Ruby: How to dynamically create a subclass of an existing class?

Question

I'm writing Ruby code that creates new classes using Object.const_set, which works great for creating new classes and instantiating instances of them. But I'd like these new classes to inherit from a class I've hardcoded already. I can't find methods to do this. Here's my code:

def create_model_class(klass_name, klass_vars)
    klass = Object.const_set(klass_name, Class.new)
    klass.class_eval do
        define_method(:initialize)
            klass_vars.each_with_index do |name, i|
                instance_variable_set("@"+name[i], name[i])
            end
        end
    end
end

I think the do after define_method has gone awol. It appears that klass_vars is a two-dimensional array. Is that intended? If klass_vars is an just array of strings, I think you need to change each_with_index to each and drop the reference to i. RE @Sergios suggestion, I believe you would add parent_class to the arguments and add (parent_class) to Class.new or add parent_class_name to the arguments and (parent_class_name.to_sym) to Class.new. — Cary Swoveland
– Cary Swoveland, Commented Oct 11, 2013 at 18:29
Hey Cary, klass_vars is a hash. I'm only partway through implementing it so I may be muddying the pool by leaving it in this example code. Thanks for the sharp eye. — Duncan Malashock
– Duncan Malashock, Commented Oct 11, 2013 at 20:23

Sergio Tulentsev · Accepted Answer · 2013-10-11 16:47:48Z

25

Class.new accepts a parameter, which will be the superclass.

Documentation: Class.new.

answered Oct 11, 2013 at 16:47

Sergio Tulentsev

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2 Comments

Cary Swoveland Over a year ago

I'd forgotten about that, Sergio. Thanks for the twig. Much better than eval "class B < A; end"

Duncan Malashock Over a year ago

1. I'm not sure where I'd call Class.new with a parameter – my guess is after I define the constant for the class name? 2. I looked at this lecture and apparently for some things in metaprogramming eval is the only way, although I understand how it can create security issues.

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