i, difference_type __n) {
+      auto __tmp = __i;
+      __tmp += __n;
+      return __tmp;
+    }
+
+    _LIBCPP_HIDE_FROM_ABI friend __iterator operator+(difference_type __n, const __iterator& __i) { return __i + __n; }
+
+    _LIBCPP_HIDE_FROM_ABI friend __iterator operator-(const __iterator& __i, difference_type __n) {
+      auto __tmp = __i;
+      __tmp -= __n;
+      return __tmp;
+    }
+
+    _LIBCPP_HIDE_FROM_ABI friend difference_type operator-(const __iterator& __x, const __iterator& __y) {
+      return difference_type(__x.__key_iter_ - __y.__key_iter_);
+    }
+  };
+
+public:
+  // [flat.map.cons], construct/copy/destroy
+  _LIBCPP_HIDE_FROM_ABI flat_map() noexcept(
+      is_nothrow_default_constructible_v<_KeyContainer> && is_nothrow_default_constructible_v<_MappedContainer> &&
+      is_nothrow_default_constructible_v<_Compare>)
+      : __containers_(), __compare_() {}
+
+  // The copy/move constructors are not specified in the spec, which means they should be defaulted.
+  // However, the move constructor can potentially leave a moved-from object in an inconsistent
+  // state if an exception is thrown.
+  _LIBCPP_HIDE_FROM_ABI flat_map(const flat_map&) = default;
+
+  _LIBCPP_HIDE_FROM_ABI flat_map(flat_map&& __other) noexcept(
+      is_nothrow_move_constructible_v<_KeyContainer> && is_nothrow_move_constructible_v<_MappedContainer> &&
+      is_nothrow_move_constructible_v<_Compare>) try
+      : __containers_(std::move(__other.__containers_)), __compare_(std::move(__other.__compare_)) {
+    __other.clear();
+  } catch (...) {
+    __other.clear();
+    throw;
+  }
+
+  template <class _Allocator>
+    requires __allocator_ctor_constraint<_Allocator>
+  _LIBCPP_HIDE_FROM_ABI flat_map(const flat_map& __other, const _Allocator& __alloc)
+      : flat_map(__ctor_uses_allocator_tag{},
+                 __alloc,
+                 __other.__containers_.keys,
+                 __other.__containers_.values,
+                 __other.__compare_) {}
+
+  template <class _Allocator>
+    requires __allocator_ctor_constraint<_Allocator>
+  _LIBCPP_HIDE_FROM_ABI flat_map(flat_map&& __other, const _Allocator& __alloc) try
+      : flat_map(__ctor_uses_allocator_tag{},
+                 __alloc,
+                 std::move(__other.__containers_.keys),
+                 std::move(__other.__containers_.values),
+                 std::move(__other.__compare_)) {
+    __other.clear();
+  } catch (...) {
+    __other.clear();
+    throw;
+  }
+
+  _LIBCPP_HIDE_FROM_ABI flat_map(
+      key_container_type __key_cont, mapped_container_type __mapped_cont, const key_compare& __comp = key_compare())
+      : __containers_{.keys = std::move(__key_cont), .values = std::move(__mapped_cont)}, __compare_(__comp) {
+    _LIBCPP_ASSERT_VALID_INPUT_RANGE(__containers_.keys.size() == __containers_.values.size(),
+                                     "flat_map keys and mapped containers have different size");
+    __sort_and_unique();
+  }
+
+  template <class _Allocator>
+    requires __allocator_ctor_constraint<_Allocator>
+  _LIBCPP_HIDE_FROM_ABI
+  flat_map(const key_container_type& __key_cont, const mapped_container_type& __mapped_cont, const _Allocator& __alloc)
+      : flat_map(__ctor_uses_allocator_tag{}, __alloc, __key_cont, __mapped_cont) {
+    _LIBCPP_ASSERT_VALID_INPUT_RANGE(__containers_.keys.size() == __containers_.values.size(),
