


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


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

SYNOPSIS
         # 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')),
               "\n";

       That prints:

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

DESCRIPTION
       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.

FUNCTIONS
       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



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


           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.

CAUTIONARY NOTES
       * 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');




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


       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
       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.

AUTHOR
       Sean M. Burke "sburke@cpan.org"








































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