Constructs an empty container with at least `n` buckets, using `hf` as the hash function, `eql` as the key equality predicate, `a` as the allocator and a maximum load factor of `1.0`.

Returns:;; The number of erased elements. Notes:;; Equivalent to: + + ```c++ auto original_size = c.size(); for (auto i = c.begin(), last = c.end(); i != last; ) { if (pred(*i)) { i = c.erase(i); } else { ++i; } } return original_size - c.size(); ```