+                                     "flat_map keys and mapped containers have different size");
+    __sort_and_unique();
+  }
+
+  template <class _Allocator>
+    requires __allocator_ctor_constraint<_Allocator>
+  _LIBCPP_HIDE_FROM_ABI
+  flat_map(const key_container_type& __key_cont,
+           const mapped_container_type& __mapped_cont,
+           const key_compare& __comp,
+           const _Allocator& __alloc)
+      : flat_map(__ctor_uses_allocator_tag{}, __alloc, __key_cont, __mapped_cont, __comp) {
+    _LIBCPP_ASSERT_VALID_INPUT_RANGE(__containers_.keys.size() == __containers_.values.size(),
+                                     "flat_map keys and mapped containers have different size");
+    __sort_and_unique();
+  }
+
+  _LIBCPP_HIDE_FROM_ABI
+  flat_map(sorted_unique_t,
+           key_container_type __key_cont,
+           mapped_container_type __mapped_cont,
+           const key_compare& __comp = key_compare())
+      : __containers_{.keys = std::move(__key_cont), .values = std::move(__mapped_cont)}, __compare_(__comp) {
+    _LIBCPP_ASSERT_VALID_INPUT_RANGE(__containers_.keys.size() == __containers_.values.size(),
+                                     "flat_map keys and mapped containers have different size");
+    _LIBCPP_ASSERT_SEMANTIC_REQUIREMENT(
+        __is_sorted_and_unique(__containers_.keys), "Either the key container is not sorted or it contains duplicates");
+  }
+
+  template <class _Allocator>
+    requires __allocator_ctor_constraint<_Allocator>
+  _LIBCPP_HIDE_FROM_ABI
+  flat_map(sorted_unique_t,
+           const key_container_type& __key_cont,
+           const mapped_container_type& __mapped_cont,
+           const _Allocator& __alloc)
+      : flat_map(__ctor_uses_allocator_tag{}, __alloc, __key_cont, __mapped_cont) {
+    _LIBCPP_ASSERT_VALID_INPUT_RANGE(__containers_.keys.size() == __containers_.values.size(),
+                                     "flat_map keys and mapped containers have different size");
+    _LIBCPP_ASSERT_SEMANTIC_REQUIREMENT(
+        __is_sorted_and_unique(__containers_.keys), "Either the key container is not sorted or it contains duplicates");
+  }
+
+  template <class _Allocator>
+    requires __allocator_ctor_constraint<_Allocator>
+  _LIBCPP_HIDE_FROM_ABI
+  flat_map(sorted_unique_t,
+           const key_container_type& __key_cont,
+           const mapped_container_type& __mapped_cont,
+           const key_compare& __comp,
+           const _Allocator& __alloc)
+      : flat_map(__ctor_uses_allocator_tag{}, __alloc, __key_cont, __mapped_cont, __comp) {
+    _LIBCPP_ASSERT_VALID_INPUT_RANGE(__containers_.keys.size() == __containers_.values.size(),
+                                     "flat_map keys and mapped containers have different size");
+    _LIBCPP_ASSERT_SEMANTIC_REQUIREMENT(
+        __is_sorted_and_unique(__containers_.keys), "Either the key container is not sorted or it contains duplicates");
+  }
+
+  _LIBCPP_HIDE_FROM_ABI explicit flat_map(const key_compare& __comp) : __containers_(), __compare_(__comp) {}
+
+  template <class _Allocator>
+    requires __allocator_ctor_constraint<_Allocator>
+  _LIBCPP_HIDE_FROM_ABI flat_map(const key_compare& __comp, const _Allocator& __alloc)
+      : flat_map(__ctor_uses_allocator_empty_tag{}, __alloc, __comp) {}
+
+  template <class _Allocator>
+    requires __allocator_ctor_constraint<_Allocator>
+  _LIBCPP_HIDE_FROM_ABI explicit flat_map(const _Allocator& __alloc)
