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

public member function

<unordered_set>

std::unordered_multiset::erase by position (1)

iterator erase ( const_iterator position );

by key (2)

size_type erase ( const key_type& k );

range (3)

iterator erase ( const_iterator first, const_iterator last );

Erase elements

Removes from the unordered_multiset container either the elements whose value is k or a range of elements ([first,last)).

This effectively reduces the container size by the number of elements removed, calling each element's destructor.

Parameters

position: Iterator pointing to a single element to be removed from the unordered_multiset.
Member type const_iterator is a forward iterator type.
k: Value of the elements to be erased.
Member type key_type is the type of the elements in the container. In unordered_multiset containers it is the same as value_type, defined as an alias of the class's first template parameter (Key).
first, last: Iterators specifying a range within the unordered_multiset container to be removed: [first,last). i.e., the range includes all the elements between first and last, including the element pointed by first but not the one pointed by last.
Notice that unordered_multiset containers do not follow any particular order to organize its elements, therefore the effect of range deletions may not be easily predictable.
Member type const_iterator is a forward iterator type.

Return value Versions (1) and (3) return an iterator pointing to the position immediately following the last of the elements erased.
Version (2) returns the number of elements erased.

Member type iterator is a forward iterator type.

Member type size_type is an unsigned integral type.

All iterators in an unordered_multiset have const access to the elements: Elements can be inserted or removed, but not modified while in the container.

Example

// unordered_multiset::erase
#include <iostream>
#include <string>
#include <unordered_set>

int main ()
{
  std::unordered_multiset<std::string> myums =
  {"fish","duck","cow","cow","pig","hen","sheep"};

  myums.erase ( myums.begin() );                    // erasing by iterator
  myums.erase ( "sheep" );                          // erasing by key
  myums.erase ( myums.find("fish"), myums.end() );  // erasing by range

  std::cout << "myums contains:";
  for ( const std::string& x: myums ) std::cout << " " << x;
  std::cout << std::endl;

  return 0;
}

Possible output:

myums contains: pig cow cow

Complexity Average case: Linear in the number of elements removed (which is constant for versions (1) and (2)).
Worst case: Linear in the container size.

Iterator validity Only the iterators and references to the elements removed are invalidated.
The rest are unaffected.

The relative order of iteration of equivalent elements not removed by the operation is preserved.

The relative order of iteration of the elements not removed by the operation is preserved.