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;
               $x = PDL::pdl(BAR,5);
               $x->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:

       •
        For Simple functions, defined as having a signature of

         func( a(), [o]b() )

        PDL will call $a->copy to create the output object.

        In the spirit of the Perl philosophy of making Easy Things Easy, This behavior enables PDL-subclassed
        objects to be written without having to overload the many simple PDL functions in this category.

        The file t/subclass4.t in the PDL Distribution tests for this behavior.  See that file for an example.

       •
        For other functions, PDL will call $class->initialize to create the output object.  Where $class is the
        class name of the first argument supplied to the function.

        For these more complex cases, it is difficult to second-guess the subclassed object's designer to know
        if a "copy" or a "initialize" is appropriate. So for these cases, $class->initialize is called by
        default. If this is not appropriate for you, overload the function in your subclass and do whatever is
        appropriate is the overloaded function's code.

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.