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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

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.second 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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