+      : flat_map(__ctor_uses_allocator_empty_tag{}, __alloc) {}
+
+  template <class _InputIterator>
+    requires __has_input_iterator_category<_InputIterator>::value
+  _LIBCPP_HIDE_FROM_ABI
+  flat_map(_InputIterator __first, _InputIterator __last, const key_compare& __comp = key_compare())
+      : __containers_(), __compare_(__comp) {
+    insert(__first, __last);
+  }
+
+  template <class _InputIterator, class _Allocator>
+    requires(__has_input_iterator_category<_InputIterator>::value && __allocator_ctor_constraint<_Allocator>)
+  _LIBCPP_HIDE_FROM_ABI
+  flat_map(_InputIterator __first, _InputIterator __last, const key_compare& __comp, const _Allocator& __alloc)
+      : flat_map(__ctor_uses_allocator_empty_tag{}, __alloc, __comp) {
+    insert(__first, __last);
+  }
+
+  template <class _InputIterator, class _Allocator>
+    requires(__has_input_iterator_category<_InputIterator>::value && __allocator_ctor_constraint<_Allocator>)
+  _LIBCPP_HIDE_FROM_ABI flat_map(_InputIterator __first, _InputIterator __last, const _Allocator& __alloc)
+      : flat_map(__ctor_uses_allocator_empty_tag{}, __alloc) {
+    insert(__first, __last);
+  }
+
+  template <_ContainerCompatibleRange<value_type> _Range>
+  _LIBCPP_HIDE_FROM_ABI flat_map(from_range_t __fr, _Range&& __rg)
+      : flat_map(__fr, std::forward<_Range>(__rg), key_compare()) {}
+
+  template <_ContainerCompatibleRange<value_type> _Range, class _Allocator>
+    requires __allocator_ctor_constraint<_Allocator>
+  _LIBCPP_HIDE_FROM_ABI flat_map(from_range_t, _Range&& __rg, const _Allocator& __alloc)
+      : flat_map(__ctor_uses_allocator_empty_tag{}, __alloc) {
+    insert_range(std::forward<_Range>(__rg));
+  }
+
+  template <_ContainerCompatibleRange<value_type> _Range>
+  _LIBCPP_HIDE_FROM_ABI flat_map(from_range_t, _Range&& __rg, const key_compare& __comp) : flat_map(__comp) {
+    insert_range(std::forward<_Range>(__rg));
+  }
+
+  template <_ContainerCompatibleRange<value_type> _Range, class _Allocator>
+    requires __allocator_ctor_constraint<_Allocator>
+  _LIBCPP_HIDE_FROM_ABI flat_map(from_range_t, _Range&& __rg, const key_compare& __comp, const _Allocator& __alloc)
+      : flat_map(__ctor_uses_allocator_empty_tag{}, __alloc, __comp) {
+    insert_range(std::forward<_Range>(__rg));
+  }
+
+  template <class _InputIterator>
+    requires __has_input_iterator_category<_InputIterator>::value
+  _LIBCPP_HIDE_FROM_ABI
+  flat_map(sorted_unique_t, _InputIterator __first, _InputIterator __last, const key_compare& __comp = key_compare())
+      : __containers_(), __compare_(__comp) {
+    insert(sorted_unique, __first, __last);
+  }
+  template <class _InputIterator, class _Allocator>
+    requires(__has_input_iterator_category<_InputIterator>::value && __allocator_ctor_constraint<_Allocator>)
+  _LIBCPP_HIDE_FROM_ABI
+  flat_map(sorted_unique_t,
+           _InputIterator __first,
+           _InputIterator __last,
+           const key_compare& __comp,
+           const _Allocator& __alloc)
+      : flat_map(__ctor_uses_allocator_empty_tag{}, __alloc, __comp) {
+    insert(sorted_unique, __first, __last);
+  }
+
+  template <class _InputIterator, class _Allocator>
+    requires(__has_input_iterator_category<_InputIterator>::value && __allocator_ctor_constraint<_Allocator>)
+  _LIBCPP_HIDE_FROM_ABI
+  flat_map(sorted_unique_t, _InputIterator __first, _InputIterator __last, const _Allocator& __alloc)
+      : flat_map(__ctor_uses_allocator_empty_tag{}, __alloc) {
