class::isa(3)
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:",
join(", ", Class::ISA::super_path('Food::Fishstick')),
"0;
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::Fun
gus, 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 ($fish
stick->calories), Perl first searches there for that
method, but if it's not there, it goes searching in its
superclasses, and so on, in a depth-first (or maybe
"height-first" is the word) search. In the above example,
it'd first look in Food::Fish, then Food, then Matter,
then Life::Fungus, then Life, then Chemicals.
This library, Class::ISA, provides functions that return
that list -- the list (in order) of names of classes Perl
would search to find a method, with no duplicates.
FUNCTIONS
- the function Class::ISA::super_path($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 con
tents of each class's $VERSION (or undef, for classes
with no $VERSION). - The code for self_and_super_versions is meant to serve
as an example for precisely the kind of tasks I antic
ipate that self_and_super_path and super_path will be
used for. You are strongly advised to read the source
for self_and_super_versions, and the comments there.
CAUTIONARY NOTES
* Class::ISA doesn't export anything. You have to address
the functions with a "Class::ISA::" on the front.
* Contrary to its name, Class::ISA isn't a class; it's
just a package. Strange, isn't it?
* Say you have a loop in the ISA tree of the class you're
calling one of the Class::ISA functions on: say that Food
inherits from Matter, but Matter inherits from Food (for
sake of argument). If Perl, while searching for a method,
actually discovers this cyclicity, it will throw a fatal
error. The functions in Class::ISA effectively ignore
this cyclicity; the Class::ISA algorithm is "never go down
the same path twice", and cyclicities are just a special
case of that.
* The Class::ISA functions just look at @ISAs. But theo
retically, I suppose, AUTOLOADs could bypass Perl's ISAbased search mechanism and do whatever they please. That
would be bad behavior, tho; and I try not to think about
that.
- * If Perl can't find a method anywhere in the ISA tree, it
then looks in the magical class UNIVERSAL. This is rarely
relevant to the tasks that I expect Class::ISA functions
to be put to, but if it matters to you, then instead of
this: - @supers = Class::Tree::super_path($class);
- do this:
@supers = (Class::Tree::super_path($class), 'UNIVER- SAL');
- 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
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"