Question

Given the simple example here:

class Base
  @tag = nil 

  def self.tag(v = nil) 
    return @tag unless v 
    @tag = v
  end 
end 

class A < Base 
  tag :A
end

class B < Base
  tag :B
end 

class C < Base; end

puts "A: #{A.tag}"
puts "B: #{B.tag}"
puts "A: #{A.tag}"
puts "C: #{C.tag}"

which works as expected

A: A
B: B 
A: A
C: 

I want to create a module that base will extend to give the same functionality but with all the tag information specified by the class. Eg.

module Tester 
  def add_ident(v); ....; end
end

class Base 
  extend Tester 

  add_ident :tag
end 

I’ve found i can do it with a straight eval, so:

def add_ident(v)
  v = v.to_s 
  eval "def self.#{v}(t = nil); return @#{v} unless t; @#{v} = t; end"
end

but i really dislike using eval string in any language.

Is there a way that i can get this functionality without using eval? I’ve gone through every combination of define_method and instance_variable_get/set i can think of and i can’t get it to work.

Ruby 1.9 without Rails.

Answer 1

You want to define a dynamic method on the singleton class of the class you’re extending. The singleton class of a class can be accessed with expression like this: class << self; self end. To open the scope of a class’s class, you can use class_eval. Putting all this together, you can write:

module Identification

  def add_identifier(identifier)
    (class << self; self end).class_eval do
      define_method(identifier) do |*args|
        value = args.first
        if value
          instance_variable_set("@#{identifier}", value)
        else
          instance_variable_get("@#{identifier}")
        end
      end
    end
  end

end

class A
  extend Identification

  add_identifier :tag
end

If you’re using recent versions of Ruby, this approach can be replaced with Module#define_singleton_method:

module Identification

  def add_identifier(identifier)
    define_singleton_method(identifier) do |value = nil|
      if value
        instance_variable_set("@#{identifier}", value)
      else
        instance_variable_get("@#{identifier}")
      end
    end
  end

end

I don’t believe you want to use self.class.send(:define_method), as shown in another answer here; this has the unintended side effect of adding the dynamic method to all child classes of self.class, which in the case of A in my example is Class.