Standard library header <flat_map> (C++23)

 

C++

Compiler support

Freestanding and hosted

Language

Standard library

Standard library headers

Named requirements

Feature test macros (C++20)

Language support library

Concepts library (C++20)

Diagnostics library

Memory management library

Metaprogramming library (C++11)

General utilities library

Containers library

Iterators library

Ranges library (C++20)

Algorithms library

Strings library

Text processing library

Numerics library

Date and time library

Input/output library

Filesystem library (C++17)

Concurrency support library (C++11)

Execution control library (C++26)

Technical specifications

Symbols index

External libraries

 

Standard library headers

Language support <cfloat> <climits> <compare> (C++20) <contracts> (C++26) <coroutine> (C++20) <csetjmp> <csignal> <cstdarg> <cstddef> <cstdint> (C++11) <cstdlib> <exception> <initializer_list> (C++11)   <limits> <new> <source_location> (C++20) <stdfloat> (C++23) <typeinfo> <version> (C++20) Concepts <concepts> (C++20) Diagnostics <cassert> <cerrno> <debugging> (C++26) <stacktrace> (C++23) <stdexcept> <system_error> (C++11) Memory management <memory> <memory_resource> (C++17) <scoped_allocator> (C++11) Metaprogramming <meta> (C++26) <ratio> (C++11) <type_traits> (C++11)

General utilities <any> (C++17) <bitset> <bit> (C++20) <charconv> (C++17) <expected> (C++23) <format> (C++20) <functional> <optional> (C++17) <stdbit.h> (C++26) <tuple> (C++11) <typeindex> (C++11) <utility> <variant> (C++17) Containers <array> (C++11) <deque> <flat_map> (C++23) <flat_set> (C++23) <forward_list> (C++11) <hive> (C++26) <inplace_vector> (C++26)   <list> <map> <mdspan> (C++23) <queue> <set> <span> (C++20) <stack> <unordered_map> (C++11) <unordered_set> (C++11) <vector> Iterators <iterator> Ranges <generator> (C++23) <ranges> (C++20)

Algorithms <algorithm> <numeric> Strings <cctype> <cstring> <cuchar> (C++11) <cwchar> <cwctype> <string_view> (C++17) <string> Text processing <clocale> <codecvt> (C++11/17/26*) <locale> <regex> (C++11) <text_encoding> (C++26)   Numerics <cfenv> (C++11) <cmath> <complex> <linalg> (C++26) <numbers> (C++20) <random> (C++11) <simd> (C++26) <stdckdint.h> (C++26) <valarray> Time <chrono> (C++11) <ctime> C compatibility <ccomplex> (C++11/17/20*) <ciso646> (until C++20) <cstdalign> (C++11/17/20*) <cstdbool> (C++11/17/20*) <ctgmath> (C++11/17/20*)

Input/output <cinttypes> (C++11) <cstdio> <filesystem> (C++17) <fstream> <iomanip> <iosfwd> <iostream> <ios> <istream> <ostream> <print> (C++23) <spanstream> (C++23) <sstream> <streambuf> <strstream> (C++98/26*) <syncstream> (C++20) Concurrency support <atomic> (C++11) <barrier> (C++20) <condition_variable> (C++11) <future> (C++11) <hazard_pointer> (C++26) <latch> (C++20) <mutex> (C++11) <rcu> (C++26) <semaphore> (C++20) <shared_mutex> (C++14) <stdatomic.h> (C++23) <stop_token> (C++20) <thread> (C++11) Execution support <execution> (C++17)

 

This header is part of the containers library.

Includes
<compare> (C++20)	Three-way comparison operator support
<initializer_list> (C++11)	std::initializer_list class template
Classes
flat_map (C++23)	adapts two containers to provide a collection of key-value pairs, sorted by unique keys (class template)
flat_multimap (C++23)	adapts two containers to provide a collection of key-value pairs, sorted by keys (class template)
std::uses_allocator<std::flat_map> (C++23)	specializes the std::uses_allocator type trait (class template specialization)
std::uses_allocator<std::flat_multimap> (C++23)	specializes the std::uses_allocator type trait (class template specialization)
Functions
erase_if(std::flat_map) (C++23)	erases all elements satisfying specific criteria (function template)
erase_if(std::flat_multimap) (C++23)	erases all elements satisfying specific criteria (function template)
Tags
sorted_uniquesorted_unique_t (C++23)	indicates that elements of a range are sorted and unique (tag)
sorted_equivalentsorted_equivalent_t (C++23)	indicates that elements of a range are sorted (uniqueness is not required) (tag)

