Index for Section 3

Alphabetical listing for C

Bottom of page

Class::ISA(3)
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 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');

  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"

Index for Section 3

Alphabetical listing for C

Top of page