Re: OO Perl - How to maintain class state when using inherited methods?

On 08/22/2006 02:53 AM, jeff_nokes@xxxxxxxxx wrote:
Hi,
Here are a couple example class files:

#Collection.pm

package Collection;

our $ClassType = 'Collection_type'; # Class variable

sub new ($;) {
my($class_name) = @_;
my $self = {};
$self->{class_name} = __PACKAGE__;
$self->{property_1} = 'foo';
bless($self, $class_name);
return($self);
}# end new()


sub serializeObject($;) {
my($self) = @_;
my $serialized_object = $ClassType . ',' .
$self->{class_name} . ',' .
$self->{property_1};
return($serialized_object);
}# end serializeObject()


sub getClassType($;) {
return($ClassType);
}# end getClassType()

1;

----------------------------------

#Addresses.pm

package Addresses;

use Collection;

our @ISA = (
'Collection',
);

our $ClassType = 'Addresses_type'; # Class variable.

1;

-------------------------------

Test script:

perl -e 'use Data::Dumper; use Collection; use Addresses;'\
-e 'my $obj_addr = Addresses->new();'\
-e 'print("\n" . Dumper($obj_addr) . "\n" . '\
-e ' "Serialized Object = " . $obj_addr->serializeObject() .
"\n" . '\
-e ' "getClassTtype = " . $obj_addr->getClassType() . "\n"'\
-e ');'

--------------------------------

Output:

$VAR1 = bless( {
'class_name' => 'Collection',
'property_1' => 'foo'
}, 'Addresses' );

Serialized Object = Collection_type,Collection,foo
getClassTtype = Collection_type

---------------------------------------

Questions:

(1) Referencing the dumped object in the above output, I can see that the namespace of the object is 'Addresses', but all of the internal properties are that of the parent class Collection.pm. Is it that the namespace was set to 'Addresses' simply due to the arrow notation implicitly invoking the parent class' new() method? And the object really has Collection properties because Perl already walked up the ISA tree into the Collection namespace?


Yes

(2) Both classes have the same class level variable $ClassType. Even though the namespace of the $obj_addr is 'Addresses', when invoking the inherited method getClassType(), the output still reflects that of the parent class' $ClassType? Why? Again, has Perl already walked up the
ISA tree to execute this inherited method, and therefore it sees the namespace 'Collection' vice 'Addresses'?


Yes

(3) If I want to keep Addresses level stateful data inside the object, is the proper way to do so to pass the data into the inherited constuctor as arguments?


No

example:

$obj_addr = Addresses->new($Addresses::ClassType);

Collection.pm: new()
------------
sub new ($;$) {
my($class_name, $stateful_data) = @_;
my $self = {};
$self->{data_type} = $stateful_data;

This is better:
$self->{data_type} = __PACKAGE__;


bless($self, $class_name);
return($self);
}# end new()


I feel like I'm missing some basic premis, that this should be able to be done without the explicit need to pass the data into the inherited constructor. Does anyone have any suggestions on how to achieve this differenlty then my example above?

Thanks in advance for any advice you can offer.

- J.


Read "perldoc perltooc" and possibly use Class::Data::Inheritable.
.

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