Synopsis

#include <compare>
#include <initializer_list>

namespace std {
  // class template flat_map
  template<class Key,
           class T,
           class Compare = less<Key>,
           class KeyContainer = vector<Key>,
           class MappedContainer = vector<T>>
  class flat_map;

  struct sorted_unique_t
  {
    explicit sorted_unique_t() = default;
  };
  inline constexpr sorted_unique_t sorted_unique{};

  template<class Key,
           class T,
           class Compare,
           class KeyContainer,
           class MappedContainer,
           class Allocator>
  struct uses_allocator<flat_map<Key, T, Compare, KeyContainer, MappedContainer>,
                        Allocator>;

  // erasure for flat_map
  template<class Key,
           class T,
           class Compare,
           class KeyContainer,
           class MappedContainer,
           class Predicate>
  constexpr typename flat_map<Key, T, Compare, KeyContainer, MappedContainer>::size_type
  erase_if(flat_map<Key, T, Compare, KeyContainer, MappedContainer>& c, Predicate pred);

  // class template flat_multimap
  template<class Key,
           class T,
           class Compare = less<Key>,
           class KeyContainer = vector<Key>,
           class MappedContainer = vector<T>>
  class flat_multimap;

  struct sorted_equivalent_t
  {
    explicit sorted_equivalent_t() = default;
  };
  inline constexpr sorted_equivalent_t sorted_equivalent{};

  template<class Key,
           class T,
           class Compare,
           class KeyContainer,
           class MappedContainer,
           class Allocator>
  struct uses_allocator<flat_multimap<Key, T, Compare, KeyContainer, MappedContainer>,
                        Allocator>;

  // erasure for flat_multimap
  template<class Key,
           class T,
           class Compare,
           class KeyContainer,
           class MappedContainer,
           class Predicate>
  constexpr
    typename flat_multimap<Key, T, Compare, KeyContainer, MappedContainer>::size_type
    erase_if(flat_multimap<Key, T, Compare, KeyContainer, MappedContainer>& c,
             Predicate pred);
}

Class template std::flat_map

namespace std {
  template<class Key,
           class T,
           class Compare = less<Key>,
           class KeyContainer = vector<Key>,
           class MappedContainer = vector<T>>
  class flat_map
  {
  public:
    // types
    using key_type = Key;
    using mapped_type = T;
    using value_type = pair<key_type, mapped_type>;
    using key_compare = 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 = /* implementation-defined */;
    using const_iterator = /* implementation-defined */;
    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*/; // exposition-only
      constexpr value_compare(key_compare c)
        : /*comp*/(c)
      {
      }                     // exposition-only

    public:
      constexpr 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;
    };

    // constructors
    constexpr flat_map()
      : flat_map(key_compare())
    {
    }

    constexpr flat_map(const flat_map&);
    constexpr flat_map(flat_map&&);
    constexpr flat_map& operator=(const flat_map&);
    constexpr flat_map& operator=(flat_map&&);

    constexpr explicit flat_map(const key_compare& comp)
      : /*c*/()
      , /*compare*/(comp)
    {
    }

    constexpr flat_map(key_container_type key_cont,
                       mapped_container_type mapped_cont,
                       const key_compare& comp = key_compare());

    constexpr flat_map(sorted_unique_t,
                       key_container_type key_cont,
                       mapped_container_type mapped_cont,
                       const key_compare& comp = key_compare());

    template<class InputIter>
    constexpr flat_map(InputIter first,
                       InputIter last,
                       const key_compare& comp = key_compare())
      : /*c*/()
      , /*compare*/(comp)
    {
      insert(first, last);
    }

    template<class InputIter>
    constexpr flat_map(sorted_unique_t,
                       InputIter first,
                       InputIter last,
                       const key_compare& comp = key_compare())
      : /*c*/()
      , /*compare*/(comp)
    {
      insert(sorted_unique, first, last);
    }

    template<container-compatible-range<value_type> R>
    constexpr flat_map(from_range_t, R&& rg)
      : flat_map(from_range, std::forward<R>(rg), key_compare())
    {
    }
    template<container-compatible-range<value_type> R>
    constexpr flat_map(from_range_t, R&& rg, const key_compare& comp)
      : flat_map(comp)
    {
      insert_range(std::forward<R>(rg));
    }

    constexpr flat_map(initializer_list<value_type> il,
                       const key_compare& comp = key_compare())
      : flat_map(il.begin(), il.end(), comp)
    {
    }

