Standard library header <unordered_set> (C++11)

From cppreference.com
< cpp‎ | header
 
 
Standard Library headers
Note: a slash '/' in a revision mark means that the header was deprecated and/or removed.
Language Support
Concepts
<concepts> (C++20)
Diagnostics
<system_error> (C++11)
General utilities
<bitset>
<tuple> (C++11)
<optional> (C++17)
<any> (C++17)
<variant> (C++17)
<type_traits> (C++11)
<ratio> (C++11)
<chrono> (C++11)
Strings
<charconv> (C++17)
<format> (C++20)

<cwchar>
<cuchar> (C++11)

Localization
<codecvt> (C++11/17)

Containers
<span> (C++20)
<array> (C++11)
<vector>
<unordered_map> (C++11)
<unordered_set> (C++11)
<stack>
Iterators
<iterator>
Ranges
<ranges> (C++20)
Algorithms
<execution> (C++17)
Numerics
<bit> (C++20)
<numbers> (C++20)
<cfenv> (C++11)
<cmath>
Input/Output
<cstdio>
<cinttypes> (C++11)
<strstream> (C++98/)

Regular expressions
<regex> (C++11)
Filesystem support
<filesystem> (C++17)
Thread support
<thread> (C++11)
<atomic> (C++11)
<mutex> (C++11)
<shared_mutex> (C++14)
<condition_variable> (C++11)  
<future> (C++11)
<stop_token> (C++20)
<semaphore> (C++20)

<latch> (C++20)
<barrier> (C++20)

C compatibility
<cstdbool> (C++11/17/20)
<ccomplex> (C++11/17/20)
<ctgmath> (C++11/17/20)
<cstdalign> (C++11/17/20)
<ciso646> (until C++20)
<stdatomic.h>
 

This header is part of the containers library.

Includes

(C++20)
Three-way comparison operator support
std::initializer_list class template

Classes

collection of unique keys, hashed by keys
(class template)
collection of keys, hashed by keys
(class template)

Functions

(removed in C++20)
compares the values in the unordered_set
(function template)
specializes the std::swap algorithm
(function template)
Erases all elements satisfying specific criteria
(function template)
(removed in C++20)
compares the values in the unordered_multiset
(function template)
specializes the std::swap algorithm
(function template)
Erases all elements satisfying specific criteria
(function template)
Range access
(C++11)(C++14)
returns an iterator to the beginning of a container or array
(function template)
(C++11)(C++14)
returns an iterator to the end of a container or array
(function template)
returns a reverse iterator to the beginning of a container or array
(function template)
(C++14)
returns a reverse end iterator for a container or array
(function template)
(C++17)(C++20)
returns the size of a container or array
(function template)
(C++17)
checks whether the container is empty
(function template)
(C++17)
obtains the pointer to the underlying array
(function template)

Synopsis

#include <compare>
#include <initializer_list>
 
namespace std {
  // class template unordered_set
  template<class Key,
           class Hash = hash<Key>,
           class Pred = equal_to<Key>,
           class Alloc = allocator<Key>>
    class unordered_set;
 
  // class template unordered_multiset
  template<class Key,
           class Hash = hash<Key>,
           class Pred = equal_to<Key>,
           class Alloc = allocator<Key>>
    class unordered_multiset;
 
  template<class Key, class Hash, class Pred, class Alloc>
    bool operator==(const unordered_set<Key, Hash, Pred, Alloc>& a,
                    const unordered_set<Key, Hash, Pred, Alloc>& b);
 
  template<class Key, class Hash, class Pred, class Alloc>
    bool operator==(const unordered_multiset<Key, Hash, Pred, Alloc>& a,
                    const unordered_multiset<Key, Hash, Pred, Alloc>& b);
 
  template<class Key, class Hash, class Pred, class Alloc>
    void swap(unordered_set<Key, Hash, Pred, Alloc>& x,
              unordered_set<Key, Hash, Pred, Alloc>& y)
      noexcept(noexcept(x.swap(y)));
 
  template<class Key, class Hash, class Pred, class Alloc>
    void swap(unordered_multiset<Key, Hash, Pred, Alloc>& x,
              unordered_multiset<Key, Hash, Pred, Alloc>& y)
      noexcept(noexcept(x.swap(y)));
 
  template<class K, class H, class P, class A, class Predicate>
    typename unordered_set<K, H, P, A>::size_type
      erase_if(unordered_set<K, H, P, A>& c, Predicate pred);
 
  template<class K, class H, class P, class A, class Predicate>
    typename unordered_multiset<K, H, P, A>::size_type
      erase_if(unordered_multiset<K, H, P, A>& c, Predicate pred);
 
  namespace pmr {
    template<class Key,
             class Hash = hash<Key>,
             class Pred = equal_to<Key>>
      using unordered_set = std::unordered_set<Key, Hash, Pred,
                                               polymorphic_allocator<Key>>;
 
    template<class Key,
             class Hash = hash<Key>,
             class Pred = equal_to<Key>>
      using unordered_multiset = std::unordered_multiset<Key, Hash, Pred,
                                                         polymorphic_allocator<Key>>;
  }
}

