STL_map——map::equal_range

Reference:
Returns a pair of iterators respectively to the first element in a map with a key that is greater than a specified key and to the first element in the map with a key that is equal to or greater than the key.

Function:
pair equal_range (
   const Key& _Key
) const;
pair equal_range (
   const Key& _Key
) const;

Parameter:
_Key:The argument key value to be compared with the sort key of an element from the map being searched.

Return Value:
A pair of iterators such that the first is the lower_bound of the key and the second is the upper_bound of the key.
To access the first iterator of a pair pr returned by the member function, use pr.first, and to dereference the lower bound iterator, use *(pr.first). To access the second iterator of a pair pr returned by the member function, use pr.second, and to dereference the upper bound iterator, use *(pr.second).

Example:

#include 

Output:
The lower bound of the element with a key of 2 in the map m1 is: 20.
The upper bound of the element with a key of 2 in the map m1 is: 30.
A direct call of upper_bound( 2 ) gives 30,
 matching the 2nd element of the pair returned by equal_range( 2 ).
The map m1 doesn't have an element with a key less than 40.