1#!/usr/local/bin/perl
2# Time-stamp: "2000-05-13 20:03:22 MDT" -*-Perl-*-
3
4package Class::ISA;
5require 5;
6use strict;
7use vars qw($Debug $VERSION);
8$VERSION = 0.32;
9$Debug = 0 unless defined $Debug;
10
11=head1 NAME
12
13Class::ISA -- report the search path for a class's ISA tree
14
15=head1 SYNOPSIS
16
17  # Suppose you go: use Food::Fishstick, and that uses and
18  # inherits from other things, which in turn use and inherit
19  # from other things.  And suppose, for sake of brevity of
20  # example, that their ISA tree is the same as:
21
22  @Food::Fishstick::ISA = qw(Food::Fish  Life::Fungus  Chemicals);
23  @Food::Fish::ISA = qw(Food);
24  @Food::ISA = qw(Matter);
25  @Life::Fungus::ISA = qw(Life);
26  @Chemicals::ISA = qw(Matter);
27  @Life::ISA = qw(Matter);
28  @Matter::ISA = qw();
29
30  use Class::ISA;
31  print "Food::Fishstick path is:\n ",
32        join(", ", Class::ISA::super_path('Food::Fishstick')),
33        "\n";
34
35That prints:
36
37  Food::Fishstick path is:
38   Food::Fish, Food, Matter, Life::Fungus, Life, Chemicals
39
40=head1 DESCRIPTION
41
42Suppose you have a class (like Food::Fish::Fishstick) that is derived,
43via its @ISA, from one or more superclasses (as Food::Fish::Fishstick
44is from Food::Fish, Life::Fungus, and Chemicals), and some of those
45superclasses may themselves each be derived, via its @ISA, from one or
46more superclasses (as above).
47
48When, then, you call a method in that class ($fishstick->calories),
49Perl first searches there for that method, but if it's not there, it
50goes searching in its superclasses, and so on, in a depth-first (or
51maybe "height-first" is the word) search.  In the above example, it'd
52first look in Food::Fish, then Food, then Matter, then Life::Fungus,
53then Life, then Chemicals.
54
55This library, Class::ISA, provides functions that return that list --
56the list (in order) of names of classes Perl would search to find a
57method, with no duplicates.
58
59=head1 FUNCTIONS
60
61=over
62
63=item the function Class::ISA::super_path($CLASS)
64
65This returns the ordered list of names of classes that Perl would
66search thru in order to find a method, with no duplicates in the list.
67$CLASS is not included in the list.  UNIVERSAL is not included -- if
68you need to consider it, add it to the end.
69
70
71=item the function Class::ISA::self_and_super_path($CLASS)
72
73Just like C<super_path>, except that $CLASS is included as the first
74element.
75
76=item the function Class::ISA::self_and_super_versions($CLASS)
77
78This returns a hash whose keys are $CLASS and its
79(super-)superclasses, and whose values are the contents of each
80class's $VERSION (or undef, for classes with no $VERSION).
81
82The code for self_and_super_versions is meant to serve as an example
83for precisely the kind of tasks I anticipate that self_and_super_path
84and super_path will be used for.  You are strongly advised to read the
85source for self_and_super_versions, and the comments there.
86
87=back
88
89=head1 CAUTIONARY NOTES
90
91* Class::ISA doesn't export anything.  You have to address the
92functions with a "Class::ISA::" on the front.
93
94* Contrary to its name, Class::ISA isn't a class; it's just a package.
95Strange, isn't it?
96
97* Say you have a loop in the ISA tree of the class you're calling one
98of the Class::ISA functions on: say that Food inherits from Matter,
99but Matter inherits from Food (for sake of argument).  If Perl, while
100searching for a method, actually discovers this cyclicity, it will
101throw a fatal error.  The functions in Class::ISA effectively ignore
102this cyclicity; the Class::ISA algorithm is "never go down the same
103path twice", and cyclicities are just a special case of that.
