22 Containers library [containers]

22.4 Associative containers [associative]

22.4.4 Class template map [map]

22.4.4.1 Overview [map.overview]

A map is an associative container that supports unique keys (contains at most one of each key value) and provides for fast retrieval of values of another type T based on the keys.

The map class supports bidirectional iterators.

A map meets all of the requirements of a container, of a reversible container ([container.requirements]), of an associative container ([associative.reqmts]), and of an allocator-aware container (Table 76).

A map also provides most operations described in [associative.reqmts] for unique keys.

This means that a map supports the a_uniq operations in [associative.reqmts] but not the a_eq operations.

For a map<Key,T> the key_type is Key and the value_type is pair<const Key,T>.

Descriptions are provided here only for operations on map that are not described in one of those tables or for operations where there is additional semantic information.

namespace std {
  template<class Key, class T, class Compare = less<Key>,
           class Allocator = allocator<pair<const Key, T>>>
  class map {
  public:
    // types
    using key_type               = Key;
    using mapped_type            = T;
    using value_type             = pair<const Key, T>;
    using key_compare            = Compare;
    using allocator_type         = Allocator;
    using pointer                = typename allocator_traits<Allocator>::pointer;
    using const_pointer          = typename allocator_traits<Allocator>::const_pointer;
    using reference              = value_type&;
    using const_reference        = const value_type&;
    using size_type              = implementation-defined; // see [container.requirements]
    using difference_type        = implementation-defined; // see [container.requirements]
    using iterator               = implementation-defined; // see [container.requirements]
    using const_iterator         = implementation-defined; // see [container.requirements]
    using reverse_iterator       = std::reverse_iterator<iterator>;
    using const_reverse_iterator = std::reverse_iterator<const_iterator>;
    using node_type              = unspecified;
    using insert_return_type     = insert-return-type<iterator, node_type>;

    class value_compare {
      friend class map;
    protected:
      Compare comp;
      value_compare(Compare c) : comp(c) {}
    public:
      bool operator()(const value_type& x, const value_type& y) const {
        return comp(x.first, y.first);
      }
    };

    // [map.cons], construct/copy/destroy
    map() : map(Compare()) { }
    explicit map(const Compare& comp, const Allocator& = Allocator());
    template<class InputIterator>
      map(InputIterator first, InputIterator last,
          const Compare& comp = Compare(), const Allocator& = Allocator());
    map(const map& x);
    map(map&& x);
    explicit map(const Allocator&);
    map(const map&, const Allocator&);
    map(map&&, const Allocator&);
    map(initializer_list<value_type>,
      const Compare& = Compare(),
      const Allocator& = Allocator());
    template<class InputIterator>
      map(InputIterator first, InputIterator last, const Allocator& a)
        : map(first, last, Compare(), a) { }
    map(initializer_list<value_type> il, const Allocator& a)
      : map(il, Compare(), a) { }
    ~map();
    map& operator=(const map& x);
    map& operator=(map&& x)
      noexcept(allocator_traits<Allocator>::is_always_equal::value &&
               is_nothrow_move_assignable_v<Compare>);
    map& operator=(initializer_list<value_type>);
    allocator_type get_allocator() const noexcept;

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

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

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

    // capacity
    [[nodiscard]] bool empty() const noexcept;
    size_type size() const noexcept;
    size_type max_size() const noexcept;

    // [map.access], element access
    mapped_type& operator[](const key_type& x);
    mapped_type& operator[](key_type&& x);
    mapped_type&       at(const key_type& x);
    const mapped_type& at(const key_type& x) const;

    // [map.modifiers], modifiers
    template<class... Args> pair<iterator, bool> emplace(Args&&... args);
    template<class... Args> iterator emplace_hint(const_iterator position, Args&&... args);
    pair<iterator, bool> insert(const value_type& x);
    pair<iterator, bool> insert(value_type&& x);
    template<class P> pair<iterator, bool> insert(P&& x);
    iterator insert(const_iterator position, const value_type& x);
    iterator insert(const_iterator position, value_type&& x);
    template<class P>
      iterator insert(const_iterator position, P&&);
    template<class InputIterator>
      void insert(InputIterator first, InputIterator last);
    void insert(initializer_list<value_type>);

    node_type extract(const_iterator position);
    node_type extract(const key_type& x);
    insert_return_type insert(node_type&& nh);
    iterator           insert(const_iterator hint, node_type&& nh);

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

    iterator  erase(iterator position);
    iterator  erase(const_iterator position);
    size_type erase(const key_type& x);
    iterator  erase(const_iterator first, const_iterator last);
    void      swap(map&)
      noexcept(allocator_traits<Allocator>::is_always_equal::value &&
               is_nothrow_swappable_v<Compare>);
    void      clear() noexcept;

    template<class C2>
      void merge(map<Key, T, C2, Allocator>& source);
    template<class C2>
      void merge(map<Key, T, C2, Allocator>&& source);
    template<class C2>
      void merge(multimap<Key, T, C2, Allocator>& source);
    template<class C2>
      void merge(multimap<Key, T, C2, Allocator>&& source);

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

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

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

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

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

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

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

  template<class InputIterator, class Compare = less<iter-key-type<InputIterator>>,
           class Allocator = allocator<iter-to-alloc-type<InputIterator>>>
    map(InputIterator, InputIterator, Compare = Compare(), Allocator = Allocator())
      -> map<iter-key-type<InputIterator>, iter-mapped-type<InputIterator>, Compare, Allocator>;

  template<class Key, class T, class Compare = less<Key>,
           class Allocator = allocator<pair<const Key, T>>>
    map(initializer_list<pair<Key, T>>, Compare = Compare(), Allocator = Allocator())
      -> map<Key, T, Compare, Allocator>;

  template<class InputIterator, class Allocator>
    map(InputIterator, InputIterator, Allocator)
      -> map<iter-key-type<InputIterator>, iter-mapped-type<InputIterator>,
             less<iter-key-type<InputIterator>>, Allocator>;

  template<class Key, class T, class Allocator>
    map(initializer_list<pair<Key, T>>, Allocator) -> map<Key, T, less<Key>, Allocator>;

  // swap
  template<class Key, class T, class Compare, class Allocator>
    void swap(map<Key, T, Compare, Allocator>& x,
              map<Key, T, Compare, Allocator>& y)
      noexcept(noexcept(x.swap(y)));
}

22.4.4.2 Constructors, copy, and assignment [map.cons]

🔗

explicit map(const Compare& comp, const Allocator& = Allocator());

Effects: Constructs an empty map using the specified comparison object and allocator.