    constexpr 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)
    {
    }

    // constructors with allocators

    template<class Alloc> constexpr explicit flat_map(const Alloc& a);
    template<class Alloc> constexpr flat_map(const key_compare& comp, const Alloc& a);
    template<class Alloc>
    constexpr flat_map(const key_container_type& key_cont,
                       const mapped_container_type& mapped_cont,
                       const Alloc& a);
    template<class Alloc>
    constexpr flat_map(const key_container_type& key_cont,
                       const mapped_container_type& mapped_cont,
                       const key_compare& comp,
                       const Alloc& a);
    template<class Alloc>
    constexpr flat_map(sorted_unique_t,
                       const key_container_type& key_cont,
                       const mapped_container_type& mapped_cont,
                       const Alloc& a);
    template<class Alloc>
    constexpr flat_map(sorted_unique_t,
                       const key_container_type& key_cont,
                       const mapped_container_type& mapped_cont,
                       const key_compare& comp,
                       const Alloc& a);
    template<class Alloc> constexpr flat_map(const flat_map&, const Alloc& a);
    template<class Alloc> constexpr flat_map(flat_map&&, const Alloc& a);
    template<class InputIter, class Alloc>
    constexpr flat_map(InputIter first, InputIter last, const Alloc& a);
    template<class InputIter, class Alloc>
    constexpr flat_map(InputIter first,
                       InputIter last,
                       const key_compare& comp,
                       const Alloc& a);
    template<class InputIter, class Alloc>
    constexpr flat_map(sorted_unique_t, InputIter first, InputIter last, const Alloc& a);
    template<class InputIter, class Alloc>
    constexpr flat_map(sorted_unique_t,
                       InputIter first,
                       InputIter last,
                       const key_compare& comp,
                       const Alloc& a);
    template<container-compatible-range<value_type> R, class Alloc>
    constexpr flat_map(from_range_t, R&& rg, const Alloc& a);
    template<container-compatible-range<value_type> R, class Alloc>
    constexpr flat_map(from_range_t, R&& rg, const key_compare& comp, const Alloc& a);
    template<class Alloc>
    constexpr flat_map(initializer_list<value_type> il, const Alloc& a);
    template<class Alloc>
    constexpr flat_map(initializer_list<value_type> il,
                       const key_compare& comp,
                       const Alloc& a);
    template<class Alloc>
    constexpr flat_map(sorted_unique_t, initializer_list<value_type> il, const Alloc& a);
    template<class Alloc>
    constexpr flat_map(sorted_unique_t,
                       initializer_list<value_type> il,
                       const key_compare& comp,
                       const Alloc& a);

    constexpr flat_map& operator=(initializer_list<value_type>);

    // iterators
    constexpr iterator begin() noexcept;
    constexpr const_iterator begin() const noexcept;
    constexpr iterator end() noexcept;
    constexpr const_iterator end() const noexcept;

    constexpr reverse_iterator rbegin() noexcept;
    constexpr const_reverse_iterator rbegin() const noexcept;
    constexpr reverse_iterator rend() noexcept;
    constexpr const_reverse_iterator rend() const noexcept;

    constexpr const_iterator cbegin() const noexcept;
    constexpr const_iterator cend() const noexcept;
    constexpr const_reverse_iterator crbegin() const noexcept;
    constexpr const_reverse_iterator crend() const noexcept;

    // capacity
    constexpr bool empty() const noexcept;
    constexpr size_type size() const noexcept;
    constexpr size_type max_size() const noexcept;

    // element access
    constexpr mapped_type& operator[](const key_type& x);
    constexpr mapped_type& operator[](key_type&& x);
    template<class K> constexpr mapped_type& operator[](K&& x);
    constexpr mapped_type& at(const key_type& x);
    constexpr const mapped_type& at(const key_type& x) const;
    template<class K> constexpr mapped_type& at(const K& x);
    template<class K> constexpr const mapped_type& at(const K& x) const;

    // modifiers
    template<class... Args> constexpr pair<iterator, bool> emplace(Args&&... args);
    template<class... Args>
    constexpr iterator emplace_hint(const_iterator position, Args&&... args);

    constexpr pair<iterator, bool> insert(const value_type& x) { return emplace(x); }
    constexpr pair<iterator, bool> insert(value_type&& x)
    {
      return emplace(std::move(x));
    }
    constexpr iterator insert(const_iterator position, const value_type& x)
    {
      return emplace_hint(position, x);
    }
    constexpr iterator insert(const_iterator position, value_type&& x)
    {
      return emplace_hint(position, std::move(x));
    }

    template<class P> constexpr pair<iterator, bool> insert(P&& x);
    template<class P> constexpr iterator insert(const_iterator position, P&&);
    template<class InputIter> constexpr void insert(InputIter first, InputIter last);
    template<class InputIter>
    constexpr void insert(sorted_unique_t, InputIter first, InputIter last);
    template<container-compatible-range<value_type> R>
    constexpr void insert_range(R&& rg);
    template<container-compatible-range<value_type> R>
    constexpr void insert_range(sorted_unique_t, R&& rg);

    constexpr void insert(initializer_list<value_type> il)
    {
      insert(il.begin(), il.end());
    }
    constexpr void insert(sorted_unique_t, initializer_list<value_type> il)
    {
      insert(sorted_unique, il.begin(), il.end());
    }

    constexpr containers extract() &&;
    constexpr void replace(key_container_type&& key_cont,
                           mapped_container_type&& mapped_cont);

