Cannot subclass Gearman::XS::Client

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

Hi, I've been trying to create a simple subclass of Gearman::XS::Client, but seem unable to do so. I can instantiate the subclass, but the object is not of the correct class:

use Test::More;
use strict;
use warnings;

{
    package My::Gearman::Client;
    use parent 'Gearman::XS::Client';

    sub foo {}
}

my $gm = My::Gearman::Client->new;

isa_ok($gm, 'Gearman::XS::Client'); # Pass
isa_ok($gm, 'My::Gearman::Client'); # Fail
can_ok($gm, 'foo'); # Fail
[download]

The code works as expected if I substitute Gearman::XS::Client with another module, such as JSON::XS. I've tried use parent, use base, manually modifying @ISA, and using extends from Mouse. I'm assuming that the problem is caused by Gearman::XS::Client, buy why?

Comment on Cannot subclass Gearman::XS::Client Select or Download Code

Replies are listed 'Best First'.
Re: Cannot subclass Gearman::XS::Client by MidLifeXis (Monsignor) on Jul 24, 2014 at 19:47 UTC
I would guess that this line in Client.xs has something to do with it. It is explicitly setting the class to the package name. As much as I dislike reblessing, perhaps implementing a `new()` method that calls `SUPER::new` with the passed parameters and then reblesses it into your class would work. You could also use delegation, and store the G::XS::C object on your object and call it when needed. One benefit that this could give is that you can change out the underlying implementation and leave your API the same. --MidLifeXis	[reply] [d/l] [select]
Re^2: Cannot subclass Gearman::XS::Client by incubusaurus (Initiate) on Jul 24, 2014 at 20:21 UTC
Thanks. It looks as though you are right. Reblessing the object works, but I'll experiment with delegation too.	[reply]
Re: Cannot subclass Gearman::XS::Client by runrig (Abbot) on Jul 24, 2014 at 19:51 UTC
Your code requires Gearman::XS::Client to bless the object into the class 'My::Gearman::Client'. Gearman::XS::Client does not necessarily do this. Although the common way to write a constructor is something along the lines of: `sub new { my $class = shift; bless {}, $class; }` [download] Not all modules will do it this way, and will bless into some constant class name. That appears to be what is happening. You can try filing a bug report (hopefully with a patch) that the module cannot be sub-classed, or you can make your library a delegator instead of a parent class.	[reply] [d/l]
Re^2: Cannot subclass Gearman::XS::Client by incubusaurus (Initiate) on Jul 24, 2014 at 20:23 UTC
I've not worked with XS code before, but this looks like an interesting little challenge for me. Time permitting, I'll have a stab at creating a patch.	[reply]