Functor

A Functor is a class (in OSG a C++ class of course) which has a definition of the operator (). That is a functor is any class which could be used as a function.

Of course the functor can then access or use any member variables of the class. A (non-const) functor could modify the values of member variables, but this is deprecated as the source of modification of the value is not obvious to the developer.