MirBSD manpage: Class::ISA(3p)

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


     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).

     @ISA stands for "is a" here, that is, a derived class "is a"

     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)

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

         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 con-
         sider 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

         The code for self_and_super_versions is meant to serve
         as an example for precisely the kind of tasks I antici-
         pate 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

     * The Class::ISA functions just look at @ISAs.  But theoret-
     ically, 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);

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

     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"

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