std::unordered_multiset<Key,Hash,KeyEqual,Allocator>::erase

	(1)
iterator erase( iterator pos );			(since C++11) (until C++23)
iterator erase( iterator pos ) requires(!std::same_as<iterator, const_iterator>);			(since C++23)
iterator erase( const_iterator pos );		(2)	(since C++11)
iterator erase( const_iterator first, const_iterator last );		(3)	(since C++11)
size_type erase( const Key& key );		(4)	(since C++11)
template< class K > size_type erase( K&& x );		(5)	(since C++23)

Removes specified elements from the container. The order of the remaining elements is preserved. (This makes it possible to erase individual elements while iterating through the container.)

1,2) Removes the element at pos. Only one overload is provided if iterator and const_iterator are the same type.

3) Removes the elements in the range [first, last), which must be a valid range in *this.

4) Removes all elements with the key equivalent to key.

5) Removes all elements 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.

References and iterators to the erased elements are invalidated. Other iterators and references are not invalidated.

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

1-3) Iterator following the last removed element.

4) Number of elements removed.

5) Number of elements removed.

Exceptions

1-3) Throws nothing.

4,5) Any exceptions thrown by the Hash and KeyEqual object.

Complexity

Given an instance c of unordered_multiset:

1,2) Average case: constant, worst case: c.size().

3) Average case: std::distance(first, last), worst case: c.size().

4) Average case: c.count(key), worst case: c.size().

5) Average case: c.count(x), worst case: c.size().

Notes

Feature-test macro	Value	Std	Feature
`__cpp_lib_associative_heterogeneous_erasure`	202110L	(C++23)	Heterogeneous erasure in associative containers and unordered associative containers; overload (5)

Example

Run this code

#include <unordered_set>
#include <iostream>
 
int main()
{
    std::unordered_multiset<int> c = {1, 2, 3, 4, 1, 2, 3, 4};
 
    auto print = [&c]
    {
        std::cout << "c = { ";
        for (int n : c)
            std::cout << n << ' ';
        std::cout << "}\n";
    };
    print();
 
    std::cout << "Erase all odd numbers:\n";
    for (auto it = c.begin(); it != c.end();)
    {
        if (*it % 2 != 0)
            it = c.erase(it);
        else
            ++it;
    }
    print();
 
    std::cout << "Erase 1, erased count: " << c.erase(1) << '\n';
    std::cout << "Erase 2, erased count: " << c.erase(2) << '\n';
    std::cout << "Erase 2, erased count: " << c.erase(2) << '\n';
    print();
}

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR	Applied to	Behavior as published	Correct behavior
LWG 2059	C++11	there was ambiguity for overload (2)	added overload (1)
LWG 2356	C++11	the order of non-equivalent elements that are not erased was not guaranteed to be preserved	required to be preserved

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