NAME

PDL::Objects -- Object-Orientation, what is it and how to exploit it


DESCRIPTION

This still needs to be written properly. [Also, is there a good reason we don't recommend storing extra object data in the header hash?]

Inheritance

There are basically two reasons for subclassing piddles. The first is simply that you want to be able to use your own routines like

        $piddle->something()

but don't want to mess up the PDL namespace (a worthy goal, indeed!). The other is that you wish to provide special handling of some functions or more information about the data the piddle contains. In the first case, you can do with

        package BAR;
        @ISA=qw/PDL/;
        sub foo {my($this) = @_; fiddle;}
        package main;
        $a = PDL::pdl(BAR,5);
        $a->foo();

However, because a PDL object is an opaque reference to a C struct, it is not possible to extend the PDL class by e.g. extra data via subclassing. To circumvent this problem PerlDL has built-in support to extent the PDL class via the has-a relation for blessed hashes. You can get the HAS-A behave like IS-A simply in that you assign the PDL object to the attribute named PDL and redefine the method initialize().

    package FOO;
    @FOO::ISA = qw(PDL);
    sub initialize {
        my $class = shift;
        my $self = {
                creation_time => time(),  # necessary extension :-)
                PDL => null,             # used to store PDL object
                };
        bless $self, $class;
    }

All PDL constructors will call initialize() to make sure that your extensions are added by all PDL constructors automatically. The PDL attribute is used by perlDL to store the PDL object and all PDL methods use this attribute automatically if they are called with a blessed hash reference instead of a PDL object (a blessed scalar).

Do remember that if you subclass a class that is subclassed from a piddle, you need to call SUPER::initialize.

NEED STUFF ABOUT CODE REFs!!

Examples

You can find some simple examples of PDL subclassing in the PDL distribution test-case files. Look in t/subclass2.t, t/subclass3.t, etc.

Output Auto-Creation and Subclassed Objects

For PDL Functions where the output is created and returned, PDL will either call the subclassed object's initialize or copy method to create the output object. (See PDL::Indexing for a discussion on Output Auto-Creation.) This behavior is summarized as follows:


AUTHOR

Copyright (C) Karl Glazebrook (kgb@aaoepp.aao.gov.au), Tuomas J. Lukka, (lukka@husc.harvard.edu) and Christian Soeller (c.soeller@auckland.ac.nz) 2000. Commercial reproduction of this documentation in a different format is forbidden.