This document defines conditionally-supported features, in the form of additional member functions on types that satisfy Protocol ([socket.reqmts.protocol]), Endpoint ([socket.reqmts.endpoint]), SettableSocketOption ([socket.reqmts.settablesocketoption]), GettableSocketOption ([socket.reqmts.gettablesocketoption]) or IoControlCommand ([socket.reqmts.iocontrolcommand]) requirements.
[ Note: This is so that, when the additional member functions are available, C++ programs can extend the library to add support for other protocols and socket options. — end note ]
For the purposes of this document, implementations that provide all of the additional member functions are known as extensible implementations.
[ Note: Implementations are encouraged to provide the additional member functions, where possible. It is intended that POSIX and Windows implementations will provide them. — end note ]