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')),
        "\n";

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 CW@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 CW@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. CW$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 CWsuper_path, except that CW$CLASS is included as the first element.

the function Class::ISA::self_and_super_versions($CLASS)

This returns a hash whose keys are CW$CLASS and its (super-)superclasses, and whose values are the contents of each class's CW$VERSION (or undef, for classes with no CW$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 CW@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 CW@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 CWsburke@cpan.org