    template<class... Args>
    constexpr pair<iterator, bool> try_emplace(const key_type& k, Args&&... args);
    template<class... Args>
    constexpr pair<iterator, bool> try_emplace(key_type&& k, Args&&... args);
    template<class K, class... Args>
    constexpr pair<iterator, bool> try_emplace(K&& k, Args&&... args);
    template<class... Args>
    constexpr iterator try_emplace(const_iterator hint,
                                   const key_type& k,
                                   Args&&... args);
    template<class... Args>
    constexpr iterator try_emplace(const_iterator hint, key_type&& k, Args&&... args);
    template<class K, class... Args>
    constexpr iterator try_emplace(const_iterator hint, K&& k, Args&&... args);
    template<class M>
    constexpr pair<iterator, bool> insert_or_assign(const key_type& k, M&& obj);
    template<class M>
    constexpr pair<iterator, bool> insert_or_assign(key_type&& k, M&& obj);
    template<class K, class M>
    constexpr pair<iterator, bool> insert_or_assign(K&& k, M&& obj);
    template<class M>
    constexpr iterator insert_or_assign(const_iterator hint, const key_type& k, M&& obj);
    template<class M>
    constexpr iterator insert_or_assign(const_iterator hint, key_type&& k, M&& obj);
    template<class K, class M>
    constexpr iterator insert_or_assign(const_iterator hint, K&& k, M&& obj);

    constexpr iterator erase(iterator position);
    constexpr iterator erase(const_iterator position);
    constexpr size_type erase(const key_type& x);
    template<class K> constexpr size_type erase(K&& x);
    constexpr iterator erase(const_iterator first, const_iterator last);

    constexpr void swap(flat_map& y) noexcept(/* see description */);
    constexpr void clear() noexcept;

    // observers
    constexpr key_compare key_comp() const;
    constexpr value_compare value_comp() const;

    constexpr const key_container_type& keys() const noexcept { return /*c*/.keys; }
    constexpr const mapped_container_type& values() const noexcept
    {
      return /*c*/.values;
    }

    // map operations
    constexpr iterator find(const key_type& x);
    constexpr const_iterator find(const key_type& x) const;
    template<class K> constexpr iterator find(const K& x);
    template<class K> constexpr const_iterator find(const K& x) const;

    constexpr size_type count(const key_type& x) const;
    template<class K> constexpr size_type count(const K& x) const;

    constexpr bool contains(const key_type& x) const;
    template<class K> constexpr bool contains(const K& x) const;

    constexpr iterator lower_bound(const key_type& x);
    constexpr const_iterator lower_bound(const key_type& x) const;
    template<class K> constexpr iterator lower_bound(const K& x);
    template<class K> constexpr const_iterator lower_bound(const K& x) const;

    constexpr iterator upper_bound(const key_type& x);
    constexpr const_iterator upper_bound(const key_type& x) const;
    template<class K> constexpr iterator upper_bound(const K& x);
    template<class K> constexpr const_iterator upper_bound(const K& x) const;

    constexpr pair<iterator, iterator> equal_range(const key_type& x);
    constexpr pair<const_iterator, const_iterator> equal_range(const key_type& x) const;
    template<class K> constexpr pair<iterator, iterator> equal_range(const K& x);
    template<class K>
    constexpr pair<const_iterator, const_iterator> equal_range(const K& x) const;

    friend constexpr bool operator==(const flat_map& x, const flat_map& y);

    friend constexpr /*synth-three-way-result*/<value_type> operator<=>(
      const flat_map& x, const flat_map& y);

    friend constexpr void swap(flat_map& x, flat_map& y) noexcept(noexcept(x.swap(y)))
    {
      x.swap(y);
    }

  private:
    containers /*c*/;        // exposition-only
    key_compare /*compare*/; // exposition-only

    struct /*key-equiv*/
    {                        // exposition-only
      constexpr /*key-equiv*/(key_compare c)
        : comp(c)
      {
      }
      constexpr bool operator()(const_reference x, const_reference y) const
      {
        return !comp(x.first, y.first) && !comp(y.first, x.first);
      }
      key_compare comp;
    };
  };

  template<class KeyContainer,
           class MappedContainer,
           class Compare = less<typename KeyContainer::value_type>>
  flat_map(KeyContainer, MappedContainer, Compare = Compare())
    -> flat_map<typename KeyContainer::value_type,
                typename MappedContainer::value_type,
                Compare,
                KeyContainer,
                MappedContainer>;

  template<class KeyContainer, class MappedContainer, class Allocator>
  flat_map(KeyContainer, MappedContainer, Allocator)
    -> flat_map<typename KeyContainer::value_type,
                typename MappedContainer::value_type,
                less<typename KeyContainer::value_type>,
                KeyContainer,
                MappedContainer>;
  template<class KeyContainer, class MappedContainer, class Compare, class Allocator>
  flat_map(KeyContainer, MappedContainer, Compare, Allocator)
    -> flat_map<typename KeyContainer::value_type,
                typename MappedContainer::value_type,
                Compare,
                KeyContainer,
                MappedContainer>;

