22 Containers library [containers]

22.5 Unordered associative containers [unord]

22.5.4 Class template unordered_­map [unord.map]

22.5.4.1 Overview [unord.map.overview]

1
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An unordered_­map is an unordered associative container that supports unique keys (an unordered_­map contains at most one of each key value) and that associates values of another type mapped_­type with the keys.
The unordered_­map class supports forward iterators.
2
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An unordered_­map meets all of the requirements of a container, of an unordered associative container, and of an allocator-aware container (Table 76).
It provides the operations described in the preceding requirements table for unique keys; that is, an unordered_­map supports the a_­uniq operations in that table, not the a_­eq operations.
For an unordered_­map<Key, T> the key type is Key, the mapped type is T, and the value type is pair<const Key, T>.
3
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Subclause [unord.map] only describes operations on unordered_­map that are not described in one of the requirement tables, or for which there is additional semantic information.
namespace std {
  template<class Key,
           class T,
           class Hash = hash<Key>,
           class Pred = equal_to<Key>,
           class Allocator = allocator<pair<const Key, T>>>
  class unordered_map {
  public:
    // types
    using key_type             = Key;
    using mapped_type          = T;
    using value_type           = pair<const Key, T>;
    using hasher               = Hash;
    using key_equal            = Pred;
    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 local_iterator       = implementation-defined; // see [container.requirements]
    using const_local_iterator = implementation-defined; // see [container.requirements]
    using node_type            = unspecified;
    using insert_return_type   = insert-return-type<iterator, node_type>;

    // [unord.map.cnstr], construct/copy/destroy
    unordered_map();
    explicit unordered_map(size_type n,
                           const hasher& hf = hasher(),
                           const key_equal& eql = key_equal(),
                           const allocator_type& a = allocator_type());
    template<class InputIterator>
      unordered_map(InputIterator f, InputIterator l,
                    size_type n = see below,
                    const hasher& hf = hasher(),
                    const key_equal& eql = key_equal(),
                    const allocator_type& a = allocator_type());
    unordered_map(const unordered_map&);
    unordered_map(unordered_map&&);
    explicit unordered_map(const Allocator&);
    unordered_map(const unordered_map&, const Allocator&);
    unordered_map(unordered_map&&, const Allocator&);
    unordered_map(initializer_list<value_type> il,
                  size_type n = see below,
                  const hasher& hf = hasher(),
                  const key_equal& eql = key_equal(),
                  const allocator_type& a = allocator_type());
    unordered_map(size_type n, const allocator_type& a)
      : unordered_map(n, hasher(), key_equal(), a) { }
    unordered_map(size_type n, const hasher& hf, const allocator_type& a)
      : unordered_map(n, hf, key_equal(), a) { }
    template<class InputIterator>
      unordered_map(InputIterator f, InputIterator l, size_type n, const allocator_type& a)
        : unordered_map(f, l, n, hasher(), key_equal(), a) { }
    template<class InputIterator>
      unordered_map(InputIterator f, InputIterator l, size_type n, const hasher& hf,
                    const allocator_type& a)
        : unordered_map(f, l, n, hf, key_equal(), a) { }
    unordered_map(initializer_list<value_type> il, size_type n, const allocator_type& a)
      : unordered_map(il, n, hasher(), key_equal(), a) { }
    unordered_map(initializer_list<value_type> il, size_type n, const hasher& hf,
                  const allocator_type& a)
      : unordered_map(il, n, hf, key_equal(), a) { }
    ~unordered_map();
    unordered_map& operator=(const unordered_map&);
    unordered_map& operator=(unordered_map&&)
      noexcept(allocator_traits<Allocator>::is_always_equal::value &&
               is_nothrow_move_assignable_v<Hash> &&
               is_nothrow_move_assignable_v<Pred>);
    unordered_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;
    const_iterator cbegin() const noexcept;
    const_iterator cend() const noexcept;

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

    // [unord.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& obj);
    pair<iterator, bool> insert(value_type&& obj);
    template<class P> pair<iterator, bool> insert(P&& obj);
    iterator       insert(const_iterator hint, const value_type& obj);
    iterator       insert(const_iterator hint, value_type&& obj);
    template<class P> iterator insert(const_iterator hint, P&& obj);
    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& k);
    iterator  erase(const_iterator first, const_iterator last);
    void      swap(unordered_map&)
      noexcept(allocator_traits<Allocator>::is_always_equal::value &&
               is_nothrow_swappable_v<Hash> &&
               is_nothrow_swappable_v<Pred>);
    void      clear() noexcept;

    template<class H2, class P2>
      void merge(unordered_map<Key, T, H2, P2, Allocator>& source);
    template<class H2, class P2>
      void merge(unordered_map<Key, T, H2, P2, Allocator>&& source);
    template<class H2, class P2>
      void merge(unordered_multimap<Key, T, H2, P2, Allocator>& source);
    template<class H2, class P2>
      void merge(unordered_multimap<Key, T, H2, P2, Allocator>&& source);

