PDL::Objects -- Object-Orientation, what is it and how to exploit it
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?]
There are basically two reasons for subclassing ndarrays. The first is simply
that you want to be able to use your own routines like
$ndarray->something()
but don't want to mess up the PDL namespace (a worthy goal, indeed!). The other
is that you wish to provide special handling of some functions or more
information about the data the ndarray contains. In the first case, you can do
with
package BAR;
@ISA=qw/PDL/;
sub foo {my($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 an ndarray, you
need to call SUPER::initialize.
NEED STUFF ABOUT CODE REFs!!
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.
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.
Copyright (C) Karl Glazebrook (
[email protected]), Tuomas J. Lukka,
(
[email protected]) and Christian Soeller (
[email protected])
2000. All rights reserved. There is no warranty. You are allowed to copy this
on the same terms as Perl itself.