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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 superclasses (as Food::Fish::Fishstick
       is from Food::Fish, Life::Fungus, and Chemicals), and some of those
       superclasses 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 Matter, 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

       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 effectively 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

       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 v5.10.1			  2009-02-12			 Class::ISA(3)

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