(In Ruby) allowing mixed-in class methods access to class constants

Question

I have a class with a constant defined for it. I then have a class method defined that accesses that class constant. This works fine. An example:

#! /usr/bin/         


        

Answer 1

Its probably worth noting that you don't need to include modules into a metaclass.

class NonInstantiableClass
    Const = "hello, world!"
    class << self
        include CommonMethods
    end
end

Ruby has the extend keyword that effectively adds the modules interface to a class, e.g.

class NonInstantiableClass
    Const = "hello, world!"
    extend CommonMethods
end

You still need to ensure you're referencing the right constant using self::Const or const_get, but extend <module> is the better way to add those methods to the class.

Answer 2

This seems to work:

#! /usr/bin/env ruby

module CommonMethods
    def shout_my_constant
        puts self::Const.upcase
    end
end

class NonInstantiableClass
    Const = "hello, world!"
    class << self
        include CommonMethods
    end
end

NonInstantiableClass.shout_my_constant

HTH

Answer 3

The problem is, if you just write Const it is evaluated on Module creation time. You have to use Module#const_get instead like this: const_get(:Const). This is evaluated at runtime when the method is executed. So this happens in your class and not in your module.