class::isa.3

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.\" ========================================================================
.\"
.IX Title "Class::ISA 3"
.TH Class::ISA 3 "2007-12-18" "perl v5.10.0" "Perl Programmers Reference Guide"
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
.\" way too many mistakes in technical documents.
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.SH "NAME"
Class::ISA \-\- report the search path for a class's ISA tree
.SH "SYNOPSIS"
.IX Header "SYNOPSIS"
.Vb 4
\&  # 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:\en ",
\&        join(", ", Class::ISA::super_path(\*(AqFood::Fishstick\*(Aq)),
\&        "\en";
.Ve
.PP
That prints:
.PP
.Vb 2
\&  Food::Fishstick path is:
\&   Food::Fish, Food, Matter, Life::Fungus, Life, Chemicals
.Ve
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
Suppose you have a class (like Food::Fish::Fishstick) that is derived,
via its \f(CW@ISA\fR, 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 \f(CW@ISA\fR, from one or
more superclasses (as above).
.PP
When, then, you call a method in that class ($fishstick\->calories),
Perl first searches there for that method, but if it's not there, it
goes searching in its superclasses, and so on, in a depth-first (or
maybe \*(L"height-first\*(R" is the word) search.  In the above example, it'd
first look in Food::Fish, then Food, then Matter, then Life::Fungus,
then Life, then Chemicals.
.PP
This library, Class::ISA, provides functions that return that list \*(--
the list (in order) of names of classes Perl would search to find a
method, with no duplicates.
.SH "FUNCTIONS"
.IX Header "FUNCTIONS"
.IP "the function Class::ISA::super_path($CLASS)" 4
.IX Item "the function Class::ISA::super_path($CLASS)"
This returns the ordered list of names of classes that Perl would
search thru in order to find a method, with no duplicates in the list.
\&\f(CW$CLASS\fR is not included in the list.  \s-1UNIVERSAL\s0 is not included \*(-- if
you need to consider it, add it to the end.
.IP "the function Class::ISA::self_and_super_path($CLASS)" 4
.IX Item "the function Class::ISA::self_and_super_path($CLASS)"
Just like \f(CW\*(C`super_path\*(C'\fR, except that \f(CW$CLASS\fR is included as the first
element.
.IP "the function Class::ISA::self_and_super_versions($CLASS)" 4
.IX Item "the function Class::ISA::self_and_super_versions($CLASS)"
This returns a hash whose keys are \f(CW$CLASS\fR and its
(super\-)superclasses, and whose values are the contents of each
class's \f(CW$VERSION\fR (or undef, for classes with no \f(CW$VERSION\fR).
.Sp
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.
.SH "CAUTIONARY NOTES"
.IX Header "CAUTIONARY NOTES"
* Class::ISA doesn't export anything.  You have to address the
functions with a \*(L"Class::ISA::\*(R" on the front.
.PP
* Contrary to its name, Class::ISA isn't a class; it's just a package.
Strange, isn't it?
.PP
* Say you have a loop in the \s-1ISA\s0 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 \*(L"never go down the same
path twice\*(R", and cyclicities are just a special case of that.
.PP
* The Class::ISA functions just look at \f(CW@ISAs\fR.  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.
.PP
* If Perl can't find a method anywhere in the \s-1ISA\s0 tree, it then looks
in the magical class \s-1UNIVERSAL\s0.  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:
.PP
.Vb 1
\&  @supers = Class::Tree::super_path($class);
.Ve
.PP
do this:
.PP
.Vb 1
\&  @supers = (Class::Tree::super_path($class), \*(AqUNIVERSAL\*(Aq);
.Ve
.PP
And don't say no-one ever told ya!
.PP
* When you call them, the Class::ISA functions look at \f(CW@ISAs\fR anew \*(--
that is, there is no memoization, and so if ISAs change during
runtime, you get the current \s-1ISA\s0 tree's path, not anything memoized.
However, changing ISAs at runtime is probably a sign that you're out
of your mind!
.SH "COPYRIGHT"
.IX Header "COPYRIGHT"
Copyright (c) 1999, 2000 Sean M. Burke. All rights reserved.
.PP
This library is free software; you can redistribute it and/or modify
it under the same terms as Perl itself.
.SH "AUTHOR"
.IX Header "AUTHOR"
Sean M. Burke \f(CW\*(C`[e-mail]\*(C'\fR