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.
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.
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
iterator. This iterator
const_local_iterator An iterator type whose category, value, difference, pointer and
const_iterator. This iterator
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]
unordered_set
unordered_set
The member types iterator and const_iterator may be aliases to the same type. This means defining a pair of function overloads using the two types as parameter types may violate the One Definition Rule. Since iterator is convertible to const_iterator, a single function with a const_iterator as parameter type will work instead.
#include <iostream>
#include <unordered_set>
void print(const auto& set)
{
for (const auto& elem : set)
std::cout << elem << ' ';
std::cout << '\n';
}
int main()
{
std::unordered_set<int> mySet{2, 7, 1, 8, 2, 8}; // creates a set of ints
print(mySet);
mySet.insert(5); // puts an element 5 in the set
print(mySet);
if (auto iter = mySet.find(5); iter != mySet.end())
mySet.erase(iter); // removes an element pointed to by iter
print(mySet);
mySet.erase(7); // removes an element 7
print(mySet);
}
Possible output:
8 1 7 2 5 8 1 7 2 8 1 7 2 8 1 2[edit] Defect reports
The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
DR Applied to Behavior as published Correct behavior LWG 2050 C++11 the definitions ofreference, const_reference, pointer
const_pointer were based on allocator_type based on value_type and
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