std::unordered_set
template < class Key, // unordered_set::key_type/value_type
class Hash = hash<Key>, // unordered_set::hasher
class Pred = equal_to<Key>, // unordered_set::key_equal
class Alloc = allocator<Key> // unordered_set::allocator_type
> class unordered_set;
Unordered Set
Unordered sets are containers that store unique elements in no particular order, and which allow for fast retrieval of individual elements based on their value.
In an unordered_set, the value of an element is at the same time its key, that identifies it uniquely. Keys are immutable(不可变), therefore, the elements in an unordered_set cannot be modified once in the container - they can be inserted and removed, though.
Internally, the elements in the unordered_set are not sorted in any particular order, but organized into buckets depending on their hash values to allow for fast access to individual elements directly by their values (with a constant average time complexity on average).
unordered_set containers are faster than set containers to access individual elements by their key, although they are generally less efficient for range iteration through a subset of their elements.
Iterators in the container are at least forward iterators.
Container properties
- Associative: Elements in associative containers are referenced by their key and not by their absolute position in the container.
- Unordered: Unordered containers organize their elements using hash tables that allow for fast access to elements by their key.
- Set: The value of an element is also the key used to identify it.
- Unique: keys No two elements in the container can have equivalent keys.
- Allocator-aware: The container uses an allocator object to dynamically handle its storage needs.
Template parameters
- Key: Type of the elements. Each element in an unordered_set is also uniquely identified by this value. Aliased as member types unordered_set::key_type and unordered_set::value_type.
- Hash: A unary function object type that takes an object of the same type as the elements as argument and returns a unique value of type size_t based on it. This can either be a class implementing a function call operator or a pointer to a function (see constructor for an example). This defaults to hash
, which returns a hash value with a probability of collision approaching 1.0/std::numeric_limits<size_t>::max(). The unordered_set object uses the hash values returned by this function to organize its elements internally, speeding up the process of locating individual elements. Aliased as member type unordered_set::hasher. - Pred: A binary predicate that takes two arguments of the same type as the elements and returns a bool. The expression pred(a,b), where pred is an object of this type and a and b are key values, shall return true if a is to be considered equivalent to b. This can either be a class implementing a function call operator or a pointer to a function (see constructor for an example). This defaults to equal_to
, which returns the same as applying the equal-to operator (a==b). The unordered_set object uses this expression to determine whether two element keys are equivalent. No two elements in an unordered_set container can have keys that yield true using this predicate. Aliased as member type unordered_set::key_equal. - Alloc: Type of the allocator object used to define the storage allocation model. By default, the allocator class template is used, which defines the simplest memory allocation model and is value-independent. Aliased as member type unordered_set::allocator_type.
In the reference for the unordered_set member functions, these same names (Key, Hash, Pred and Alloc) are assumed for the template parameters.
Member types
The following aliases are member types of unordered_set. They are widely used as parameter and return types by member functions:
| member type | definition | notes |
|---|---|---|
| key_type | the first template parameter (Key) | |
| value_type | the first template parameter (Key) | The same as key_type |
| hasher | the second template parameter (Hash) | defaults to: hash<key_type> |
| key_equal | the third template parameter (Pred) | defaults to: equal_to<key_type> |
| allocator_type | the fourth template parameter (Alloc) | defaults to: allocator<value_type> |
| reference | Alloc::reference | |
| const_reference | Alloc::const_reference | |
| pointer | Alloc::pointer | for the default allocator: value_type* |
| const_pointer | Alloc::const_pointer | for the default allocator: const value_type* |
| iterator | a forward iterator to const value_type | * convertible to const_iterator |
| const_iterator | a forward iterator to const value_type | * |
| local_iterator | a forward iterator to const value_type | * convertible to const_local_iterator |
| const_local_iterator | a forward iterator to const value_type | * |
| size_type | an unsigned integral type | usually the same as size_t |
| difference_type | a signed integral type | usually the same as ptrdiff_t |
*Note: All iterators in an unordered_set point to const elements. Whether the const_ member type is the same type as its non-const_ counterpart depends on the particular library implementation, but programs should not rely on them being different to overload functions: const_iterator is more generic, since iterator is always convertible to it.
The same applies to local_ and non-local_ iterator types: they may either be the same type or not, but a program should not rely on them being different.
Member functions
- (constructor):Construct unordered_set (public member function )
- (destructor): Destroy unordered set (public member function)
- operator= Assign content (public member function )
Capacity
empty, size, max_size
Iterators
begin, end, cbegin, cend
Element lookup
- find: Get iterator to element (public member function)
- count: Count elements with a specific key (public member function)
- equal_range: Get range of elements with a specific key (public member function)
Modifiers
- emplace: Construct and insert element (public member function )
- emplace_hint: Construct and insert element with hint (public member function)
- insert: Insert elements (public member function )
- erase: Erase elements (public member function )
- clear: Clear content (public member function)
- swap: Swap content (public member function)
Buckets
- bucket_count: Return number of buckets (public member function)
- max_bucket_count: Return maximum number of buckets (public member function)
- bucket_size: Return bucket size (public member type)
- bucket: Locate element's bucket (public member function)
Hash policy
- load_factor: Return load factor (public member function)
- max_load_factor: Get or set maximum load factor (public member function)
- rehash: Set number of buckets (public member function )
- reserve: Request a capacity change (public member function)
Observers
- hash_function: Get hash function (public member type )
- key_eq: Get key equivalence predicate (public member type)
- get_allocator: Get allocator (public member function)
Non-member function overloads
- operators (unordered_set) Relational operators for unordered_set (function template )
- swap (unordered_set) Exchanges contents of two unordered_set containers (function template )
Code Example
#include <iostream>
#include <string>
#include <unordered_set>
using namespace std;
template<class T>
T cmerge(T a, T b){
T t(a);
t.insert(b.begin(), b.end());
return t;
}
int main(int argc, char **argv)
{
unordered_set<string> first1;
unordered_set<string> first2( {"one", "two", "three"} );
unordered_set<string> first3( {"red", "green", "blue"} );
unordered_set<string> first4( first2 );
unordered_set<string> first5( cmerge(first4,first3) );
unordered_set<string> first6( first5.begin(), first5.end() );
cout << "
First6 set: ";
for(const string& x: first6 ){
cout << " " << x;
}
return 0;
/** other function please to see the unordered_map */