  template<class KeyContainer,
           class MappedContainer,
           class Compare = less<typename KeyContainer::value_type>>
  flat_map(sorted_unique_t, KeyContainer, MappedContainer, Compare = Compare())
    -> flat_map<typename KeyContainer::value_type,
                typename MappedContainer::value_type,
                Compare,
                KeyContainer,
                MappedContainer>;

  template<class KeyContainer, class MappedContainer, class Allocator>
  flat_map(sorted_unique_t, KeyContainer, MappedContainer, Allocator)
    -> flat_map<typename KeyContainer::value_type,
                typename MappedContainer::value_type,
                less<typename KeyContainer::value_type>,
                KeyContainer,
                MappedContainer>;
  template<class KeyContainer, class MappedContainer, class Compare, class Allocator>
  flat_map(sorted_unique_t, KeyContainer, MappedContainer, Compare, Allocator)
    -> flat_map<typename KeyContainer::value_type,
                typename MappedContainer::value_type,
                Compare,
                KeyContainer,
                MappedContainer>;

  template<class InputIter, class Compare = less</*iter-key-type*/<InputIter>>>
  flat_map(InputIter, InputIter, Compare = Compare())
    -> flat_map</*iter-key-type*/<InputIter>, /*iter-mapped-type*/<InputIter>, Compare>;

  template<class InputIter, class Compare = less</*iter-key-type*/<InputIter>>>
  flat_map(sorted_unique_t, InputIter, InputIter, Compare = Compare())
    -> flat_map</*iter-key-type*/<InputIter>, /*iter-mapped-type*/<InputIter>, Compare>;

  template<ranges::input_range R,
           class Compare = less</*range-key-type*/<R>>,
           class Allocator = allocator<byte>>
  flat_map(from_range_t, R&&, Compare = Compare(), Allocator = Allocator()) -> flat_map<
    /*range-key-type*/<R>,
    /*range-mapped-type*/<R>,
    Compare,
    vector</*range-key-type*/<R>, /*alloc-rebind*/<Allocator, /*range-key-type*/<R>>>,
    vector</*range-mapped-type*/<R>,
           /*alloc-rebind*/<Allocator, /*range-mapped-type*/<R>>>>;

  template<ranges::input_range R, class Allocator>
  flat_map(from_range_t, R&&, Allocator) -> flat_map<
    /*range-key-type*/<R>,
    /*range-mapped-type*/<R>,
    less</*range-key-type*/<R>>,
    vector</*range-key-type*/<R>, /*alloc-rebind*/<Allocator, /*range-key-type*/<R>>>,
    vector</*range-mapped-type*/<R>,
           /*alloc-rebind*/<Allocator, /*range-mapped-type*/<R>>>>;

  template<class Key, class T, class Compare = less<Key>>
  flat_map(initializer_list<pair<Key, T>>, Compare = Compare())
    -> flat_map<Key, T, Compare>;

  template<class Key, class T, class Compare = less<Key>>
  flat_map(sorted_unique_t, initializer_list<pair<Key, T>>, Compare = Compare())
    -> flat_map<Key, T, Compare>;

  template<class Key,
           class T,
           class Compare,
           class KeyContainer,
           class MappedContainer,
           class Allocator>
  struct uses_allocator<flat_map<Key, T, Compare, KeyContainer, MappedContainer>,
                        Allocator>
    : bool_constant<uses_allocator_v<KeyContainer, Allocator> &&
                    uses_allocator_v<MappedContainer, Allocator>>
  {};
}

Class template std::flat_multimap

namespace std {
  template<class Key,
           class T,
           class Compare = less<Key>,
           class KeyContainer = vector<Key>,
           class MappedContainer = vector<T>>
  class flat_multimap
  {
  public:
    // types
    using key_type = Key;
    using mapped_type = T;
    using value_type = pair<key_type, mapped_type>;
    using key_compare = 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 = /* implementation-defined */;
    using const_iterator = /* implementation-defined */;
    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*/; // exposition-only
      constexpr value_compare(key_compare c)
        : /*comp*/(c)
      {
      }                     // exposition-only

    public:
      constexpr 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;
    };

