Class::ISA(3pm) Perl Programmers Reference Guide Class::ISA(3pm)
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 ($fishstick->calories), Perl first searches there for
that method, but if it's not there, it goes searching in its superclasses, and so on, in a depth-first depthfirst
first (or maybe "height-first" is the word) search. In the above example, it'd first look in
Food::Fish, then Food, then Matter, then Life::Fungus, then Life, then Chemicals.
This library, Class::ISA, provides functions that return that list -- the list (in order) of names of
classes Perl would search to find a method, with no duplicates.
FUNCTIONS
the function Class::ISA::super_path($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 $VERSION).
The code for self_and_super_versions is meant to serve as an example for precisely the kind of
tasks I anticipate that self_and_super_path and super_path will be used for. You are strongly
advised to read the source for self_and_super_versions, and the comments there.
CAUTIONARY NOTES
* Class::ISA doesn't export anything. You have to address the functions with a "Class::ISA::" on the
front.
* Contrary to its name, Class::ISA isn't a class; it's just a package. Strange, isn't it?
* Say you have a loop in the ISA tree of the class you're calling one of the Class::ISA functions on:
say that Food inherits from Matter, but Matter inherits from Food (for sake of argument). If Perl,
while searching for a method, actually discovers this cyclicity, it will throw a fatal error. The
functions in Class::ISA effectively ignore this cyclicity; the Class::ISA algorithm is "never go down
the same path twice", and cyclicities are just a special case of that.
* The Class::ISA functions just look at @ISAs. But theoretically, I suppose, AUTOLOADs could bypass
Perl's ISA-based search mechanism and do whatever they please. That would be bad behavior, tho; and
I try not to think about that.
* If Perl can't find a method anywhere in the ISA tree, it then looks in the magical class UNIVERSAL.
This is rarely relevant to the tasks that I expect Class::ISA functions to be put to, but if it mat-ters matters
ters to you, then instead of this:
@supers = Class::Tree::super_path($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 memoiza-tion, memoization,
tion, and so if ISAs change during runtime, you get the current ISA tree's path, not anything memo-ized. memoized.
ized. 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"
perl v5.8.8 2001-09-21 Class::ISA(3pm)
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