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Manual Reference Pages  -  CLASS::ISA (3)

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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 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. $CLASS is not included in the list. UNIVERSAL is not included — if you need to consider it, add it to the end.
the function Class::ISA::self_and_super_path($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.


* 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 (c) 1999-2009 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


Maintained by Steffen Mueller
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perl v5.20.3 CLASS::ISA (3) 2009-09-29

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