Create an empty FlatHashSet object. Optionally specify a capacity indicating the minimum initial size of the underlying array of entries of this container. If capacity is not supplied or is 0, no memory is allocated. Optionally specify a hash functor used to generate the hash values associated with the elements in this container. If hash is not supplied, a default‐constructed object of the (template parameter) type HASH is used. Optionally specify an equality functor equal used to determine whether two elements are equivalent. If equal is not supplied, a default‐constructed object of the (template parameter) type EQUAL is used. Optionally specify a basicAllocator used to supply memory. If basicAllocator is not supplied or is 0, the currently installed default allocator is used.