Class::ISA(3) Perl Programmers Reference Guide Class::ISA(3)NAMEClass::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();
print "Food::Fishstick path is:\n ",
join(", ", Class::ISA::super_path('Food::Fishstick')),
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 $CLASS is included as the first
the function Class::ISA::self_and_super_versions($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);
@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
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 "email@example.com"
perl v5.10.0 2007-12-18 Class::ISA(3)