Passing values into a class created with Class.new

Question

I have a list of names and values I'm trying to read in and turn into classes so I'm using Class.new.

The end result I want is a number of classes that work as if defined like:

module MyMod
  class AA < Base
    def self.value
      value1
    end
  end

  class AB < Base
    def self.value
      value2
    end
  end

  ...
end

My current code looks like:

name = 'AA'
value = 'test'
MyMod.const_set name, Class.new(Base) do
  ???
end

Setting the name works great, but haven't figured out what I need in the block for get value in. Calling def doesn't work because the closure for value gets lost.

I have managed to get things working with:

temp = const_set name, Class.new(Base)
temp.define_singleton_method(:value) { value }

However, it seems like there should be a way to do it with the block of Class.new. Also, I'm really not sure define_singleton_method is actually putting the method in the right place. It works in my tests, but I'm not sure if the method is actually where I think it is or somewhere else up the call chain. I've tried various combinations of class_variable_set, attr_reader, class_eval, instance_eval, and others, but it got to a point where it was just guess and check. I think I still haven't quite wrapped my head around metaprogramming :-/

Can you provide some context why you want to do this? Maybe there's another way. I love metaprogramming (I'm working with Smalltalk), but generating classes on the fly (I tried that) was rarely the optimal solution. — ewernli
– ewernli, Commented Nov 23, 2012 at 13:09
@ewernli I have a short list of valid name:value pairs. I had created a class for them and kept the list of all the valid instances that were generated from reading a file. I didn't want arbitrary instance being created so had a lookup method on that list instead of an initializer. I was about to make another class that would encapsulate an "instance" of the first class storing a reference to it and some additional data. Based on that naming I realised making classes and having actual instances of them might be better. I think it models the domain well, but haven't considered performance. — Yogh
– Yogh, Commented Nov 23, 2012 at 20:23

score 4 · Accepted Answer · 2012-11-23 13:40:39Z

4

if i correctly understood your question, this should work for you:

class Base
end

class AA < Base

  name = :Blah
  klass = self.const_set name, Class.new(Base)

  class << klass
    def value
      __method__
    end
  end

end

p AA::Blah.value
#=> :value

UPDATE: seems you want it defined in the block:

class Base
end

class AA < Base

  name = :Blah
  klass = Class.new(Base) do

    class << self
      def value
        __method__
      end
    end

  end
  self.const_set name, klass

end

p AA::Blah.value

you trying this:

const_set name, Class.new(Base) do
  ...
end

it does not work cause the block is referring to const_set rather than to Class.new

If you prefer define_singleton_method over class << self:

class Base
end

class AA < Base

  name = :Blah
  klass = Class.new(Base) do

    self.define_singleton_method :value do
      __method__
    end

  end
  self.const_set name, klass

end

And finally if you really want to define them at once, use brackets instead of do...end:

class Base
end

class AA < Base

  name = :Blah
  self.const_set name, Class.new(Base) {

    self.define_singleton_method :value do
      __method__
    end

  }

end

Here is a working demo

edited Nov 23, 2012 at 13:40

answered Nov 23, 2012 at 13:10

user904990

Sign up to request clarification or add additional context in comments.

2 Comments

Yogh Over a year ago

The crux of the solution was "it does not work cause the block is referring to const_set rather than to Class.new". Thanks! Refactorings have rendered the rest obsolete, but they were useful examples.

Yogh Over a year ago

Further refactorings have rendered the rest of this very useful so thank you again!

Collectives™ on Stack Overflow

Passing values into a class created with Class.new

1 Answer 1

Here is a working demo

2 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Here is a working demo

2 Comments

Your Answer

Sign up or log in

Post as a guest

Related