Inheritance not working

Mandor has asked for the wisdom of the Perl Monks concerning the following question:

While I have been using the OOP interface of many modules
(even wrote some simple OOP Modules myself) I have ignored
inheritance up to recently.

After reading the inheritance examples in the perldocs I thought
it would be a nice idea to test my new knowledge on the local
web project I am working on. After restructuring some things
I have finally come up with something like this.

file - Project\Client.pm

package Project::Client;
use strict;

sub new
{
    my ($class, %args) = shift;
    $class = ref ($class) || $class;

    my $self = {};
    
    ...
        
    bless ($self, $class);
    return $self;
}

...

file - Project\Client\Mode.pm

package Project::Client::Mode;
@ISA = ('Project::Client');
use strict;

...

file - Project\Client\Mode\TestMode.pm

package Project::Client::Mode::TestMode;
@ISA = ('Project::Client::Mode');
use strict;

...
[download]

Now I expect the following to work.


file - client.pl

use strict;
use lib '...';

my $mode = Project::Client::Mode::TestMode -> new (...);

...
[download]

But I get the dreaded 'Can't locate object method "new" via package "Project::Client::Mode::TestMode"'
message. What am I doing wrong?

Comment on Inheritance not working Select or Download Code

Replies are listed 'Best First'.
•Re: Inheritance not working by merlyn (Sage) on Mar 23, 2002 at 15:58 UTC
I prefer the "load and set inheritance" call of base for OO modules, so in `Project/Client/Mode.pm`, you merely put: `use base qw(Project::Client);` [download] -- Randal L. Schwartz, Perl hacker	[reply] [d/l]
Re: Inheritance not working by mattriff (Chaplain) on Mar 23, 2002 at 15:51 UTC
You need to "require" the modules that you are inheriting from. In Project::Client::Mode::TestMode you need: `require Project::Client::Mode;` [download] And, in Project::Client::Mode you need: `require Project::Client;` [download] That should take care of the error. - Matt Riffle	[reply] [d/l] [select]
Re: Inheritance not working by Kanji (Parson) on Mar 23, 2002 at 15:52 UTC
You need to `require` or `use` the modules you're inheriting from, so that Perl has access to it's code. `package Project::Client::Mode; require Project::Client; # <--- @ISA = ('Project::Client'); use strict; ...` [download] You might also want to check out the Inheritance section of perltoot, and `base` (which will combine the `require` and `@ISA`-population for you in one easy step). --k.	[reply] [d/l]
Re: Inheritance not working by jepri (Parson) on Mar 23, 2002 at 16:38 UTC
You also need to use `our @ISA=etc.` . If you move the `use strict` up one line you will see what I mean :) ____________________ Jeremy I didn't believe in evil until I dated it.	[reply] [d/l] [select]
Re: Re: Inheritance not working by chromatic (Archbishop) on Mar 23, 2002 at 18:22 UTC
Post hoc, ergo propter hoc. If he's not running under strict yet, any non-lexical variable declared will be autovivified as a package global. If he does swap the two lines, he will have to use our or vars, but that's because strict catches that sort of thing, not because it's necessary for inheritance purposes.	[reply]
Re: Inheritance not working by dreadpiratepeter (Priest) on Mar 23, 2002 at 15:53 UTC
You need to: `use Project::Client::Node::TestNode;` [download] in client.pl, or else the new subroutine never hits the symbol table. Perl will not go out and find packages just because you try to use a method in them. You must explicitly load them. UPDATE: I missed the requires in the parent classes. I tend to use `use base`, which does the requires automatically. Hope this helps. -pete "I am Jack's utter lack of disbelief"	[reply] [d/l] [select]
Re: Inheritance not working by trs80 (Priest) on Mar 23, 2002 at 19:45 UTC
I struggled with this when first doing OOP and I agree with merlyn, using `use base qw( Project::Client );` [download] style inheritance has proven the best solution for what I have needed. It removes the need to mess with ISA and/or require the module. It makes easy things easy :)	[reply] [d/l]