Create an empty FlatHashMap 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 keys of 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 the keys of 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.