Class template std::unordered_set

namespace std {
  template<class Key,
           class Hash = hash<Key>,
           class Pred = equal_to<Key>,
           class Allocator = allocator<Key>>
  class unordered_set {
  public:
    // types
    using key_type             = Key;
    using value_type           = Key;
    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 */;
    using difference_type      = /* implementation-defined */;
 
    using iterator             = /* implementation-defined */;
    using const_iterator       = /* implementation-defined */;
    using local_iterator       = /* implementation-defined */;
    using const_local_iterator = /* implementation-defined */;
    using node_type            = /* unspecified */;
    using insert_return_type   = /*insert-return-type*/<iterator, node_type>;
 
    // construct/copy/destroy
    unordered_set();
    explicit unordered_set(size_type n,
                           const hasher& hf = hasher(),
                           const key_equal& eql = key_equal(),
                           const allocator_type& a = allocator_type());
    template<class InputIt>
      unordered_set(InputIt f, InputIt l,
                    size_type n = /* see description */,
                    const hasher& hf = hasher(),
                    const key_equal& eql = key_equal(),
                    const allocator_type& a = allocator_type());
    unordered_set(const unordered_set&);
    unordered_set(unordered_set&&);
    explicit unordered_set(const Allocator&);
    unordered_set(const unordered_set&, const Allocator&);
    unordered_set(unordered_set&&, const Allocator&);
    unordered_set(initializer_list<value_type> il,
                  size_type n = /* see description */,
                  const hasher& hf = hasher(),
                  const key_equal& eql = key_equal(),
                  const allocator_type& a = allocator_type());
    unordered_set(size_type n, const allocator_type& a)
      : unordered_set(n, hasher(), key_equal(), a) { }
    unordered_set(size_type n, const hasher& hf, const allocator_type& a)
      : unordered_set(n, hf, key_equal(), a) { }
    template<class InputIt>
      unordered_set(InputIt f, InputIt l, size_type n, const allocator_type& a)
        : unordered_set(f, l, n, hasher(), key_equal(), a) { }
    template<class InputIt>
      unordered_set(InputIt f, InputIt l, size_type n, const hasher& hf,
                    const allocator_type& a)
      : unordered_set(f, l, n, hf, key_equal(), a) { }
    unordered_set(initializer_list<value_type> il, size_type n, const allocator_type& a)
      : unordered_set(il, n, hasher(), key_equal(), a) { }
    unordered_set(initializer_list<value_type> il, size_type n, const hasher& hf,
                  const allocator_type& a)
      : unordered_set(il, n, hf, key_equal(), a) { }
    ~unordered_set();
    unordered_set& operator=(const unordered_set&);
    unordered_set& operator=(unordered_set&&)
      noexcept(allocator_traits<Allocator>::is_always_equal::value &&
               is_nothrow_move_assignable_v<Hash> &&
               is_nothrow_move_assignable_v<Pred>);
    unordered_set& 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;
 
    // 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);
    iterator insert(const_iterator hint, const value_type& obj);
    iterator insert(const_iterator hint, value_type&& obj);
    template<class InputIt> void insert(InputIt first, InputIt last);
    void insert(initializer_list<value_type>);
 
    node_type extract(const_iterator position);
    node_type extract(const key_type& x);
    template<class K> node_type extract(K&& x);
    insert_return_type insert(node_type&& nh);
    iterator           insert(const_iterator hint, node_type&& nh);
 
    iterator  erase(iterator position);
    iterator  erase(const_iterator position);
    size_type erase(const key_type& k);
    template<class K> size_type erase(K&& x);
    iterator  erase(const_iterator first, const_iterator last);
    void      swap(unordered_set&)
      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_set<Key, H2, P2, Allocator>& source);
    template<class H2, class P2>
      void merge(unordered_set<Key, H2, P2, Allocator>&& source);
    template<class H2, class P2>
      void merge(unordered_multiset<Key, H2, P2, Allocator>& source);
    template<class H2, class P2>
      void merge(unordered_multiset<Key, H2, P2, Allocator>&& source);
 