    // constructors
    constexpr flat_multimap()
      : flat_multimap(key_compare())
    {
    }

    constexpr flat_multimap(const flat_multimap&);
    constexpr flat_multimap(flat_multimap&&);
    constexpr flat_multimap& operator=(const flat_multimap&);
    constexpr flat_multimap& operator=(flat_multimap&&);

    constexpr explicit flat_multimap(const key_compare& comp)
      : /*c*/()
      , /*compare*/(comp)
    {
    }

    constexpr flat_multimap(key_container_type key_cont,
                            mapped_container_type mapped_cont,
                            const key_compare& comp = key_compare());

    constexpr flat_multimap(sorted_equivalent_t,
                            key_container_type key_cont,
                            mapped_container_type mapped_cont,
                            const key_compare& comp = key_compare());

    template<class InputIter>
    constexpr flat_multimap(InputIter first,
                            InputIter last,
                            const key_compare& comp = key_compare())
      : /*c*/()
      , /*compare*/(comp)
    {
      insert(first, last);
    }

    template<class InputIter>
    constexpr flat_multimap(sorted_equivalent_t,
                            InputIter first,
                            InputIter last,
                            const key_compare& comp = key_compare())
      : /*c*/()
      , /*compare*/(comp)
    {
      insert(sorted_equivalent, first, last);
    }

    template<container-compatible-range<value_type> R>
    constexpr flat_multimap(from_range_t, R&& rg)
      : flat_multimap(from_range, std::forward<R>(rg), key_compare())
    {
    }
    template<container-compatible-range<value_type> R>
    constexpr flat_multimap(from_range_t, R&& rg, const key_compare& comp)
      : flat_multimap(comp)
    {
      insert_range(std::forward<R>(rg));
    }

    constexpr flat_multimap(initializer_list<value_type> il,
                            const key_compare& comp = key_compare())
      : flat_multimap(il.begin(), il.end(), comp)
    {
    }

    constexpr flat_multimap(sorted_equivalent_t,
                            initializer_list<value_type> il,
                            const key_compare& comp = key_compare())
      : flat_multimap(sorted_equivalent, il.begin(), il.end(), comp)
    {
    }

    // constructors with allocators

    template<class Alloc> constexpr explicit flat_multimap(const Alloc& a);
    template<class Alloc>
    constexpr flat_multimap(const key_compare& comp, const Alloc& a);
    template<class Alloc>
    constexpr flat_multimap(const key_container_type& key_cont,
                            const mapped_container_type& mapped_cont,
                            const Alloc& a);
    template<class Alloc>
    constexpr flat_multimap(const key_container_type& key_cont,
                            const mapped_container_type& mapped_cont,
                            const key_compare& comp,
                            const Alloc& a);
    template<class Alloc>
    constexpr flat_multimap(sorted_equivalent_t,
                            const key_container_type& key_cont,
                            const mapped_container_type& mapped_cont,
                            const Alloc& a);
    template<class Alloc>
    constexpr flat_multimap(sorted_equivalent_t,
                            const key_container_type& key_cont,
                            const mapped_container_type& mapped_cont,
                            const key_compare& comp,
                            const Alloc& a);
    template<class Alloc> constexpr flat_multimap(const flat_multimap&, const Alloc& a);
    template<class Alloc> constexpr flat_multimap(flat_multimap&&, const Alloc& a);
    template<class InputIter, class Alloc>
    constexpr flat_multimap(InputIter first, InputIter last, const Alloc& a);
    template<class InputIter, class Alloc>
    constexpr flat_multimap(InputIter first,
                            InputIter last,
                            const key_compare& comp,
                            const Alloc& a);
    template<class InputIter, class Alloc>
    constexpr flat_multimap(sorted_equivalent_t,
                            InputIter first,
                            InputIter last,
                            const Alloc& a);
    template<class InputIter, class Alloc>
    constexpr flat_multimap(sorted_equivalent_t,
                            InputIter first,
                            InputIter last,
                            const key_compare& comp,
                            const Alloc& a);
    template<container-compatible-range<value_type> R, class Alloc>
    constexpr flat_multimap(from_range_t, R&& rg, const Alloc& a);
    template<container-compatible-range<value_type> R, class Alloc>
    constexpr flat_multimap(from_range_t,
                            R&& rg,
                            const key_compare& comp,
                            const Alloc& a);
    template<class Alloc>
    constexpr flat_multimap(initializer_list<value_type> il, const Alloc& a);
    template<class Alloc>
    constexpr flat_multimap(initializer_list<value_type> il,
                            const key_compare& comp,
                            const Alloc& a);
    template<class Alloc>
    constexpr flat_multimap(sorted_equivalent_t,
                            initializer_list<value_type> il,
                            const Alloc& a);
    template<class Alloc>
    constexpr flat_multimap(sorted_equivalent_t,
                            initializer_list<value_type> il,
                            const key_compare& comp,
                            const Alloc& a);

    flat_multimap& operator=(initializer_list<value_type>);

