std::unordered_multimap erase() method

since C++23

// (1) Non const version only
iterator erase( iterator pos );
iterator erase( const_iterator pos );

// (2) Non const version only
iterator erase( const_iterator first, const_iterator last );

// (3) Non const version only
size_type erase( const Key& key );

// (4) Non const version only
template< class K >
size_type erase( K&& x );

since C++11

// (1) Non const version only
iterator erase( iterator pos );
iterator erase( const_iterator pos );

// (2) Non const version only
iterator erase( const_iterator first, const_iterator last );

// (3) Non const version only
size_type erase( const Key& key );

• (1) Removes the element at pos.
• (2) Removes the elements in the range [ first; last ), which must be a valid range in *this.
• (3) Removes the element (if one exists) with the key equivalent to key.
• (4) Removes the element (if one exists) with key that compares equivalent to the value x. This overload participates in overload resolution only if Hash::is_transparent and KeyEqual::is_transparent are valid and each denotes a type, and neither iterator nor const_iterator is implicitly convertible from K. This assumes that such Hash is callable with both K and Key type, and that the KeyEqual is transparent, which, together, allows calling this function without constructing an instance of Key.

The order of the elements that are not erased is preserved (this makes it possible to erase individual elements while iterating through the container).

Invalidation

References and iterators to the erased elements are invalidated.

Other iterators and references are not invalidated.

Valid iterator

The iterator pos must be valid and dereferenceable. Thus the end() iterator (which is valid, but is not dereferenceable) cannot be used as a value for pos.

Parameters

• pos - iterator to the element to remove
• first, last - range of elements to remove
• key - key value of the elements to remove
• x - a value of any type that can be transparently compared with a key denoting the elements to remove

Return value

• (1-2) - Iterator following the last removed element.
• (3-4) - Number of elements removed (0 or 1).

Complexity

• (1)
  Average case, constant - O(1).
  Worst case, linear in size of the container - O(size()).

• (2)
  Average case, linear in std::distance(first, last) - O(N), where N is std::distance(first, last).
  Worst case, linear in size of the container - O(size()).

• (3)
  Average case, count(key) - O(N), where N is count(key).
  Worst case, linear in size of the container - O(size()).

• (4)
  Average case, count(x) - O(N), where N is count(x).
  Worst case, linear in size of the container - O(size()).

Exceptions

• (1-2) (none)
• (3-4) Any exceptions thrown by the Hash and KeyEqual object.

Notes

Feature testing macro: __cpp_lib_associative_heterogeneous_erasure (for overload (4)).

Example

Main.cpp

#include <unordered_map>
#include <iostream>
int main()
{
    std::unordered_multimap<int, std::string> c = {
        {1, "one" }, {2, "two" }, {3, "three"},
        {4, "four"}, {5, "five"}, {6, "six"  }
    };
 
    // erase all odd numbers from c
    for(auto it = c.begin(); it != c.end(); ) {
        if(it->first % 2 != 0)
            it = c.erase(it);
        else
            ++it;
    }
 
    for(auto& p : c) {
        std::cout << p.second << ' ';
    }
}

Possible output

two four six