In the Apache C++ Standard Library, the names of entities that start with two underscores refer to implementation details. They are workarounds for missing library facilities, caused by the lack of compiler support, and are not meant to be used by client code. These entities, without the underscore notation, were part of the original Hewlett-Packard Standard Template Library implementation, but were not ratified as part of the C++ Standard. They are documented in Part III, "Obsolete Classes Reference," but we do not encourage their use.