+    insert(sorted_unique, __first, __last);
+  }
+
+  _LIBCPP_HIDE_FROM_ABI flat_map(initializer_list<value_type> __il, const key_compare& __comp = key_compare())
+      : flat_map(__il.begin(), __il.end(), __comp) {}
+
+  template <class _Allocator>
+    requires __allocator_ctor_constraint<_Allocator>
+  _LIBCPP_HIDE_FROM_ABI
+  flat_map(initializer_list<value_type> __il, const key_compare& __comp, const _Allocator& __alloc)
+      : flat_map(__il.begin(), __il.end(), __comp, __alloc) {}
+
+  template <class _Allocator>
+    requires __allocator_ctor_constraint<_Allocator>
+  _LIBCPP_HIDE_FROM_ABI flat_map(initializer_list<value_type> __il, const _Allocator& __alloc)
+      : flat_map(__il.begin(), __il.end(), __alloc) {}
+
+  _LIBCPP_HIDE_FROM_ABI
+  flat_map(sorted_unique_t, initializer_list<value_type> __il, const key_compare& __comp = key_compare())
+      : flat_map(sorted_unique, __il.begin(), __il.end(), __comp) {}
+
+  template <class _Allocator>
+    requires __allocator_ctor_constraint<_Allocator>
+  _LIBCPP_HIDE_FROM_ABI
+  flat_map(sorted_unique_t, initializer_list<value_type> __il, const key_compare& __comp, const _Allocator& __alloc)
+      : flat_map(sorted_unique, __il.begin(), __il.end(), __comp, __alloc) {}
+
+  template <class _Allocator>
+    requires __allocator_ctor_constraint<_Allocator>
+  _LIBCPP_HIDE_FROM_ABI flat_map(sorted_unique_t, initializer_list<value_type> __il, const _Allocator& __alloc)
+      : flat_map(sorted_unique, __il.begin(), __il.end(), __alloc) {}
+
+  _LIBCPP_HIDE_FROM_ABI flat_map& operator=(initializer_list<value_type> __il) {
+    clear();
+    insert(__il);
+    return *this;
+  }
+
+  // copy/move assignment are not specified in the spec (defaulted)
+  // but move assignment can potentially leave moved from object in an inconsistent
+  // state if an exception is thrown
+  _LIBCPP_HIDE_FROM_ABI flat_map& operator=(const flat_map&) = default;
+
+  _LIBCPP_HIDE_FROM_ABI flat_map& operator=(flat_map&& __other) noexcept(
+      is_nothrow_move_assignable_v<_KeyContainer> && is_nothrow_move_assignable_v<_MappedContainer> &&
+      is_nothrow_move_assignable_v<_Compare>) {
+    auto __clear_other_guard = std::__make_scope_guard([&]() noexcept { __other.clear() /* noexcept */; });
+    auto __clear_self_guard  = std::__make_exception_guard([&]() noexcept { clear() /* noexcept */; });
+    __containers_            = std::move(__other.__containers_);
+    __compare_               = std::move(__other.__compare_);
+    __clear_self_guard.__complete();
+    return *this;
+  }
+
+  // iterators
+  _LIBCPP_HIDE_FROM_ABI iterator begin() noexcept {
+    return iterator(__containers_.keys.begin(), __containers_.values.begin());
+  }
+
+  _LIBCPP_HIDE_FROM_ABI const_iterator begin() const noexcept {
+    return const_iterator(__containers_.keys.begin(), __containers_.values.begin());
+  }
+
+  _LIBCPP_HIDE_FROM_ABI iterator end() noexcept {
+    return iterator(__containers_.keys.end(), __containers_.values.end());
+  }
+
+  _LIBCPP_HIDE_FROM_ABI const_iterator end() const noexcept {
+    return const_iterator(__containers_.keys.end(), __containers_.values.end());
+  }
+
+  _LIBCPP_HIDE_FROM_ABI reverse_iterator rbegin() noexcept { return reverse_iterator(end()); }
+  _LIBCPP_HIDE_FROM_ABI const_reverse_iterator rbegin() const noexcept { return const_reverse_iterator(end()); }