    // observers
    hasher hash_function() const;
    key_equal key_eq() const;
 
    // set 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;
    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;
 
    // 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 InputIt,
           class Hash = hash</*iter-value-type*/<InputIt>>,
           class Pred = equal_to</*iter-value-type*/<InputIt>>,
           class Allocator = allocator</*iter-value-type*/<InputIt>>>
    unordered_set(InputIt, InputIt,
                  typename /* see description */::size_type = /* see description */,
                  Hash = Hash(), Pred = Pred(), Allocator = Allocator())
      -> unordered_set</*iter-value-type*/<InputIt>,
                       Hash, Pred, Allocator>;
 
  template<class T, class Hash = hash<T>,
           class Pred = equal_to<T>, class Allocator = allocator<T>>
    unordered_set(initializer_list<T>,
                  typename /* see description */::size_type = /* see description */,
                  Hash = Hash(), Pred = Pred(), Allocator = Allocator())
      -> unordered_set<T, Hash, Pred, Allocator>;
 
  template<class InputIt, class Allocator>
    unordered_set(InputIt, InputIt, typename /* see description */::size_type, Allocator)
      -> unordered_set</*iter-value-type*/<InputIt>,
                       hash</*iter-value-type*/<InputIt>>,
                       equal_to</*iter-value-type*/<InputIt>>,
                       Allocator>;
 
  template<class InputIt, class Hash, class Allocator>
    unordered_set(InputIt, InputIt, typename /* see description */::size_type,
                  Hash, Allocator)
      -> unordered_set</*iter-value-type*/<InputIt>, Hash,
                       equal_to</*iter-value-type*/<InputIt>>,
                       Allocator>;
 
  template<class T, class Allocator>
    unordered_set(initializer_list<T>, typename /* see description */::size_type,
                  Allocator)
      -> unordered_set<T, hash<T>, equal_to<T>, Allocator>;
 
  template<class T, class Hash, class Allocator>
    unordered_set(initializer_list<T>, typename /* see description */::size_type,
                  Hash, Allocator)
      -> unordered_set<T, Hash, equal_to<T>, Allocator>;
 
  // swap
  template<class Key, class Hash, class Pred, class Alloc>
    void swap(unordered_set<Key, Hash, Pred, Alloc>& x,
              unordered_set<Key, Hash, Pred, Alloc>& y)
      noexcept(noexcept(x.swap(y)));
}

Class template std::unordered_multiset

namespace std {
  template<class Key,
           class Hash = hash<Key>,
           class Pred = equal_to<Key>,
           class Allocator = allocator<Key>>
  class unordered_multiset {
  public:
    // types
    using key_type             = Key;
    using value_type           = Key;
    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 */;
    using difference_type      = /* implementation-defined */;
 
    using iterator             = /* implementation-defined */;
    using const_iterator       = /* implementation-defined */;
    using local_iterator       = /* implementation-defined */;
    using const_local_iterator = /* implementation-defined */;
    using node_type            = /* unspecified */;
 
    // construct/copy/destroy
    unordered_multiset();
    explicit unordered_multiset(size_type n,
                                const hasher& hf = hasher(),
                                const key_equal& eql = key_equal(),
                                const allocator_type& a = allocator_type());
    template<class InputIt>
      unordered_multiset(InputIt f, InputIt l,
                         size_type n = /* see description */,
                         const hasher& hf = hasher(),
                         const key_equal& eql = key_equal(),
                         const allocator_type& a = allocator_type());
    unordered_multiset(const unordered_multiset&);
    unordered_multiset(unordered_multiset&&);
    explicit unordered_multiset(const Allocator&);
    unordered_multiset(const unordered_multiset&, const Allocator&);
    unordered_multiset(unordered_multiset&&, const Allocator&);
    unordered_multiset(initializer_list<value_type> il,
                       size_type n = /* see description */,
                       const hasher& hf = hasher(),
                       const key_equal& eql = key_equal(),
                       const allocator_type& a = allocator_type());
    unordered_multiset(size_type n, const allocator_type& a)
      : unordered_multiset(n, hasher(), key_equal(), a) { }
    unordered_multiset(size_type n, const hasher& hf, const allocator_type& a)
      : unordered_multiset(n, hf, key_equal(), a) { }
    template<class InputIt>
      unordered_multiset(InputIt f, InputIt l, size_type n, const allocator_type& a)
        : unordered_multiset(f, l, n, hasher(), key_equal(), a) { }
    template<class InputIt>
      unordered_multiset(InputIt f, InputIt l, size_type n, const hasher& hf,
                         const allocator_type& a)
      : unordered_multiset(f, l, n, hf, key_equal(), a) { }
    unordered_multiset(initializer_list<value_type> il, size_type n,
                       const allocator_type& a)
      : unordered_multiset(il, n, hasher(), key_equal(), a) { }
    unordered_multiset(initializer_list<value_type> il, size_type n, const hasher& hf,
                       const allocator_type& a)
      : unordered_multiset(il, n, hf, key_equal(), a) { }
    ~unordered_multiset();
    unordered_multiset& operator=(const unordered_multiset&);
    unordered_multiset& operator=(unordered_multiset&&)
      noexcept(allocator_traits<Allocator>::is_always_equal::value &&
               is_nothrow_move_assignable_v<Hash> &&
               is_nothrow_move_assignable_v<Pred>);
    unordered_multiset& 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;
 
