OBJECTS(S)     User Contributed Perl Documentation     OBJECTS(S)



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

DESCRIPTION
       This still needs to be written properly.

       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 prob-
       lem 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(w);
           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 extentions are added by all PDL constructors
       automaticly.   The "PDL" attribute is used by perlDL to
       store the PDL object and all PDL methods use this
       attribute automaticly 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 sub-
       classed 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/sub-
       class2.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-Cre-
       ation.) This behavior is summarized as follows:

       o
        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.

       o
        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.



perl v5.6.1                 2000-05-26                 OBJECTS(S)