OBJECTS(1) User Contributed Perl Documentation OBJECTS(1)NAMEPDL::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:
·
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.
perl v5.20.2 2015-05-24 OBJECTS(1)