std::unordered_set
std::unordered_set<unordered_set>
class Key,
class Hash = std::hash <Key>,
class KeyEqual = std::equal_to <Key>,
class Allocator = std::allocator <Key>
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>>;
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.
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
Reason: Add descriptions of the template parameters.
[edit] Member types
local_iterator
An iterator type whose category, value, difference, pointer andreference types are the same as
iterator. This iteratorcan be used to iterate through a single bucket but not across buckets[edit]
const_local_iterator
An iterator type whose category, value, difference, pointer andreference types are the same as
const_iterator. This iteratorcan be used to iterate through a single bucket but not across buckets[edit]
insert_return_type (since C++17)
type describing the result of inserting a node_type, a specialization oftemplate<class Iter, class NodeType>
struct /*unspecified*/
{
Iter position;
bool inserted;
NodeType node;
};
instantiated with template arguments iterator and node_type.[edit]
[edit] Member functions
Iterators
Capacity
Modifiers
Lookup
Bucket interface
Hash policy
(public member function) [edit]
(public member function) [edit]
Observers
[edit] Non-member functions
(function template) [edit]
Deduction guides
(since C++17)[edit] Notes
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.
| Feature-test macro | Value | Std | Feature |
|---|---|---|---|
__cpp_lib_containers_ranges |
202202L |
(C++23) | Ranges construction and insertion for containers |
__cpp_lib_constexpr_unordered_set |
202502L |
(C++26) | constexpr std::unordered_set
|
[edit] Example
#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 of reference, const_reference, pointerand const_pointer were based on allocator_type
|
based on value_type andstd::allocator_traits |