2.14 Unification and foreign frames (version 2)

As documented with PL_unify(), if a unification call fails and control isn't made immediately to Prolog, any changes made by unification must be undone. The functions PL_open_foreign_frame(), PL_rewind_foreign_frame(), and PL_close_foreign_frame() are encapsulated in the class PlFrame, whose destructor calls PL_close_foreign_frame(). Using this, the example code with PL_unify() can be written:

{ PlFrame frame;
  ...
  if ( !t1.unify_term(t2) )
    frame.rewind();
  ...
}

Note that PlTerm::unify_term() checks for an exception and throws an exception to Prolog; if you with to handle exceptions, you must call PL_unify_term(t1.C_,t2.C_).