+  _LIBCPP_HIDE_FROM_ABI reverse_iterator rend() noexcept { return reverse_iterator(begin()); }
+  _LIBCPP_HIDE_FROM_ABI const_reverse_iterator rend() const noexcept { return const_reverse_iterator(begin()); }
+
+  _LIBCPP_HIDE_FROM_ABI const_iterator cbegin() const noexcept { return begin(); }
+  _LIBCPP_HIDE_FROM_ABI const_iterator cend() const noexcept { return end(); }
+  _LIBCPP_HIDE_FROM_ABI const_reverse_iterator crbegin() const noexcept { return const_reverse_iterator(end()); }
+  _LIBCPP_HIDE_FROM_ABI const_reverse_iterator crend() const noexcept { return const_reverse_iterator(begin()); }
+
+  // [flat.map.capacity], capacity
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI bool empty() const noexcept { return __containers_.keys.empty(); }
+
+  _LIBCPP_HIDE_FROM_ABI size_type size() const noexcept { return __containers_.keys.size(); }
+
+  _LIBCPP_HIDE_FROM_ABI size_type max_size() const noexcept {
+    return std::min<size_type>(__containers_.keys.max_size(), __containers_.values.max_size());
+  }
+
+  // [flat.map.access], element access
+  _LIBCPP_HIDE_FROM_ABI mapped_type& operator[](const key_type& __x)
+    requires is_constructible_v<mapped_type>
+  {
+    return try_emplace(__x).first->second;
+  }
+
+  _LIBCPP_HIDE_FROM_ABI mapped_type& operator[](key_type&& __x)
+    requires is_constructible_v<mapped_type>
+  {
+    return try_emplace(std::move(__x)).first->second;
+  }
+
+  template <class _Kp>
+    requires(__is_compare_transparent && is_constructible_v<key_type, _Kp> && is_constructible_v<mapped_type> &&
+             !is_convertible_v<_Kp &&, const_iterator> && !is_convertible_v<_Kp &&, iterator>)
+  _LIBCPP_HIDE_FROM_ABI mapped_type& operator[](_Kp&& __x) {
+    return try_emplace(std::forward<_Kp>(__x)).first->second;
+  }
+
+  _LIBCPP_HIDE_FROM_ABI mapped_type& at(const key_type& __x) {
+    auto __it = find(__x);
+    if (__it == end()) {
+      std::__throw_out_of_range("flat_map::at(const key_type&): Key does not exist");
+    }
+    return (*__it).second;
+  }
+
+  _LIBCPP_HIDE_FROM_ABI const mapped_type& at(const key_type& __x) const {
+    auto __it = find(__x);
+    if (__it == end()) {
+      std::__throw_out_of_range("flat_map::at(const key_type&) const: Key does not exist");
+    }
+    return (*__it).second;
+  }
+
+  template <class _Kp>
+    requires __is_compare_transparent
+  _LIBCPP_HIDE_FROM_ABI mapped_type& at(const _Kp& __x) {
+    static_assert(requires { find(__x); }, "flat_map::at(const K& x): find(x) needs to be well-formed");
+    auto __it = find(__x);
+    if (__it == end()) {
+      std::__throw_out_of_range("flat_map::at(const K&): Key does not exist");
+    }
+    return (*__it).second;
+  }
+
+  template <class _Kp>
+    requires __is_compare_transparent
+  _LIBCPP_HIDE_FROM_ABI const mapped_type& at(const _Kp& __x) const {
+    static_assert(requires { find(__x); }, "flat_map::at(const K& x) const: find(x) needs to be well-formed");
+    auto __it = find(__x);
+    if (__it == end()) {
+      std::__throw_out_of_range("flat_map::at(const K&) const: Key does not exist");
+    }
+    return (*__it).second;
+  }
+
+  // [flat.map.modifiers], modifiers
+  template <class... _Args>
+    requires is_constructible_v<pair<key_type, mapped_type>, _Args...>
+  // todo: LWG
+  // insufficiently constrained. key and values need to be move constructible
+  _LIBCPP_HIDE_FROM_ABI pair<iterator, bool> emplace(_Args&&... __args) {