104
105* The Class::ISA functions just look at @ISAs.  But theoretically, I
106suppose, AUTOLOADs could bypass Perl's ISA-based search mechanism and
107do whatever they please.  That would be bad behavior, tho; and I try
108not to think about that.
109
110* If Perl can't find a method anywhere in the ISA tree, it then looks
111in the magical class UNIVERSAL.  This is rarely relevant to the tasks
112that I expect Class::ISA functions to be put to, but if it matters to
113you, then instead of this:
114
115  @supers = Class::Tree::super_path($class);
116
117do this:
118
119  @supers = (Class::Tree::super_path($class), 'UNIVERSAL');
120
121And don't say no-one ever told ya!
122
123* When you call them, the Class::ISA functions look at @ISAs anew --
124that is, there is no memoization, and so if ISAs change during
125runtime, you get the current ISA tree's path, not anything memoized.
126However, changing ISAs at runtime is probably a sign that you're out
127of your mind!
128
129=head1 COPYRIGHT
130
131Copyright (c) 1999, 2000 Sean M. Burke. All rights reserved.
132
133This library is free software; you can redistribute it and/or modify
134it under the same terms as Perl itself.
135
136=head1 AUTHOR
137
138Sean M. Burke C<sburke@cpan.org>
139
140=cut
141
142###########################################################################
143
144sub self_and_super_versions {
145  no strict 'refs';
146  map {
147        $_ => (defined(${"$_\::VERSION"}) ? ${"$_\::VERSION"} : undef)
148      } self_and_super_path($_[0])
149}
150
151# Also consider magic like:
152#   no strict 'refs';
153#   my %class2SomeHashr =
154#     map { defined(%{"$_\::SomeHash"}) ? ($_ => \%{"$_\::SomeHash"}) : () }
155#         Class::ISA::self_and_super_path($class);
156# to get a hash of refs to all the defined (and non-empty) hashes in
157# $class and its superclasses.
158#
159# Or even consider this incantation for doing something like hash-data
160# inheritance:
161#   no strict 'refs';
162#   %union_hash =
163#     map { defined(%{"$_\::SomeHash"}) ? %{"$_\::SomeHash"}) : () }
164#         reverse(Class::ISA::self_and_super_path($class));
165# Consider that reverse() is necessary because with
166#   %foo = ('a', 'wun', 'b', 'tiw', 'a', 'foist');
167# $foo{'a'} is 'foist', not 'wun'.
168
169###########################################################################
170sub super_path {
171  my @ret = &self_and_super_path(@_);
172  shift @ret if @ret;
173  return @ret;
174}
175
176#--------------------------------------------------------------------------
177sub self_and_super_path {
178  # Assumption: searching is depth-first.
179  # Assumption: '' (empty string) can't be a class package name.
180  # Note: 'UNIVERSAL' is not given any special treatment.
181  return () unless @_;
182
183  my @out = ();
184
185  my @in_stack = ($_[0]);
186  my %seen = ($_[0] => 1);
187
188  my $current;
189  while(@in_stack) {
190    next unless defined($current = shift @in_stack) && length($current);
191    print "At $current\n" if $Debug;
192    push @out, $current;
193    no strict 'refs';
194    unshift @in_stack,
195      map
196        { my $c = $_; # copy, to avoid being destructive
197          substr($c,0,2) = "main::" if substr($c,0,2) eq '::';
198           # Canonize the :: -> main::, ::foo -> main::foo thing.
199           # Should I ever canonize the Foo'Bar = Foo::Bar thing?
200          $seen{$c}++ ? () : $c;
201        }
202        @{"$current\::ISA"}
203    ;
204    # I.e., if this class has any parents (at least, ones I've never seen
205    # before), push them, in order, onto the stack of classes I need to
206    # explore.
207  }
208
209  return @out;
210}
211#--------------------------------------------------------------------------
2121;
213
214__END__
215