Complexity: Constant.

🔗

template<class InputIterator>
  map(InputIterator first, InputIterator last,
      const Compare& comp = Compare(), const Allocator& = Allocator());

Effects: Constructs an empty map using the specified comparison object and allocator, and inserts elements from the range [first, last).

Complexity: Linear in N if the range [first, last) is already sorted using comp and otherwise

N log N

, where N is last - first.

22.4.4.3 Element access [map.access]

🔗

mapped_type& operator[](const key_type& x);

Effects: Equivalent to: return try_emplace(x).first->second;

🔗

mapped_type& operator[](key_type&& x);

Effects: Equivalent to: return try_emplace(move(x)).first->second;

🔗

mapped_type&       at(const key_type& x);
const mapped_type& at(const key_type& x) const;

Returns: A reference to the mapped_type corresponding to x in *this.

Throws: An exception object of type out_of_range if no such element is present.

Complexity: Logarithmic.

22.4.4.4 Modifiers [map.modifiers]

🔗

template<class P>
  pair<iterator, bool> insert(P&& x);
template<class P>
  iterator insert(const_iterator position, P&& x);

Constraints: is_constructible_v<value_type, P&&> is true.

Effects: The first form is equivalent to return emplace(std::forward<P>(x)).

The second form is equivalent to return emplace_hint(position, std::forward<P>(x)).

🔗

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

Preconditions: value_type is Cpp17EmplaceConstructible into map from piecewise_construct, forward_as_tuple(k), forward_as_tuple(std::forward<Args>(args)...).

Effects: If the map already contains an element whose key is equivalent to k, there is no effect.

Otherwise inserts an object of type value_type constructed with piecewise_construct, forward_as_tuple(k), forward_as_tuple(std::forward<Args>(args)...).

Returns: In the first overload, the bool component of the returned pair is true if and only if the insertion took place.

The returned iterator points to the map element whose key is equivalent to k.

Complexity: The same as emplace and emplace_hint, respectively.

🔗

template<class... Args>
  pair<iterator, bool> try_emplace(key_type&& k, Args&&... args);
template<class... Args>
  iterator try_emplace(const_iterator hint, key_type&& k, Args&&... args);

Preconditions: value_type is Cpp17EmplaceConstructible into map from piecewise_construct, forward_as_tuple(std::move(k)), forward_as_tuple(std::forward<Args>(args)...).

Effects: If the map already contains an element whose key is equivalent to k, there is no effect.

Otherwise inserts an object of type value_type constructed with piecewise_construct, forward_as_tuple(std::move(k)), forward_as_tuple(std::forward<Args>(args)...).

Returns: In the first overload, the bool component of the returned pair is true if and only if the insertion took place.

The returned iterator points to the map element whose key is equivalent to k.

Complexity: The same as emplace and emplace_hint, respectively.

🔗

template<class M>
  pair<iterator, bool> insert_or_assign(const key_type& k, M&& obj);
template<class M>
  iterator insert_or_assign(const_iterator hint, const key_type& k, M&& obj);

Mandates: is_assignable_v<mapped_type&, M&&> is true.

Preconditions: value_type is Cpp17EmplaceConstructible into map from k, forward<M>(obj).

Effects: If the map already contains an element e whose key is equivalent to k, assigns std::forward<M>(obj) to e.second.

Otherwise inserts an object of type value_type constructed with k, std::forward<M>(obj).

Returns: In the first overload, the bool component of the returned pair is true if and only if the insertion took place.

The returned iterator points to the map element whose key is equivalent to k.

Complexity: The same as emplace and emplace_hint, respectively.

🔗

template<class M>
  pair<iterator, bool> insert_or_assign(key_type&& k, M&& obj);
template<class M>
  iterator insert_or_assign(const_iterator hint, key_type&& k, M&& obj);

Mandates: is_assignable_v<mapped_type&, M&&> is true.

Preconditions: value_type is Cpp17EmplaceConstructible into map from move(k), forward<M>(obj).

Effects: If the map already contains an element e whose key is equivalent to k, assigns std::forward<M>(obj) to e.second.

Otherwise inserts an object of type value_type constructed with std::move(k), std::forward<M>(obj).

Returns: In the first overload, the bool component of the returned pair is true if and only if the insertion took place.

The returned iterator points to the map element whose key is equivalent to k.

Complexity: The same as emplace and emplace_hint, respectively.

22.4.4.5 Erasure [map.erasure]

🔗

template<class Key, class T, class Compare, class Allocator, class Predicate>
  typename map<Key, T, Compare, Allocator>::size_type
    erase_if(map<Key, T, Compare, Allocator>& c, Predicate pred);

Effects: Equivalent to:

auto original_size = c.size();
for (auto i = c.begin(), last = c.end(); i != last; ) {
  if (pred(*i)) {
    i = c.erase(i);
  } else {
    ++i;
  }
}
return original_size - c.size();