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[derivation]
16 Library introduction [library]
16.4 Library-wide requirements [requirements]
16.4.6 Conforming implementations [conforming]
16.4.6.13 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 asvirtual shall be virtual;
Every base class not specified asvirtual shall not be virtual;
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 assize_t ([support.types]) andstreamoff ([stream.types]). â end note]
All types specified in the C++ standard library shall be non-final types unless otherwise specified.