    // observers
    hasher hash_function() const;
    key_equal key_eq() const;

    // map operations
    iterator         find(const key_type& k);
    const_iterator   find(const key_type& k) const;
    template<class K>
      iterator       find(const K& k);
    template<class K>
      const_iterator find(const K& k) const;
    template<class K>
    size_type        count(const key_type& k) const;
    template<class K>
      size_type      count(const K& k) const;
    bool             contains(const key_type& k) const;
    template<class K>
      bool           contains(const K& k) const;
    pair<iterator, iterator>               equal_range(const key_type& k);
    pair<const_iterator, const_iterator>   equal_range(const key_type& k) const;
    template<class K>
      pair<iterator, iterator>             equal_range(const K& k);
    template<class K>
      pair<const_iterator, const_iterator> equal_range(const K& k) const;

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

    // bucket interface
    size_type bucket_count() const noexcept;
    size_type max_bucket_count() const noexcept;
    size_type bucket_size(size_type n) const;
    size_type bucket(const key_type& k) const;
    local_iterator begin(size_type n);
    const_local_iterator begin(size_type n) const;
    local_iterator end(size_type n);
    const_local_iterator end(size_type n) const;
    const_local_iterator cbegin(size_type n) const;
    const_local_iterator cend(size_type n) const;

    // hash policy
    float load_factor() const noexcept;
    float max_load_factor() const noexcept;
    void max_load_factor(float z);
    void rehash(size_type n);
    void reserve(size_type n);
  };

  template<class InputIterator,
           class Hash = hash<iter-key-type<InputIterator>>,
           class Pred = equal_to<iter-key-type<InputIterator>>,
           class Allocator = allocator<iter-to-alloc-type<InputIterator>>>
    unordered_map(InputIterator, InputIterator, typename see below::size_type = see below,
                  Hash = Hash(), Pred = Pred(), Allocator = Allocator())
      -> unordered_map<iter-key-type<InputIterator>, iter-mapped-type<InputIterator>, Hash, Pred,
                       Allocator>;

  template<class Key, class T, class Hash = hash<Key>,
           class Pred = equal_to<Key>, class Allocator = allocator<pair<const Key, T>>>
    unordered_map(initializer_list<pair<Key, T>>,
                  typename see below::size_type = see below, Hash = Hash(),
                  Pred = Pred(), Allocator = Allocator())
      -> unordered_map<Key, T, Hash, Pred, Allocator>;

  template<class InputIterator, class Allocator>
    unordered_map(InputIterator, InputIterator, typename see below::size_type, Allocator)
      -> unordered_map<iter-key-type<InputIterator>, iter-mapped-type<InputIterator>,
                       hash<iter-key-type<InputIterator>>,
                       equal_to<iter-key-type<InputIterator>>, Allocator>;

  template<class InputIterator, class Allocator>
    unordered_map(InputIterator, InputIterator, Allocator)
      -> unordered_map<iter-key-type<InputIterator>, iter-mapped-type<InputIterator>,
                       hash<iter-key-type<InputIterator>>,
                       equal_to<iter-key-type<InputIterator>>, Allocator>;

  template<class InputIterator, class Hash, class Allocator>
    unordered_map(InputIterator, InputIterator, typename see below::size_type, Hash, Allocator)
      -> unordered_map<iter-key-type<InputIterator>, iter-mapped-type<InputIterator>, Hash,
                       equal_to<iter-key-type<InputIterator>>, Allocator>;

  template<class Key, class T, class Allocator>
    unordered_map(initializer_list<pair<Key, T>>, typename see below::size_type,
                  Allocator)
      -> unordered_map<Key, T, hash<Key>, equal_to<Key>, Allocator>;

  template<class Key, class T, class Allocator>
    unordered_map(initializer_list<pair<Key, T>>, Allocator)
      -> unordered_map<Key, T, hash<Key>, equal_to<Key>, Allocator>;

  template<class Key, class T, class Hash, class Allocator>
    unordered_map(initializer_list<pair<Key, T>>, typename see below::size_type, Hash,
                  Allocator)
      -> unordered_map<Key, T, Hash, equal_to<Key>, Allocator>;

  // swap
  template<class Key, class T, class Hash, class Pred, class Alloc>
    void swap(unordered_map<Key, T, Hash, Pred, Alloc>& x,
              unordered_map<Key, T, Hash, Pred, Alloc>& y)
      noexcept(noexcept(x.swap(y)));
}
4
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A size_­type parameter type in an unordered_­map deduction guide refers to the size_­type member type of the type deduced by the deduction guide.

