20 Library introduction [library]

20.5 Library-wide requirements [requirements]

20.5.5 Conforming implementations [conforming] Derived classes [derivation]

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:
  • Every base class described as virtual shall be virtual;
  • 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.
There is an implicit exception to this rule for types that are described as synonyms for basic integral types, such as size_­t and streamoff.