Class::ISA(3p) Perl Programmers Reference Guide Class::ISA(3p)
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).
@ISA stands for "is a" here, that is, a derived class "is a"
superclass.
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)
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Class::ISA(3p) Perl Programmers Reference Guide Class::ISA(3p)
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. $CLASS is not included in the
list. UNIVERSAL is not included -- if you need to con-
sider it, add it to the end.
the function Class::ISA::self_and_super_path($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
$VERSION).
The code for self_and_super_versions is meant to serve
as an example for precisely the kind of tasks I antici-
pate that self_and_super_path and super_path will be
used for. You are strongly advised to read the source
for self_and_super_versions, and the comments there.
* Class::ISA doesn't export anything. You have to address
the functions with a "Class::ISA::" on the front.
* Contrary to its name, Class::ISA isn't a class; it's just
a package. Strange, isn't it?
* Say you have a loop in the ISA tree of the class you're
calling one of the Class::ISA functions on: say that Food
inherits from Matter, but Matter inherits from Food (for
sake of argument). If Perl, while searching for a method,
actually discovers this cyclicity, it will throw a fatal
error. The functions in Class::ISA effectively ignore this
cyclicity; the Class::ISA algorithm is "never go down the
same path twice", and cyclicities are just a special case of
that.
* The Class::ISA functions just look at @ISAs. But theoret-
ically, I suppose, AUTOLOADs could bypass Perl's ISA-based
search mechanism and do whatever they please. That would be
bad behavior, tho; and I try not to think about that.
* If Perl can't find a method anywhere in the ISA tree, it
then looks in the magical class UNIVERSAL. This is rarely
relevant to the tasks that I expect Class::ISA functions to
be put to, but if it matters to you, then instead of this:
@supers = Class::Tree::super_path($class);
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Class::ISA(3p) Perl Programmers Reference Guide Class::ISA(3p)
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, 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 "sburke@cpan.org"
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