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std::unordered_set

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C++
 
Containers library
 
std::unordered_set
Member types
Member functions
Non-member functions
(until C++20)
Deduction guides (C++17)
 
Defined in header <unordered_set>
template<

    class Key,
    class Hash = std::hash<Key>,
    class KeyEqual = std::equal_to<Key>,
    class Allocator = std::allocator<Key>

> class unordered_set;
 (1) (since C++11)
namespace pmr {

    template<
        class Key,
        class Hash = std::hash<Key>,
        class Pred = std::equal_to<Key>
    > using unordered_set = std::unordered_set<Key, Hash, Pred,
                                std::pmr::polymorphic_allocator<Key>>;

}
 (2) (since C++17)

std::unordered_set is an associative container that contains a set of unique objects of type Key. Search, insertion, and removal have average constant-time complexity.

Internally, the elements are not sorted in any particular order, but organized into buckets. Which bucket an element is placed into depends entirely on the hash of its value. This allows fast access to individual elements, since once a hash is computed, it refers to the exact bucket the element is placed into.

Container elements may not be modified (even by non const iterators) since modification could change an element's hash and corrupt the container.

std::unordered_set meets the requirements of Container, AllocatorAwareContainer, UnorderedAssociativeContainer.

All member functions of std::unordered_set are constexpr: it is possible to create and use std::unordered_set objects in the evaluation of a constant expression.

However, std::unordered_set objects generally cannot be constexpr, because any dynamically allocated storage must be released in the same evaluation of constant expression.

 (since C++26)

Contents

[edit] Iterator invalidation

Operations Invalidated
All read only operations, swap, std::swap Never
clear, rehash, reserve, operator= Always
insert, emplace, emplace_hint Only if causes rehash
erase Only to the element erased

[edit] Notes

  • The swap functions do not invalidate any of the iterators inside the container, but they do invalidate the iterator marking the end of the swap region.
  • References and pointers to data stored in the container are only invalidated by erasing that element, even when the corresponding iterator is invalidated.
  • After container move assignment, unless elementwise move assignment is forced by incompatible allocators, references, pointers, and iterators (other than the past-the-end iterator) to moved-from container remain valid, but refer to elements that are now in *this.

[edit] Template parameters

This section is incomplete
Reason: Add descriptions of the template parameters.

[edit] Member types

Type Definition
key_type Key[edit]
value_type Key[edit]
size_type Unsigned integer type (usually std::size_t)[edit]
difference_type Signed integer type (usually std::ptrdiff_t)[edit]
hasher Hash[edit]
key_equal KeyEqual[edit]
allocator_type Allocator[edit]
reference value_type&[edit]
const_reference const value_type&[edit]
pointer std::allocator_traits<Allocator>::pointer[edit]
const_pointer std::allocator_traits<Allocator>::const_pointer[edit]
iterator Constant LegacyForwardIterator and ConstexprIterator(since C++26) to value_type[edit]
const_iterator LegacyForwardIterator and ConstexprIterator(since C++26) to const value_type[edit]
local_iterator An iterator type whose category, value, difference, pointer and
reference types are the same as iterator. This iterator
can be used to iterate through a single bucket but not across buckets[edit]
const_local_iterator An iterator type whose category, value, difference, pointer and
reference types are the same as const_iterator. This iterator
can be used to iterate through a single bucket but not across buckets[edit]
node_type (since C++17) a specialization of node handle representing a container node[edit]
insert_return_type (since C++17) type describing the result of inserting a node_type, a specialization of

template<class Iter, class NodeType>
struct /*unspecified*/
{
    Iter     position;
    bool     inserted;
    NodeType node;
};
instantiated with template arguments iterator and node_type.[edit]

[edit] Member functions

 constructs the unordered_set
(public member function) [edit]
destructs the unordered_set
(public member function) [edit]
assigns values to the container
(public member function) [edit]
returns the associated allocator
(public member function) [edit]
Iterators
returns an iterator to the beginning
(public member function) [edit]
returns an iterator to the end
(public member function) [edit]
Capacity
checks whether the container is empty
(public member function) [edit]
returns the number of elements
(public member function) [edit]
returns the maximum possible number of elements
(public member function) [edit]
Modifiers
clears the contents
(public member function) [edit]
inserts elements or nodes(since C++17)
(public member function) [edit]
(C++23)
 inserts a range of elements
(public member function) [edit]
constructs element in-place
(public member function) [edit]
constructs elements in-place using a hint
(public member function) [edit]
erases elements
(public member function) [edit]
swaps the contents
(public member function) [edit]
(C++17)
 extracts nodes from the container
(public member function)