---
name: "Class::ISA"
section: "3perl"
platform: "linux"
package: "perl-doc"
version: "5.10.1-17squeeze6"
release: "Debian 6.0 (Squeeze)"
platforms: ["linux"]
url: "https://new.carta.tech/Class::ISA"
---

# Class::ISA(3perl)

## 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:\n ",
        join(", ", Class::ISA::super_path('Food::Fishstick')),
        "\n";
```

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::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.

## 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 `$CLASS` and its (super-)superclasses, and whose values are the contents of each class's `$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.

## 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 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

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"`