16 Library introduction [library]

An implementation may derive any class in the C++ standard library from a class with a name reserved to the implementation.

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:

• (3.1)
  Every base class described as virtual shall be virtual;
• (3.2)
  Every base class not specified as virtual shall not be virtual;
• (3.3)
  Unless explicitly stated otherwise, types with distinct names shall be distinct types.
  [Note 1: 

  There is an implicit exception to this rule for types that are described as synonyms ([dcl.typedef], [namespace.udecl]), such as size_t ([support.types]) and streamoff ([stream.types]).

  — end note]

All types specified in the C++ standard library shall be non-final types unless otherwise specified.