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Showing content from https://cplusplus.com/reference/unordered_map/unordered_multimap/erase/ below:

public member function

<unordered_map>

std::unordered_multimap::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_multimap container either the elements whose key is k or those in a range ([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_multimap.
Member type const_iterator is a forward iterator type.
k: Key of the elements to be erased.
Member type key_type is the type of the keys for the elements in the container, defined in unordered_multimap as an alias of its first template parameter (Key).
first, last: Iterators specifying a range within the unordered_multimap 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_multimap 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.

Example

// unordered_multimap::erase
#include <iostream>
#include <unordered_map>

int main ()
{
  std::unordered_multimap<std::string,std::string> myumm = {
         {"strawberry","red"},
         {"banana","yellow"},
         {"orange","orange"},
         {"lemon","yellow"},
         {"apple","red"},
         {"apple","green"},
         {"pear","green"},
  };


  // erase examples:
  myumm.erase ( myumm.begin() );     // erasing by iterator
  myumm.erase ("apple");             // erasing by key (erases 2 elements)
  myumm.erase ( myumm.find("orange"), myumm.end() ); // erasing by range

  // show content:
  for ( auto& x: myumm )
    std::cout << x.first << ": " << x.second << std::endl;

  return 0;
}

Possible output:

banana: yellow
strawberry: red

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.