22.5.4.2 Constructors [unord.map.cnstr]

🔗
unordered_map() : unordered_map(size_type(see below)) { } explicit unordered_map(size_type n, const hasher& hf = hasher(), const key_equal& eql = key_equal(), const allocator_type& a = allocator_type());
1
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Effects: Constructs an empty unordered_­map using the specified hash function, key equality predicate, and allocator, and using at least n buckets.
For the default constructor, the number of buckets is implementation-defined.
max_­load_­factor() returns 1.0.
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Complexity: Constant.
🔗
template<class InputIterator> unordered_map(InputIterator f, InputIterator l, size_type n = see below, const hasher& hf = hasher(), const key_equal& eql = key_equal(), const allocator_type& a = allocator_type()); unordered_map(initializer_list<value_type> il, size_type n = see below, const hasher& hf = hasher(), const key_equal& eql = key_equal(), const allocator_type& a = allocator_type());
3
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Effects: Constructs an empty unordered_­map using the specified hash function, key equality predicate, and allocator, and using at least n buckets.
If n is not provided, the number of buckets is implementation-defined.
Then inserts elements from the range [f, l) for the first form, or from the range [il.begin(), il.end()) for the second form.
max_­load_­factor() returns 1.0.
4
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Complexity: Average case linear, worst case quadratic.

22.5.4.3 Element access [unord.map.elem]

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mapped_type& operator[](const key_type& k);
1
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Effects: Equivalent to: return try_­emplace(k).first->second;
🔗
mapped_type& operator[](key_type&& k);
2
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Effects: Equivalent to: return try_­emplace(move(k)).first->second;
🔗
mapped_type& at(const key_type& k); const mapped_type& at(const key_type& k) const;
3
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Returns: A reference to x.second, where x is the (unique) element whose key is equivalent to k.
4
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Throws: An exception object of type out_­of_­range if no such element is present.

22.5.4.4 Modifiers [unord.map.modifiers]

🔗
template<class P> pair<iterator, bool> insert(P&& obj);
1
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Constraints: is_­constructible_­v<value_­type, P&&> is true.
2
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Effects: Equivalent to: return emplace(std​::​forward<P>(obj));
🔗
template<class P> iterator insert(const_iterator hint, P&& obj);
3
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Constraints: is_­constructible_­v<value_­type, P&&> is true.
4
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Effects: Equivalent to: return emplace_­hint(hint, std​::​forward<P>(obj));
🔗
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);
5
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Preconditions: value_­type is Cpp17EmplaceConstructible into unordered_­map from piecewise_­construct, forward_­as_­tuple(k), forward_­as_­tuple(std​::​forward<Args>(args)...).
6
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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)...).
7
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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.
8
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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);
9
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Preconditions: value_­type is Cpp17EmplaceConstructible into unordered_­map from piecewise_­construct, forward_­as_­tuple(std​::​move(k)), forward_­as_­tuple(std​::​forward<Args>(args)...).
10
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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)...).
11
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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.
12
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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);
13
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Mandates: is_­assignable_­v<mapped_­type&, M&&> is true.
14
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Preconditions: value_­type is Cpp17EmplaceConstructible into unordered_­map from k, std​::​forward<M>(obj).
15
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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).
16
#
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.
17
#
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);
18
#
Mandates: is_­assignable_­v<mapped_­type&, M&&> is true.
Preconditions: value_­type is Cpp17EmplaceConstructible into unordered_­map from std​::​move(k), std​::​​forward<M>(obj).
19
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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).
20
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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.
21
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Complexity: The same as emplace and emplace_­hint, respectively.

22.5.4.5 Erasure [unord.map.erasure]

🔗
template<class K, class T, class H, class P, class A, class Predicate> typename unordered_map<K, T, H, P, A>::size_type erase_if(unordered_map<K, T, H, P, A>& c, Predicate pred);
1
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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();