+    std::pair<key_type, mapped_type> __pair(std::forward<_Args>(__args)...);
+    return __try_emplace(std::move(__pair.first), std::move(__pair.second));
+  }
+
+  template <class... _Args>
+    requires is_constructible_v<pair<key_type, mapped_type>, _Args...>
+  _LIBCPP_HIDE_FROM_ABI iterator emplace_hint(const_iterator __hint, _Args&&... __args) {
+    std::pair<key_type, mapped_type> __pair(std::forward<_Args>(__args)...);
+    return __try_emplace_hint(__hint, std::move(__pair.first), std::move(__pair.second)).first;
+  }
+
+  _LIBCPP_HIDE_FROM_ABI pair<iterator, bool> insert(const value_type& __x) { return emplace(__x); }
+
+  _LIBCPP_HIDE_FROM_ABI pair<iterator, bool> insert(value_type&& __x) { return emplace(std::move(__x)); }
+
+  _LIBCPP_HIDE_FROM_ABI iterator insert(const_iterator __hint, const value_type& __x) {
+    return emplace_hint(__hint, __x);
+  }
+
+  _LIBCPP_HIDE_FROM_ABI iterator insert(const_iterator __hint, value_type&& __x) {
+    return emplace_hint(__hint, std::move(__x));
+  }
+
+  template <class _Pp>
+    requires is_constructible_v<pair<key_type, mapped_type>, _Pp>
+  _LIBCPP_HIDE_FROM_ABI pair<iterator, bool> insert(_Pp&& __x) {
+    return emplace(std::forward<_Pp>(__x));
+  }
+
+  template <class _Pp>
+    requires is_constructible_v<pair<key_type, mapped_type>, _Pp>
+  _LIBCPP_HIDE_FROM_ABI iterator insert(const_iterator __hint, _Pp&& __x) {
+    return emplace_hint(__hint, std::forward<_Pp>(__x));
+  }
+
+  template <class _InputIterator>
+    requires __has_input_iterator_category<_InputIterator>::value
+  _LIBCPP_HIDE_FROM_ABI void insert(_InputIterator __first, _InputIterator __last) {
+    if constexpr (sized_sentinel_for<_InputIterator, _InputIterator>) {
+      __reserve(__last - __first);
+    }
+    __append_sort_merge_unique</*WasSorted = */ false>(std::move(__first), std::move(__last));
+  }
+
+  template <class _InputIterator>
+    requires __has_input_iterator_category<_InputIterator>::value
+  void insert(sorted_unique_t, _InputIterator __first, _InputIterator __last) {
+    if constexpr (sized_sentinel_for<_InputIterator, _InputIterator>) {
+      __reserve(__last - __first);
+    }
+
+    __append_sort_merge_unique</*WasSorted = */ true>(std::move(__first), std::move(__last));
+  }
+
+  template <_ContainerCompatibleRange<value_type> _Range>
+  _LIBCPP_HIDE_FROM_ABI void insert_range(_Range&& __range) {
+    if constexpr (ranges::sized_range<_Range>) {
+      __reserve(ranges::size(__range));
+    }
+
+    __append_sort_merge_unique</*WasSorted = */ false>(ranges::begin(__range), ranges::end(__range));
+  }
+
+  _LIBCPP_HIDE_FROM_ABI void insert(initializer_list<value_type> __il) { insert(__il.begin(), __il.end()); }
+
+  _LIBCPP_HIDE_FROM_ABI void insert(sorted_unique_t, initializer_list<value_type> __il) {
+    insert(sorted_unique, __il.begin(), __il.end());
+  }
+
+  _LIBCPP_HIDE_FROM_ABI containers extract() && {
+    auto __guard = std::__make_scope_guard([&]() noexcept { clear() /* noexcept */; });
+    auto __ret   = std::move(__containers_);
+    return __ret;
+  }
+
+  _LIBCPP_HIDE_FROM_ABI void replace(key_container_type&& __key_cont, mapped_container_type&& __mapped_cont) {
+    _LIBCPP_ASSERT_VALID_INPUT_RANGE(
+        __key_cont.size() == __mapped_cont.size(), "flat_map keys and mapped containers have different size");
+
+    _LIBCPP_ASSERT_SEMANTIC_REQUIREMENT(
+        __is_sorted_and_unique(__key_cont), "Either the key container is not sorted or it contains duplicates");
