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RedHat 9 (Linux i386) - man page for class::isa (redhat section 3pm)

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

       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 exam-
       ple, 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 element.

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

	   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 com-
	   ments 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 cyclic-
       ity; 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 "sburke@cpan.org"

perl v5.8.0				    2002-06-01				  Class::ISA(3pm)

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