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Class::ISA(3)          Perl Programmers Reference Guide          Class::ISA(3)

       Class::ISA -- report the search path for a class's ISA tree

         # Suppose you go: use Food::Fishstick, and that uses and
         # inherits from other things, which in turn use and inherit
         # from other things.  And suppose, for sake of brevity of
         # example, that their ISA tree is the same as:

         @Food::Fishstick::ISA = qw(Food::Fish  Life::Fungus  Chemicals);
         @Food::Fish::ISA = qw(Food);
         @Food::ISA = qw(Matter);
         @Life::Fungus::ISA = qw(Life);
         @Chemicals::ISA = qw(Matter);
         @Life::ISA = qw(Matter);
         @Matter::ISA = qw();

         use Class::ISA;
         print "Food::Fishstick path is:\n ",
               join(", ", Class::ISA::super_path('Food::Fishstick')),

       That prints:

         Food::Fishstick path is:
          Food::Fish, Food, Matter, Life::Fungus, Life, Chemicals

       Suppose you have a class (like Food::Fish::Fishstick) that is derived, via its @ISA, from one or more super-
       classes (as Food::Fish::Fishstick is from Food::Fish, Life::Fungus, and Chemicals), and some of those super-
       classes may themselves each be derived, via its @ISA, from one or more superclasses (as above).

       When, then, you call a method in that class ($fishstick->calories), Perl first searches there for that method,
       but if it's not there, it goes searching in its superclasses, and so on, in a depth-first (or maybe
       "height-first" is the word) search.  In the above example, it'd first look in Food::Fish, then Food, then Mat-
       ter, then Life::Fungus, then Life, then Chemicals.

       This library, Class::ISA, provides functions that return that list -- the list (in order) of names of classes
       Perl would search to find a method, with no duplicates.

       the function Class::ISA::super_path($CLASS)
           This returns the ordered list of names of classes that Perl would search thru in order to find a method,
           with no duplicates in the list.  $CLASS is not included in the list.  UNIVERSAL is not included -- if you
           need to consider it, add it to the end.

       the function Class::ISA::self_and_super_path($CLASS)
           Just like "super_path", except that $CLASS is included as the first element.

       the function Class::ISA::self_and_super_versions($CLASS)
           This returns a hash whose keys are $CLASS and its (super-)superclasses, and whose values are the contents
           of each class's $VERSION (or undef, for classes with no $VERSION).

           The code for self_and_super_versions is meant to serve as an example for precisely the kind of tasks I
           anticipate that self_and_super_path and super_path will be used for.  You are strongly advised to read the
           source for self_and_super_versions, and the comments there.

       * Class::ISA doesn't export anything.  You have to address the functions with a "Class::ISA::" on the front.

       * Contrary to its name, Class::ISA isn't a class; it's just a package.  Strange, isn't it?

       * Say you have a loop in the ISA tree of the class you're calling one of the Class::ISA functions on: say that
       Food inherits from Matter, but Matter inherits from Food (for sake of argument).  If Perl, while searching for
       a method, actually discovers this cyclicity, it will throw a fatal error.  The functions in Class::ISA effec-
       tively ignore this cyclicity; the Class::ISA algorithm is "never go down the same path twice", and cyclicities
       are just a special case of that.

       * The Class::ISA functions just look at @ISAs.  But theoretically, I suppose, AUTOLOADs could bypass Perl's
       ISA-based search mechanism and do whatever they please.  That would be bad behavior, tho; and I try not to
       think about that.

       * If Perl can't find a method anywhere in the ISA tree, it then looks in the magical class UNIVERSAL.  This is
       rarely relevant to the tasks that I expect Class::ISA functions to be put to, but if it matters to you, then
       instead of this:

         @supers = Class::Tree::super_path($class);

       do this:

         @supers = (Class::Tree::super_path($class), 'UNIVERSAL');

       And don't say no-one ever told ya!

       * When you call them, the Class::ISA functions look at @ISAs anew -- that is, there is no memoization, and so
       if ISAs change during runtime, you get the current ISA tree's path, not anything memoized.  However, changing
       ISAs at runtime is probably a sign that you're out of your mind!

       Copyright (c) 1999, 2000 Sean M. Burke. All rights reserved.

       This library is free software; you can redistribute it and/or modify it under the same terms as Perl itself.

       Sean M. Burke ""

perl v5.8.8                       2001-09-21                     Class::ISA(3)