+    auto __guard         = std::__make_exception_guard([&]() noexcept { clear() /* noexcept */; });
+    __containers_.keys   = std::move(__key_cont);
+    __containers_.values = std::move(__mapped_cont);
+    __guard.__complete();
+  }
+
+  template <class... _Args>
+    requires is_constructible_v<mapped_type, _Args...>
+  _LIBCPP_HIDE_FROM_ABI pair<iterator, bool> try_emplace(const key_type& __key, _Args&&... __args) {
+    return __try_emplace(__key, std::forward<_Args>(__args)...);
+  }
+
+  template <class... _Args>
+    requires is_constructible_v<mapped_type, _Args...>
+  _LIBCPP_HIDE_FROM_ABI pair<iterator, bool> try_emplace(key_type&& __key, _Args&&... __args) {
+    return __try_emplace(std::move(__key), std::forward<_Args>(__args)...);
+  }
+
+  template <class _Kp, class... _Args>
+    requires(__is_compare_transparent && is_constructible_v<key_type, _Kp> &&
+             is_constructible_v<mapped_type, _Args...> && !is_convertible_v<_Kp &&, const_iterator> &&
+             !is_convertible_v<_Kp &&, iterator>)
+  _LIBCPP_HIDE_FROM_ABI pair<iterator, bool> try_emplace(_Kp&& __key, _Args&&... __args) {
+    return __try_emplace(std::forward<_Kp>(__key), std::forward<_Args>(__args)...);
+  }
+
+  template <class... _Args>
+    requires is_constructible_v<mapped_type, _Args...>
+  _LIBCPP_HIDE_FROM_ABI iterator try_emplace(const_iterator __hint, const key_type& __key, _Args&&... __args) {
+    return __try_emplace_hint(__hint, __key, std::forward<_Args>(__args)...).first;
+  }
+
+  template <class... _Args>
+    requires is_constructible_v<mapped_type, _Args...>
+  _LIBCPP_HIDE_FROM_ABI iterator try_emplace(const_iterator __hint, key_type&& __key, _Args&&... __args) {
+    return __try_emplace_hint(__hint, std::move(__key), std::forward<_Args>(__args)...).first;
+  }
+
+  template <class _Kp, class... _Args>
+    requires __is_compare_transparent && is_constructible_v<key_type, _Kp> && is_constructible_v<mapped_type, _Args...>
+  _LIBCPP_HIDE_FROM_ABI iterator try_emplace(const_iterator __hint, _Kp&& __key, _Args&&... __args) {
+    return __try_emplace_hint(__hint, std::forward<_Kp>(__key), std::forward<_Args>(__args)...).first;
+  }
+
+  template <class _Mapped>
+    requires is_assignable_v<mapped_type&, _Mapped> && is_constructible_v<mapped_type, _Mapped>
+  _LIBCPP_HIDE_FROM_ABI pair<iterator, bool> insert_or_assign(const key_type& __key, _Mapped&& __obj) {
+    return __insert_or_assign(__key, std::forward<_Mapped>(__obj));
+  }
+
+  template <class _Mapped>
+    requires is_assignable_v<mapped_type&, _Mapped> && is_constructible_v<mapped_type, _Mapped>
+  _LIBCPP_HIDE_FROM_ABI pair<iterator, bool> insert_or_assign(key_type&& __key, _Mapped&& __obj) {
+    return __insert_or_assign(std::move(__key), std::forward<_Mapped>(__obj));
+  }
+
+  template <class _Kp, class _Mapped>
+    requires __is_compare_transparent && is_constructible_v<key_type, _Kp> && is_assignable_v<mapped_type&, _Mapped> &&
+             is_constructible_v<mapped_type, _Mapped>
+  _LIBCPP_HIDE_FROM_ABI pair<iterator, bool> insert_or_assign(_Kp&& __key, _Mapped&& __obj) {
+    return __insert_or_assign(std::forward<_Kp>(__key), std::forward<_Mapped>(__obj));
+  }
+
+  template <class _Mapped>
+    requires is_assignable_v<mapped_type&, _Mapped> && is_constructible_v<mapped_type, _Mapped>
+  _LIBCPP_HIDE_FROM_ABI iterator insert_or_assign(const_iterator __hint, const key_type& __key, _Mapped&& __obj) {
