Certain classes defined in the C++ standard library are required to be derived from other classes in the C++ standard library. An implementation may derive such a class directly from the required base or indirectly through a hierarchy of base classes with names reserved to the implementation.
In any case:
Every base class not specified as virtual shall not be virtual;
Unless explicitly stated otherwise, types with distinct names shall be distinct types.182
All types specified in the C++ standard library shall be non-final types unless otherwise specified.