# hash_map<Key, Data, HashFcn, EqualKey, Alloc>

Category: containers | Component type: type

### Description

`Hash_map` is a Hashed Associative Container that associates objects of type `Key` with objects of type `Data`. `Hash_map` is a Pair Associative Container, meaning that its value type is `pair<const Key, Data>`. It is also a Unique Associative Container, meaning that no two elements have keys that compare equal using `EqualKey`.

Looking up an element in a `hash_map` by its key is efficient, so `hash_map` is useful for "dictionaries" where the order of elements is irrelevant. If it is important for the elements to be in a particular order, however, then `map` is more appropriate.

### Example

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

    int main()
    {
      hash_map<const char*, int, hash<const char*>, eqstr> months;

      months["january"] = 31;
      months["february"] = 28;
      months["march"] = 31;
      months["april"] = 30;
      months["may"] = 31;
      months["june"] = 30;
      months["july"] = 31;
      months["august"] = 31;
      months["september"] = 30;
      months["october"] = 31;
      months["november"] = 30;
      months["december"] = 31;

      cout << "september -> " << months["september"] << endl;
      cout << "april     -> " << months["april"] << endl;
      cout << "june      -> " << months["june"] << endl;
      cout << "november  -> " << months["november"] << endl;
    }

### Definition

Defined in the header hash_map, and in the backward-compatibility header hash_map.h. This class is an SGI extension; it is not part of the C++ standard.

### Template parameters

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

### Model of

Unique Hashed Associative Container, Pair 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
--- | --- | ---
`key_type` | Associative Container | The `hash_map`'s key type, `Key`.
`data_type` | Pair Associative Container | The type of object associated with the keys.
`value_type` | Pair Associative Container | The type of object, `pair<const key_type, data_type>`, stored in the hash_map.
`hasher` | Hashed Associative Container | The `hash_map`'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_map`. [1]
`const_iterator` | Container | Const iterator used to iterate through a `hash_map`.
`iterator begin()` | Container | Returns an `iterator` pointing to the beginning of the `hash_map`.
`iterator end()` | Container | Returns an `iterator` pointing to the end of the `hash_map`.
`const_iterator begin() const` | Container | Returns an `const_iterator` pointing to the beginning of the `hash_map`.
`const_iterator end() const` | Container | Returns an `const_iterator` pointing to the end of the `hash_map`.
`size_type size() const` | Container | Returns the size of the `hash_map`.
`size_type max_size() const` | Container | Returns the largest possible size of the `hash_map`.
`bool empty() const` | Container | `true` if the `hash_map`'s size is `0`.
`size_type bucket_count() const` | Hashed Associative Container | Returns the number of buckets used by the `hash_map`.
`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_map`.
`key_equal key_eq() const` | Hashed Associative Container | Returns the `key_equal` object used by the `hash_map`.
`hash_map()` | Container | Creates an empty `hash_map`.
`hash_map(size_type n)` | Hashed Associative Container | Creates an empty `hash_map` with at least `n` buckets.
    hash_map(size_type n,
             const hasher& h)
| Hashed Associative Container | Creates an empty `hash_map` with at least `n` buckets, using `h` as the hash function.
    hash_map(size_type n,
             const hasher& h,
             const key_equal& k)
| Hashed Associative Container | Creates an empty `hash_map` with at least `n` buckets, using `h` as the hash function and `k` as the key equal function.
    template <class InputIterator>
    hash_map(InputIterator f, InputIterator l)
    [2]
| Unique Hashed Associative Container | Creates a hash_map with a copy of a range.
    template <class InputIterator>
    hash_map(InputIterator f, InputIterator l,
             size_type n)
    [2]
| Unique Hashed Associative Container | Creates a hash_map with a copy of a range and a bucket count of at least `n`.
    template <class InputIterator>
    hash_map(InputIterator f, InputIterator l,
             size_type n, const hasher& h)
    [2]
| Unique Hashed Associative Container | Creates a hash_map with a copy of a range and a bucket count of at least `n`, using `h` as the hash function.
    template <class InputIterator>
    hash_map(InputIterator f, InputIterator l,
             size_type n, const hasher& h,
             const key_equal& k)
    [2]
| Unique Hashed Associative Container | Creates a hash_map 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_map(const hash_map&)` | Container | The copy constructor.
`hash_map& operator=(const hash_map&)` | Container | The assignment operator
`void swap(hash_map&)` | Container | Swaps the contents of two hash_maps.
    pair<iterator, bool>
    insert(const value_type& x)
| Unique Associative Container | Inserts `x` into the `hash_map`.
    template <class InputIterator>
    void insert(InputIterator f, InputIterator l)
    [2]
| Unique Associative Container | Inserts a range into the `hash_map`.
`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.
`void clear()` | Associative Container | Erases all of the elements.
`const_iterator find(const key_type& k) const` | Associative Container | Finds an element whose key is `k`.
`iterator find(const key_type& k)` | 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<const_iterator, const_iterator>
    equal_range(const key_type& k) const
| Associative Container | Finds a range containing all elements whose key is `k`.
    pair<iterator, iterator>
    equal_range(const key_type& k)
| Associative Container | Finds a range containing all elements whose key is `k`.
    data_type&
    operator[](const key_type& k) [3]
| `hash_map` | See below.
    bool operator==(const hash_map&,
                    const hash_map&)
| Hashed Associative Container | Tests two hash_maps for equality. This is a global function, not a member function.

### New members

These members are not defined in the Unique Hashed Associative Container and Pair Associative Container requirements, but are specific to `hash_map`.

Member | Description
--- | ---
    data_type&
    operator[](const key_type& k) [3]
| Returns a reference to the object that is associated with a particular key. If the `hash_map` does not already contain such an object, `operator[]` inserts the default object `data_type()`. [3]

### Notes

[1] `Hash_map::iterator` is not a mutable iterator, because `hash_map::value_type` is not Assignable. That is, if `i` is of type `hash_map::iterator` and `p` is of type `hash_map::value_type`, then `*i = p` is not a valid expression. However, `hash_map::iterator` isn't a constant iterator either, because it can be used to modify the object that it points to. Using the same notation as above, `(*i).second = p` is a valid expression.

[2] This member function relies on member template functions, which at present (early 1998) are not supported by all 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_map::const_iterator`.

[3] Since `operator[]` might insert a new element into the `hash_map`, it can't possibly be a `const` member function. Note that the definition of `operator[]` is extremely simple: `m[k]` is equivalent to `(*((m.insert(value_type(k, data_type()))).first)).second`. Strictly speaking, this member function is unnecessary: it exists only for convenience.

### See also

Associative Container, Hashed Associative Container, Pair Associative Container, Unique Hashed Associative Container, `set`, `map` `multiset`, `multimap`, `hash_set`, `hash_multiset`, `hash_multimap`

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