hash_set<Key, HashFcn, EqualKey, Alloc>


Category: containers	Component type: type

Description

Hash_set is a Hashed Associative Container that stores objects of type Key. Hash_set is a Simple Associative Container, meaning that its value type, as well as its key type, is Key. It is also a Unique Associative Container, meaning that no two elements compare equal using the Binary Predicate EqualKey.

Hash_set is useful in applications where it is important to be able to search for an element quickly. If it is important for the elements to be in a particular order, however, then set is more appropriate.

Example

struct eqstr
{
  bool operator()(const char* s1, const char* s2) const
  {
    return strcmp(s1, s2) == 0;
  }
};

void lookup(const hash_set<const char*, hash<const char*>, eqstr>& Set,
            const char* word)
{
  hash_set<const char*, hash<const char*>, eqstr>::const_iterator it
    = Set.find(word);
  cout << word << ": "
       << (it != Set.end() ? "present" : "not present")
       << endl;
}

int main()
{
  hash_set<const char*, hash<const char*>, eqstr> Set;
  Set.insert("kiwi");
  Set.insert("plum");
  Set.insert("apple");
  Set.insert("mango");
  Set.insert("apricot");
  Set.insert("banana");

  lookup(Set, "mango");
  lookup(Set, "apple");
  lookup(Set, "durian");
}

Definition

Hash_set is declared in hash_set.h. The implementation is in hash_set.h and hashtable.h.

Template parameters

Parameter	Description	Default
`Key`	The hash_set's key type and value type. This is also defined as `hash_set::key_type` and `hash_set::value_type`
`HashFcn`	The Hash Function used by the hash_set. This is also defined as `hash_set::hasher`.	`hash<Key>`
`EqualKey`	The hash_set's key equality function: a binary predicate that determines whether two keys are equal. This is also defined as `hash_set::key_equal`.	`equal_to<Key>`
`Alloc`	The `hash_set`'s allocator, used for all internal memory management.	`alloc`

Model of

Unique Hashed Associative Container, Simple Associative Container

Type requirements

Key is Assignable.
EqualKey is a Binary Predicate whose argument type is Key.
EqualKey is an equivalence relation.
Alloc is an Allocator.

Public base classes

None.

Members

Member	Where defined	Description
`value_type`	Container	The type of object, `T`, stored in the hash_set.
`key_type`	Associative Container	The key type associated with `value_type`.
`hasher`	Hashed Associative Container	The `hash_set`'s Hash Function.
`key_equal`	Hashed Associative Container	Function object that compares keys for equality.
`pointer`	Container	Pointer to `T`.
`reference`	Container	Reference to `T`
`const_reference`	Container	Const reference to `T`
`size_type`	Container	An unsigned integral type.
`difference_type`	Container	A signed integral type.
`iterator`	Container	Iterator used to iterate through a `hash_set`.
`const_iterator`	Container	Const iterator used to iterate through a `hash_set`. (`Iterator` and `const_iterator` are the same type.)
`iterator begin() const`	Container	Returns an `iterator` pointing to the beginning of the `hash_set`.
`iterator end() const`	Container	Returns an `iterator` pointing to the end of the `hash_set`.
`size_type size() const`	Container	Returns the size of the `hash_set`.
`size_type max_size() const`	Container	Returns the largest possible size of the `hash_set`.
`bool empty() const`	Container	`true` if the `hash_set`'s size is `0`.
`size_type bucket_count() const`	Hashed Associative Container	Returns the number of buckets used by the `hash_set`.
`void resize(size_type n)`	Hashed Associative Container	Increases the bucket count to at least `n`.
`hasher hash_funct() const`	Hashed Associative Container	Returns the `hasher` object used by the `hash_set`.
`key_equal key_eq() const`	Hashed Associative Container	Returns the `key_equal` object used by the `hash_set`.
`hash_set()`	Container	Creates an empty `hash_set`.
`hash_set(size_type n)`	Hashed Associative Container	Creates an empty `hash_set` with at least `n` buckets.
hash_set(size_type n, const hasher& h)	Hashed Associative Container	Creates an empty `hash_set` with at least `n` buckets, using `h` as the hash function.
hash_set(size_type n, const hasher& h, const key_equal& k)	Hashed Associative Container	Creates an empty `hash_set` with at least `n` buckets, using `h` as the hash function and `k` as the key equal function.
template <class InputIterator> hash_set(InputIterator f, InputIterator l) [1]	Unique Hashed Associative Container	Creates a hash_set with a copy of a range.
template <class InputIterator> hash_set(InputIterator f, InputIterator l, size_type n) [1]	Unique Hashed Associative Container	Creates a hash_set with a copy of a range and a bucket count of at least `n`.
template <class InputIterator> hash_set(InputIterator f, InputIterator l, size_type n, const hasher& h) [1]	Unique Hashed Associative Container	Creates a hash_set with a copy of a range and a bucket count of at least `n`, using `h` as the hash function.
hash_set(InputIterator f, InputIterator l, size_type n, const hasher& h, const key_equal& k) [1]	Unique Hashed Associative Container	Creates a hash_set with a copy of a range and a bucket count of at least `n`, using `h` as the hash function and `k` as the key equal function.
`hash_set(const hash_set&)`	Container	The copy constructor.
`hash_set& operator=(const hash_set&)`	Container	The assignment operator
`void swap(hash_set&)`	Container	Swaps the contents of two hash_sets.
pair<iterator, bool> insert(const value_type& x)	Unique Associative Container	Inserts `x` into the `hash_set`.
template <class InputIterator> void insert(InputIterator f, InputIterator l) [1]	Unique Associative Container	Inserts a range into the `hash_set`.
`void erase(iterator pos)`	Associative Container	Erases the element pointed to by `pos`.
`size_type erase(const key_type& k)`	Associative Container	Erases the element whose key is `k`.
`void erase(iterator first, iterator last)`	Associative Container	Erases all elements in a range.
`iterator find(const key_type& k) const`	Associative Container	Finds an element whose key is `k`.
`size_type count(const key_type& k) const`	Unique Associative Container	Counts the number of elements whose key is `k`.
pair<iterator, iterator> equal_range(const key_type& k) const	Associative Container	Finds a range containing all elements whose key is `k`.
bool operator==(const hash_set&, const hash_set&)	Hashed Associative Container	Tests two hash_sets for equality. This is a global function, not a member function.

New members

All of hash_set's members are defined in the Unique Hashed Associative Container and Simple Associative Container requirements. Hash_set does not introduce any new members.

Notes

[1] This member function relies on member template functions, which at present (June 1997) are supported by very few compilers. If your compiler supports member templates, you can call this function with any type of input iterator. If your compiler does not yet support member templates, though, then the arguments must either be of type const value_type* or of type hash_set::const_iterator.