c++ std::unordered_set

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 << "\nFirst6 set: ";
    for(const string& x: first6 ){
        cout << "  " << x;
    }

    return 0;

    /** other function please to see the unordered_map */

Reference

cplusplus

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