    // modifiers
    template<class... Args> iterator emplace(Args&&... args);
    template<class... Args> iterator emplace_hint(const_iterator position, Args&&... args);
    iterator insert(const value_type& obj);
    iterator insert(value_type&& obj);
    iterator insert(const_iterator hint, const value_type& obj);
    iterator insert(const_iterator hint, value_type&& obj);
    template<class InputIt> void insert(InputIt first, InputIt last);
    void insert(initializer_list<value_type>);
 
    node_type extract(const_iterator position);
    node_type extract(const key_type& x);
    template<class K> node_type extract(K&& x);
    iterator insert(node_type&& nh);
    iterator insert(const_iterator hint, node_type&& nh);
 
    iterator  erase(iterator position);
    iterator  erase(const_iterator position);
    size_type erase(const key_type& k);
    template<class K> size_type erase(K&& x);
    iterator  erase(const_iterator first, const_iterator last);
    void      swap(unordered_multiset&)
      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_multiset<Key, H2, P2, Allocator>& source);
    template<class H2, class P2>
      void merge(unordered_multiset<Key, H2, P2, Allocator>&& source);
    template<class H2, class P2>
      void merge(unordered_set<Key, H2, P2, Allocator>& source);
    template<class H2, class P2>
      void merge(unordered_set<Key, H2, P2, Allocator>&& source);
 
    // observers
    hasher hash_function() const;
    key_equal key_eq() const;
 
    // set 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;
    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;
 
    // 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 InputIt,
           class Hash = hash</*iter-value-type*/<InputIt>>,
           class Pred = equal_to</*iter-value-type*/<InputIt>>,
           class Allocator = allocator</*iter-value-type*/<InputIt>>>
    unordered_multiset(InputIt, InputIt,
                       /* see description */::size_type = /* see description */,
                       Hash = Hash(), Pred = Pred(), Allocator = Allocator())
      -> unordered_multiset</*iter-value-type*/<InputIt>,
                            Hash, Pred, Allocator>;
 
  template<class T, class Hash = hash<T>,
           class Pred = equal_to<T>, class Allocator = allocator<T>>
    unordered_multiset(initializer_list<T>,
                       typename /* see description */::size_type = /* see description */,
                       Hash = Hash(), Pred = Pred(), Allocator = Allocator())
      -> unordered_multiset<T, Hash, Pred, Allocator>;
 
  template<class InputIt, class Allocator>
    unordered_multiset(InputIt, InputIt, typename /* see description */::size_type,
                       Allocator)
      -> unordered_multiset</*iter-value-type*/<InputIt>,
                            hash</*iter-value-type*/<InputIt>>,
                            equal_to</*iter-value-type*/<InputIt>>,
                            Allocator>;
 
  template<class InputIt, class Hash, class Allocator>
    unordered_multiset(InputIt, InputIt, typename /* see description */::size_type,
                       Hash, Allocator)
      -> unordered_multiset</*iter-value-type*/<InputIt>, Hash,
                            equal_to</*iter-value-type*/<InputIt>>,
                            Allocator>;
 
  template<class T, class Allocator>
    unordered_multiset(initializer_list<T>, typename /* see description */::size_type,
                       Allocator)
      -> unordered_multiset<T, hash<T>, equal_to<T>, Allocator>;
 
  template<class T, class Hash, class Allocator>
    unordered_multiset(initializer_list<T>, typename /* see description */::size_type,
                       Hash, Allocator)
      -> unordered_multiset<T, Hash, equal_to<T>, Allocator>;
 
  // swap
  template<class Key, class Hash, class Pred, class Alloc>
    void swap(unordered_multiset<Key, Hash, Pred, Alloc>& x,
              unordered_multiset<Key, Hash, Pred, Alloc>& y)
      noexcept(noexcept(x.swap(y)));
}