NAME

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

DESCRIPTION

       This describes how to subclass PDL objects for fun and profit.

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

         $ndarray->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 ndarray contains.  The first case you can do with

         package BAR;
         our @ISA=qw/PDL/;
         sub foo {my($this) = @_; fiddle;}

         package main;
         $x = BAR->pdl(5);
         $x->foo;

       However, because a PDL object is an opaque reference to a C struct, it is not instantly
       possible to extend the PDL class by e.g. extra data via subclassing.

   Perl values into underlying C values
       "typemap"s turns Perl values into the C entities needed by the C code. PDL's one uses
       "SvPDLV" to extract a "pdl*" from a Perl value. That knows how to deal with:

       •   "simple" blessed scalars that have a C pointer

       •   blessed hash-refs that have a "PDL" member that's a C pointer as above

       •   blessed hash-refs that have a "PDL" member that's a Perl code-ref.  It will be called,
           and is expected to return an ndarray that conforms to one of cases 1-2 or 4-5 above.
           As of 2.094, the original hash-ref will be passed as the first argument. This means
           you can give a method as shown below, rather than needing to make a closure each time.
           Example from t/subclass.t:

             package PDL::Derived2;
             # This is a array of ones of dim 'Coeff'
             # All that is stored initially is "Coeff", the
             # PDL array is only realised when a boring PDL
             # function is called on it. One can imagine methods
             # in PDL::Derived2 doing manipulation on the Coeffs
             # rather than actualizing the data.
             our @ISA = qw/PDL/;
             sub new {
               my $class = shift;
               bless {
                 Coeff=>shift,
                 PDL=>\&cache,
                 SomethingElse=>42,
               }, $class;
             }
             # Actualize the value (demonstrating cacheing)
             # One can imagine expiring the cache if say, Coeffs change
             sub cache {
               my $self = shift;
               return $self->{Cache} if exists $self->{Cache};
               $self->{Cache} = PDL->ones(@$self{qw(Coeff Coeff)})+2;
             }

       •   "simple" Perl data, either a scalar or an array-reference

       •   blessed hash-refs that are a Math::Complex (or subclass) object - special case of the
           "scalar" above

       The rest of this document deals with the second case above.

   Inheritance
       To enable subclassing, make a package and bless a hash-ref into it with a "PDL" member.
       Make that package inherit from "PDL", and redefine the method "initialize".

         package FOO;
         our ISA = qw(PDL::Hash);
         sub initialize {
           my $class = shift;
           my $self = $class->SUPER::initialize(@_);
           $self->{creation_time} = time(); # necessary extension :-)
           $self;
         }

       All PDL constructors will call initialize() to make sure that your extensions are added by
       all PDL constructors automatically.

       Do remember that if you subclass a class that is subclassed from an ndarray, you need to
       call "SUPER::initialize". Make sure it is callable as an instance method, e.g. by copying
       data from $class if ref $class is true.

   Examples
       You can find some simple examples of PDL subclassing in the PDL distribution test-case
       files. Look in t/subclass.t.

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

       PDL will call "initialize" on the first input argument if it is an object, else on "PDL"
       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.

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.  All
       rights reserved. There is no warranty. You are allowed to copy this on the same terms as
       Perl itself.