    // iterators
    constexpr iterator begin() noexcept;
    constexpr const_iterator begin() const noexcept;
    constexpr iterator end() noexcept;
    constexpr const_iterator end() const noexcept;

    constexpr reverse_iterator rbegin() noexcept;
    constexpr const_reverse_iterator rbegin() const noexcept;
    constexpr reverse_iterator rend() noexcept;
    constexpr const_reverse_iterator rend() const noexcept;

    constexpr const_iterator cbegin() const noexcept;
    constexpr const_iterator cend() const noexcept;
    constexpr const_reverse_iterator crbegin() const noexcept;
    constexpr const_reverse_iterator crend() const noexcept;

    // capacity
    constexpr bool empty() const noexcept;
    constexpr size_type size() const noexcept;
    constexpr size_type max_size() const noexcept;

    // modifiers
    template<class... Args> constexpr iterator emplace(Args&&... args);
    template<class... Args>
    constexpr iterator emplace_hint(const_iterator position, Args&&... args);

    constexpr iterator insert(const value_type& x) { return emplace(x); }
    constexpr iterator insert(value_type&& x) { return emplace(std::move(x)); }
    constexpr iterator insert(const_iterator position, const value_type& x)
    {
      return emplace_hint(position, x);
    }
    constexpr iterator insert(const_iterator position, value_type&& x)
    {
      return emplace_hint(position, std::move(x));
    }

    template<class P> constexpr iterator insert(P&& x);
    template<class P> constexpr iterator insert(const_iterator position, P&&);
    template<class InputIter> constexpr void insert(InputIter first, InputIter last);
    template<class InputIter>
    constexpr void insert(sorted_equivalent_t, InputIter first, InputIter last);
    template<container-compatible-range<value_type> R>
    constexpr void insert_range(R&& rg);
    template<container-compatible-range<value_type> R>
    constexpr void insert_range(sorted_equivalent_t, R&& rg);

    constexpr void insert(initializer_list<value_type> il)
    {
      insert(il.begin(), il.end());
    }
    constexpr void insert(sorted_equivalent_t, initializer_list<value_type> il)
    {
      insert(sorted_equivalent, il.begin(), il.end());
    }

    constexpr containers extract() &&;
    constexpr void replace(key_container_type&& key_cont,
                           mapped_container_type&& mapped_cont);

    constexpr iterator erase(iterator position);
    constexpr iterator erase(const_iterator position);
    constexpr size_type erase(const key_type& x);
    template<class K> constexpr size_type erase(K&& x);
    constexpr iterator erase(const_iterator first, const_iterator last);

    constexpr void swap(flat_multimap&) noexcept(
      is_nothrow_swappable_v<key_container_type>&& is_nothrow_swappable_v<
        mapped_container_type>&& is_nothrow_swappable_v<key_compare>);
    constexpr void clear() noexcept;

    // observers
    constexpr key_compare key_comp() const;
    constexpr value_compare value_comp() const;

    constexpr const key_container_type& keys() const noexcept { return /*c*/.keys; }
    constexpr const mapped_container_type& values() const noexcept
    {
      return /*c*/.values;
    }

    // map operations
    constexpr iterator find(const key_type& x);
    constexpr const_iterator find(const key_type& x) const;
    template<class K> constexpr iterator find(const K& x);
    template<class K> constexpr const_iterator find(const K& x) const;

    constexpr size_type count(const key_type& x) const;
    template<class K> constexpr size_type count(const K& x) const;

    constexpr bool contains(const key_type& x) const;
    template<class K> constexpr bool contains(const K& x) const;

    constexpr iterator lower_bound(const key_type& x);
    constexpr const_iterator lower_bound(const key_type& x) const;
    template<class K> constexpr iterator lower_bound(const K& x);
    template<class K> constexpr const_iterator lower_bound(const K& x) const;

    constexpr iterator upper_bound(const key_type& x);
    constexpr const_iterator upper_bound(const key_type& x) const;
    template<class K> constexpr iterator upper_bound(const K& x);
    template<class K> constexpr const_iterator upper_bound(const K& x) const;

    constexpr pair<iterator, iterator> equal_range(const key_type& x);
    constexpr pair<const_iterator, const_iterator> equal_range(const key_type& x) const;
    template<class K> constexpr pair<iterator, iterator> equal_range(const K& x);
    template<class K>
    constexpr pair<const_iterator, const_iterator> equal_range(const K& x) const;

    friend constexpr bool operator==(const flat_multimap& x, const flat_multimap& y);

    friend constexpr /*synth-three-way-result*/<value_type> operator<=>(
      const flat_multimap& x, const flat_multimap& y);

    friend constexpr void swap(flat_multimap& x,
                               flat_multimap& y) noexcept(noexcept(x.swap(y)))
    {
      x.swap(y);
    }

  private:
    containers /*c*/;        // exposition-only
    key_compare /*compare*/; // exposition-only
  };

