std::unordered_multiset<Key,Hash,KeyEqual,Allocator>::clear (3) - Linux Manuals
void clear() noexcept; (since C++11)
Erases all elements from the container. After this call, size() returns zero.
Invalidates any references, pointers, or iterators referring to contained elements. May also invalidate past-the-end iterators.
Linear in the size of the container, i.e., the number of elements.
The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
DR Applied to Behavior as published Correct behavior
LWG_2550 C++11 for unordered associative containers, unclear if complexity is linear in the number of elements or buckets clarified that it's linear in the number of elements
erase (public member function)