A helper (mixin) class to provide external references in an R/S-Plus portable way.
A cleaner mechanism would use external references, but historically this class has existed mainly for R/S-Plus portability.
Objects from the Class
A virtual Class: No objects may be created from it.
Object of class
"integer"this is an integer vector holding an opaque reference into a C struct (may or may not be a C pointer, may or may not have length one).