  template<class KeyContainer,
           class MappedContainer,
           class Compare = less<typename KeyContainer::value_type>>
  flat_multimap(KeyContainer, MappedContainer, Compare = Compare())
    -> flat_multimap<typename KeyContainer::value_type,
                     typename MappedContainer::value_type,
                     Compare,
                     KeyContainer,
                     MappedContainer>;

  template<class KeyContainer, class MappedContainer, class Allocator>
  flat_multimap(KeyContainer, MappedContainer, Allocator)
    -> flat_multimap<typename KeyContainer::value_type,
                     typename MappedContainer::value_type,
                     less<typename KeyContainer::value_type>,
                     KeyContainer,
                     MappedContainer>;
  template<class KeyContainer, class MappedContainer, class Compare, class Allocator>
  flat_multimap(KeyContainer, MappedContainer, Compare, Allocator)
    -> flat_multimap<typename KeyContainer::value_type,
                     typename MappedContainer::value_type,
                     Compare,
                     KeyContainer,
                     MappedContainer>;

  template<class KeyContainer,
           class MappedContainer,
           class Compare = less<typename KeyContainer::value_type>>
  flat_multimap(sorted_equivalent_t, KeyContainer, MappedContainer, Compare = Compare())
    -> flat_multimap<typename KeyContainer::value_type,
                     typename MappedContainer::value_type,
                     Compare,
                     KeyContainer,
                     MappedContainer>;

  template<class KeyContainer, class MappedContainer, class Allocator>
  flat_multimap(sorted_equivalent_t, KeyContainer, MappedContainer, Allocator)
    -> flat_multimap<typename KeyContainer::value_type,
                     typename MappedContainer::value_type,
                     less<typename KeyContainer::value_type>,
                     KeyContainer,
                     MappedContainer>;
  template<class KeyContainer, class MappedContainer, class Compare, class Allocator>
  flat_multimap(sorted_equivalent_t, KeyContainer, MappedContainer, Compare, Allocator)
    -> flat_multimap<typename KeyContainer::value_type,
                     typename MappedContainer::value_type,
                     Compare,
                     KeyContainer,
                     MappedContainer>;

  template<class InputIter, class Compare = less</*iter-key-type*/<InputIter>>>
  flat_multimap(InputIter, InputIter, Compare = Compare())
    -> flat_multimap</*iter-key-type*/<InputIter>,
                     /*iter-mapped-type*/<InputIter>,
                     Compare>;

  template<class InputIter, class Compare = less</*iter-key-type*/<InputIter>>>
  flat_multimap(sorted_equivalent_t, InputIter, InputIter, Compare = Compare())
    -> flat_multimap</*iter-key-type*/<InputIter>,
                     /*iter-mapped-type*/<InputIter>,
                     Compare>;

  template<ranges::input_range R,
           class Compare = less</*range-key-type*/<R>>,
           class Allocator = allocator<byte>>
  flat_multimap(from_range_t, R&&, Compare = Compare(), Allocator = Allocator())
    -> flat_multimap<
      /*range-key-type*/<R>,
      /*range-mapped-type*/<R>,
      Compare,
      vector</*range-key-type*/<R>, /*alloc-rebind*/<Allocator, /*range-key-type*/<R>>>,
      vector</*range-mapped-type*/<R>,
             /*alloc-rebind*/<Allocator, /*range-mapped-type*/<R>>>>;

  template<ranges::input_range R, class Allocator>
  flat_multimap(from_range_t, R&&, Allocator) -> flat_multimap<
    /*range-key-type*/<R>,
    /*range-mapped-type*/<R>,
    less</*range-key-type*/<R>>,
    vector</*range-key-type*/<R>, /*alloc-rebind*/<Allocator, /*range-key-type*/<R>>>,
    vector</*range-mapped-type*/<R>,
           /*alloc-rebind*/<Allocator, /*range-mapped-type*/<R>>>>;

  template<class Key, class T, class Compare = less<Key>>
  flat_multimap(initializer_list<pair<Key, T>>, Compare = Compare())
    -> flat_multimap<Key, T, Compare>;

  template<class Key, class T, class Compare = less<Key>>
  flat_multimap(sorted_equivalent_t, initializer_list<pair<Key, T>>, Compare = Compare())
    -> flat_multimap<Key, T, Compare>;

  template<class Key,
           class T,
           class Compare,
           class KeyContainer,
           class MappedContainer,
           class Allocator>
  struct uses_allocator<flat_multimap<Key, T, Compare, KeyContainer, MappedContainer>,
                        Allocator>
    : bool_constant<uses_allocator_v<KeyContainer, Allocator> &&
                    uses_allocator_v<MappedContainer, Allocator>>
  {};
}

References

• C++23 standard (ISO/IEC 14882:2024):

• 24.6.4 Header <flat_map> synopsis [flat.map.syn]

• 24.6.9.2 Definition [flat.map.defn]

• 24.6.10.2 Definition [flat.multimap.defn]