public member function
<map>

std::map::equal_range

pair<const_iterator,const_iterator> equal_range (const key_type& k) const;
pair<iterator,iterator>             equal_range (const key_type& k);
Get range of equal elements
Returns the bounds of a range that includes all the elements in the container which have a key equivalent to k.

Because the elements in a map container have unique keys, the range returned will contain a single element at most.

If no matches are found, the range returned has a length of zero, with both iterators pointing to the first element that has a key considered to go after k according to the container's internal comparison object (key_comp).

Two keys are considered equivalent if the container's comparison object returns false reflexively (i.e., no matter the order in which the keys are passed as arguments).

Parameters

k
Key to search for.
Member type key_type is the type of the elements in the container, defined in map as an alias of its first template parameter (Key).

Return value

The function returns a pair, whose member pair::first is the lower bound of the range (the same as lower_bound), and pair::second is the upper bound (the same as upper_bound).

If the map object is const-qualified, the function returns a pair of const_iterator. Otherwise, it returns a pair of iterator.

Member types iterator and const_iterator are bidirectional iterator types pointing to elements (of type value_type).
Notice that value_type in map containers is itself also a pair type: pair<const key_type, mapped_type>.

Example

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// map::equal_range
#include <iostream>
#include <map>

int main ()
{
  std::map<char,int> mymap;

  mymap['a']=10;
  mymap['b']=20;
  mymap['c']=30;

  std::pair<std::map<char,int>::iterator,std::map<char,int>::iterator> ret;
  ret = mymap.equal_range('b');

  std::cout << "lower bound points to: ";
  std::cout << ret.first->first << " => " << ret.first->second << '\n';

  std::cout << "upper bound points to: ";
  std::cout << ret.second->first << " => " << ret.second->second << '\n';

  return 0;
}


lower bound points to: 'b' => 20
upper bound points to: 'c' => 30

Complexity

Logarithmic in size.

Iterator validity

No changes.

Data races

The container is accessed (neither the const nor the non-const versions modify the container).
No mapped values are accessed: concurrently accessing or modifying elements is safe.

Exception safety

Strong guarantee: if an exception is thrown, there are no changes in the container.

See also

  • 9316991614 2018-02-25
  • 191841613 2018-02-25
  • 16411612 2018-02-25
  • 6081981611 2018-02-25
  • 1784561610 2018-02-25
  • 4651609 2018-02-25
  • 9348111608 2018-02-25
  • 2513771607 2018-02-25
  • 6478461606 2018-02-24
  • 4985791605 2018-02-24
  • 5637141604 2018-02-24
  • 282181603 2018-02-24
  • 6217941602 2018-02-24
  • 5076141601 2018-02-24
  • 714281600 2018-02-24
  • 6607141599 2018-02-24
  • 949041598 2018-02-24
  • 6809961597 2018-02-24
  • 671871596 2018-02-24
  • 7107821595 2018-02-24