+    return __insert_or_assign(__hint, __key, std::forward<_Mapped>(__obj));
+  }
+
+  template <class _Mapped>
+    requires is_assignable_v<mapped_type&, _Mapped> && is_constructible_v<mapped_type, _Mapped>
+  _LIBCPP_HIDE_FROM_ABI iterator insert_or_assign(const_iterator __hint, key_type&& __key, _Mapped&& __obj) {
+    return __insert_or_assign(__hint, std::move(__key), std::forward<_Mapped>(__obj));
+  }
+
+  template <class _Kp, class _Mapped>
+    requires __is_compare_transparent && is_constructible_v<key_type, _Kp> && is_assignable_v<mapped_type&, _Mapped> &&
+             is_constructible_v<mapped_type, _Mapped>
+  _LIBCPP_HIDE_FROM_ABI iterator insert_or_assign(const_iterator __hint, _Kp&& __key, _Mapped&& __obj) {
+    return __insert_or_assign(__hint, std::forward<_Kp>(__key), std::forward<_Mapped>(__obj));
+  }
+
+  _LIBCPP_HIDE_FROM_ABI iterator erase(iterator __position) {
+    return __erase(__position.__key_iter_, __position.__mapped_iter_);
+  }
+
+  _LIBCPP_HIDE_FROM_ABI iterator erase(const_iterator __position) {
+    return __erase(__position.__key_iter_, __position.__mapped_iter_);
+  }
+
+  _LIBCPP_HIDE_FROM_ABI size_type erase(const key_type& __x) {
+    auto __iter = find(__x);
+    if (__iter != end()) {
+      erase(__iter);
+      return 1;
+    }
+    return 0;
+  }
+
+  template <class _Kp>
+    requires(__is_compare_transparent && !is_convertible_v<_Kp &&, iterator> &&
+             !is_convertible_v<_Kp &&, const_iterator>)
+  _LIBCPP_HIDE_FROM_ABI size_type erase(_Kp&& __x) {
+    auto [__first, __last] = equal_range(__x);
+    auto __res             = __last - __first;
+    erase(__first, __last);
+    return __res;
+  }
+
+  _LIBCPP_HIDE_FROM_ABI iterator erase(const_iterator __first, const_iterator __last) {
+    auto __on_failure = std::__make_exception_guard([&]() noexcept { clear() /* noexcept */; });
+    auto __key_it     = __containers_.keys.erase(__first.__key_iter_, __last.__key_iter_);
+    auto __mapped_it  = __containers_.values.erase(__first.__mapped_iter_, __last.__mapped_iter_);
+    __on_failure.__complete();
+    return iterator(std::move(__key_it), std::move(__mapped_it));
+  }
+
+  _LIBCPP_HIDE_FROM_ABI void swap(flat_map& __y) noexcept {
+    // warning: The spec has unconditional noexcept, which means that
+    // if any of the following functions throw an exception,
+    // std::terminate will be called
+    ranges::swap(__compare_, __y.__compare_);
+    ranges::swap(__containers_.keys, __y.__containers_.keys);
+    ranges::swap(__containers_.values, __y.__containers_.values);
+  }
+
+  _LIBCPP_HIDE_FROM_ABI void clear() noexcept {
+    __containers_.keys.clear();
+    __containers_.values.clear();
+  }
+
+  // observers
+  _LIBCPP_HIDE_FROM_ABI key_compare key_comp() const { return __compare_; }
+  _LIBCPP_HIDE_FROM_ABI value_compare value_comp() const { return value_compare(__compare_); }
+
+  // todo: can flat_map | std::views::keys be specialised?
+  _LIBCPP_HIDE_FROM_ABI const key_container_type& keys() const noexcept { return __containers_.keys; }
+  _LIBCPP_HIDE_FROM_ABI const mapped_container_type& values() const noexcept { return __containers_.values; }
----------------
ldionne wrote:

I think that's an interesting suggestion. I think that would have to be a paper.

https://github.com/llvm/llvm-project/pull/98643


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