Provides a C++ Standard Library-compliant hash function object suitable for use with hash or unordered containers. The primary template definition is not defined. Specializations of this class are expected to:

• Conform to the interface of std::unary_function<T, size_t>. Specifically they must provide a typedef of T to argument_type and of size_t to result_type.
• Define operator() const such that it takes a single argument of type argument_type and returns result_type.
• Be copy constructible and destructible.