# map<Key, Data, Compare, Alloc>

Category: containers | Component type: type

### Description

`Map` is a Sorted Associative Container that associates objects of type `Key` with objects of type `Data`. `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 the same key.

`Map` has the important property that inserting a new element into a `map` does not invalidate iterators that point to existing elements. Erasing an element from a `map` also does not invalidate any iterators, except, of course, for iterators that actually point to the element that is being erased.

### Example

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

    int main()
    {
      map<const char*, int, ltstr> 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 << "june -> " << months["june"] << endl;
      map<const char*, int, ltstr>::iterator cur  = months.find("june");
      map<const char*, int, ltstr>::iterator prev = cur;
      map<const char*, int, ltstr>::iterator next = cur;
      ++next;
      --prev;
      cout << "Previous (in alphabetical order) is " << (*prev).first << endl;
      cout << "Next (in alphabetical order) is " << (*next).first << endl;
    }

### Definition

Defined in the standard header map, and in the nonstandard backward-compatibility header map.h.

### Template parameters

Parameter | Description | Default
--- | --- | ---
`Key` | The map's key type. This is also defined as `map::key_type`. |
`Data` | The map's data type. This is also defined as `map::data_type`. |
`Compare` | The key comparison function, a Strict Weak Ordering whose argument type is `key_type`; it returns `true` if its first argument is less than its second argument, and `false` otherwise. This is also defined as `map::key_compare`. | `less<Key>`
`Alloc` | The `map`'s allocator, used for all internal memory management. | `alloc`

### Model of

Unique Sorted Associative Container, Pair Associative Container

### Type requirements

- `Data` is Assignable.
- `Compare` is a Strict Weak Ordering whose argument type is `Key`.
- `Alloc` is an Allocator.

### Public base classes

None.

### Members

Member | Where defined | Description
--- | --- | ---
`key_type` | Associative Container | The `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 map.
`key_compare` | Sorted Associative Container | Function object that compares two keys for ordering.
`value_compare` | Sorted Associative Container | Function object that compares two values for ordering.
`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 `map`. [1]
`const_iterator` | Container | Const iterator used to iterate through a `map`.
`reverse_iterator` | Reversible Container | Iterator used to iterate backwards through a `map`. [1]
`const_reverse_iterator` | Reversible Container | Const iterator used to iterate backwards through a `map`.
`iterator begin()` | Container | Returns an `iterator` pointing to the beginning of the `map`.
`iterator end()` | Container | Returns an `iterator` pointing to the end of the `map`.
`const_iterator begin() const` | Container | Returns a `const_iterator` pointing to the beginning of the `map`.
`const_iterator end() const` | Container | Returns a `const_iterator` pointing to the end of the `map`.
`reverse_iterator rbegin()` | Reversible Container | Returns a `reverse_iterator` pointing to the beginning of the reversed map.
`reverse_iterator rend()` | Reversible Container | Returns a `reverse_iterator` pointing to the end of the reversed map.
`const_reverse_iterator rbegin() const` | Reversible Container | Returns a `const_reverse_iterator` pointing to the beginning of the reversed map.
`const_reverse_iterator rend() const` | Reversible Container | Returns a `const_reverse_iterator` pointing to the end of the reversed map.
`size_type size() const` | Container | Returns the size of the `map`.
`size_type max_size() const` | Container | Returns the largest possible size of the `map`.
`bool empty() const` | Container | `true` if the `map`'s size is `0`.
`key_compare key_comp() const` | Sorted Associative Container | Returns the `key_compare` object used by the `map`.
`value_compare value_comp() const` | Sorted Associative Container | Returns the `value_compare` object used by the `map`.
`map()` | Container | Creates an empty `map`.
`map(const key_compare& comp)` | Sorted Associative Container | Creates an empty `map`, using `comp` as the `key_compare` object.
    template <class InputIterator>
    map(InputIterator f, InputIterator l)
    [2]
| Unique Sorted Associative Container | Creates a map with a copy of a range.
    template <class InputIterator>
    map(InputIterator f, InputIterator l,
        const key_compare& comp)
    [2]
| Unique Sorted Associative Container | Creates a map with a copy of a range, using `comp` as the `key_compare` object.
`map(const map&)` | Container | The copy constructor.
`map& operator=(const map&)` | Container | The assignment operator
`void swap(map&)` | Container | Swaps the contents of two maps.
    pair<iterator, bool>
    insert(const value_type& x)
| Unique Associative Container | Inserts `x` into the `map`.
    iterator insert(iterator pos,
                    const value_type& x)
| Unique Sorted Associative Container | Inserts `x` into the `map`, using `pos` as a hint to where it will be inserted.
    template <class InputIterator>
    void insert(InputIterator, InputIterator)
    [2]
| Unique Sorted Associative Container | Inserts a range into the `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.
`iterator find(const key_type& k)` | Associative Container | Finds an element whose key is `k`.
`const_iterator find(const key_type& k) const` | Associative Container | Finds an element whose key is `k`.
`size_type count(const key_type& k)` | Unique Associative Container | Counts the number of elements whose key is `k`.
`iterator lower_bound(const key_type& k)` | Sorted Associative Container | Finds the first element whose key is not less than `k`.
`const_iterator lower_bound(const key_type& k) const` | Sorted Associative Container | Finds the first element whose key is not less than `k`.
`iterator upper_bound(const key_type& k)` | Sorted Associative Container | Finds the first element whose key greater than `k`.
`const_iterator upper_bound(const key_type& k) const` | Sorted Associative Container | Finds the first element whose key greater than `k`.
    pair<iterator, iterator>
    equal_range(const key_type& k)
| Sorted Associative Container | Finds a range containing all elements whose key is `k`.
    pair<const_iterator, const_iterator>
    equal_range(const key_type& k) const
| Sorted Associative Container | Finds a range containing all elements whose key is `k`.
    data_type&
    operator[](const key_type& k) [3]
| `map` | See below.
    bool operator==(const map&,
                    const map&)
| Forward Container | Tests two maps for equality. This is a global function, not a member function.
    bool operator<(const map&,
                   const map&)
| Forward Container | Lexicographical comparison. This is a global function, not a member function.

### New members

These members are not defined in the Unique Sorted Associative Container and Pair Associative Container requirements, but are unique to `map`:

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

### Notes

[1] `Map::iterator` is not a mutable iterator, because `map::value_type` is not Assignable. That is, if `i` is of type `map::iterator` and `p` is of type `map::value_type`, then `*i = p` is not a valid expression. However, `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. The same point applies to `map::reverse_iterator`.

[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 `map::const_iterator`.

[3] Since `operator[]` might insert a new element into the `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, Sorted Associative Container, Pair Associative Container, Unique Sorted Associative Container, `set` `multiset`, `multimap`, `hash_set`, `hash_map`, `hash_multiset`, `hash_multimap`,

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