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
@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 "
[email protected]"
Maintained by Steffen Mueller "
[email protected]".