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Showing content from https://cplusplus.com/reference/unordered_set/unordered_multiset/equal_range/ below:

public member function

<unordered_set>

std::unordered_multiset::equal_range 
pair<iterator,iterator>  equal_range ( const key_type& k );pair<const_iterator,const_iterator>  equal_range ( const key_type& k ) const;

Get range of elements with specific key

Returns the bounds of a range that includes all the elements in the container that compare equal to k.

If k does not match any element in the container, the range returned has end as both its lower and upper range bounds.



Parameters
k
Value to be compared.
Member type key_type is the type of the elements in the container. In unordered_set containers it is the same as value_type, defined as an alias of the class's first template parameter (Key).

Return value The function returns a pair, where its member pair::first is an iterator to the lower bound of the range, and pair::second is an iterator to its upper bound. The elements in the range are those between these two iterators, including pair::first, but not pair::second.

Member types iterator and const_iterator are forward iterator types.



Example 
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// unordered_multiset::equal_range
#include <iostream>
#include <string>
#include <unordered_set>

int main ()
{
  std::unordered_multiset<std::string> myums =
    {"cow","pig","pig","chicken","pig","chicken"};

  auto myrange = myums.equal_range("pig");

  std::cout << "These pigs were found:";

  while ( myrange.first != myrange.second ) {
    std::cout << " " << *myrange.first++;
  }

  std::cout << std::endl;

  return 0;
}

Output: 
These pigs were found: pig pig pig


Complexity Average case: constant.
Worst case: linear in container size.

Iterator validity No changes.

See also
unordered_multiset::count
Count elements with a specific key (public member function)
unordered_multiset::find
Get iterator to element (public member function)
unordered_multiset::begin
Return iterator to beginning (public member type)

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