Re: Inheritance when subclass passed as a parameter

I see a couple of issues; I think you're mixing some concepts.

As choroba and hippo said, you use useall to locate the class name of the appropriate "plugin", but then don't instantiate an object of that class, therefore template never gets populated. You could change $self->{service} = $plugin to $self->{service} = $plugin->new to fix that. If you go with this suggestion:
- You'll need to access the template as ...->{service}->{template} (or with an appropriate accessor), both in Service::populate and in your tests.
- It doesn't really make sense to me to have the Service/*.pms be subclasses of Service.
You're using the first parameter of Service->new to locate a subclass. Why not just do e.g. Service::XML->new instead? Then, in your subclasses, you'll need to call the superclass constructor, as in my $self = $class->SUPER::new(@_); $self->{template} = ....
Another option would be to set up a factory method. Although it could be called new, IMO that messes with inheritance a bit. So you could set up a method such as Service->create that simply dispatches to the constructor of the appropriate subclass; then each subclass constructor could call the superclass constructor like in my previous point.

Another small issue in your sample code: You pass the constructor two arguments, but only use the first.

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Re^2: Inheritance when subclass passed as a parameter by davies (Monsignor) on Feb 05, 2020 at 11:09 UTC
Thanks a lot. I'm pretty sure that a factory method is what I need. I've never used them before, so I have some googling to do. Are there any especially good (or bad) docs? I am certainly confused (which is why I'm coming here for help) and the fog is starting to lift thanks to everyone's help. My problem with `Service::XML->new` is that I really need to call it as `Service::$var->new`, as I don't know which service will be used until I get the data. I tried various forms of that and got various syntax errors. Obviously I will have to do lots of error checking in case a service is requested that has no subclass, but I'm trying to get the basics working first. Which I won't if I make trivial blunders like the character class Choroba pointed out. Regards, John Davies Update: https://www.perl.com/pub/2003/08/15/design3.html/ looks helpful to me, although I think there are some more modern constructs I could use.	[reply] [d/l] [select]
Re^3: Inheritance when subclass passed as a parameter by haukex (Archbishop) on Feb 05, 2020 at 11:27 UTC
I'm pretty sure that a factory method is what I need. I've never used them before, so I have some googling to do. Are there any especially good (or bad) docs? I'm not sure, probably Wikipedia is a good start - but the basic concept is pretty simple, it's a class method in a central place somewhere that simply dispatches to the appropriate constructor depending on the arguments. The advantage is that the code calling the factory doesn't have to have any knowledge of the subclasses that exist, or their names. In a strongly typed language the factory method would simply be returning an object that is-a `Service`. My problem with `Service::XML->new` is that I really need to call it as `Service::$var->new`, as I don't know which service will be used until I get the data. I tried various forms of that and got various syntax errors. That'll only work if the entire class name is a string, as in `my $class="Service::$var"; my $obj=$class->new;`. Which is certainly an option, but if you want some flexibility in `$var` and/or better error messages in case of invalid `$var` values, then a factory method is probably better. These tests pass: Read more... code (2 kB)	[reply] [d/l] [select]
Re^4: Inheritance when subclass passed as a parameter by Fletch (Bishop) on Feb 05, 2020 at 16:27 UTC
wiki.c2.com is also another good design patterns resource (along with the original GangOfFour book ISBN 9780201633610). The cake is a lie. The cake is a lie. The cake is a lie.	[reply]