Question

I’m new to Ruby and confused about how I can make a method in Class similar to :attr_accessor, in that it adds methods to a user class, but so these added methods have access to a pre-initialized instance variable. It’s difficult for me to explain so here is a greatly simplified sample of my efforts:

class Class
    def super_accessor_wow(attr_name)
        attr_name = attr_name.to_s 
        new_var_name = "@crazy_var_name"

        instance_variable_set(new_var_name, ["hi", "everyone"])

        module_eval(%Q/
            def super_#{attr_name}()
                return @#{attr_name}
            end

            def super_#{attr_name}=(value)
                @#{attr_name} = value
            end

            def greetings
                return #{new_var_name}
            end
        /)
    end
end

This is how I’m trying to use the new method on Class to modify my own class:

class Foo
    super_accessor_wow(:bar)
end

foo1 = Foo.new()
foo1.super_bar = 1000

puts foo1.super_bar
puts foo1.greetings.inspect

The first puts prints '1000'

The second puts prints 'nil', so my instance_variable_set call in super_accessor_wow seemingly has no effect.

I expected that the second puts would print '['hi', 'everyone']' by the way. All of this code is contained in a single Ruby file.

Answer 1

Your instance_variable_set is called when you call super_accessor_wow during the class definition. No instance of the class exists yet. You create an instance of the class when you call new. You could add your @crazy_var_name initialization to the constructor, or you could define it in the greetings method:

Put the default in a class variable, and initialize the instance variable in the constructor (be aware that this creates a constructor for your class, and if you then create your own constructor, it will override this one):

class Class
    def super_accessor_wow(attr_name)
        attr_name = attr_name.to_s
        new_var_name = "@crazy_var_name"
        new_var_name_default = "@#{new_var_name}"

        module_eval(%Q/
            #{new_var_name_default} = ["hi", "everyone"]

            def initialize()
              #{new_var_name} = #{new_var_name_default}
            end

            def super_#{attr_name}()
                return @#{attr_name}
            end

            def super_#{attr_name}=(value)
                @#{attr_name} = value
            end

            def greetings
                return #{new_var_name}
            end
        /)
    end
end

class Foo
    super_accessor_wow(:bar)
end

foo1 = Foo.new()
foo1.super_bar = 1000

puts foo1.super_bar
puts foo1.greetings.inspect
puts Foo.class_variable_get('@@crazy_var_name').inspect
puts foo1.instance_variable_get('@crazy_var_name').inspect

Outputs:

1000
["hi", "everyone"]
["hi", "everyone"]
["hi", "everyone"]

Define it in the greetings method:

class Class
    def super_accessor_wow(attr_name)
        attr_name = attr_name.to_s
        new_var_name = "@crazy_var_name"

        module_eval(%Q/
            def super_#{attr_name}()
                return @#{attr_name}
            end

            def super_#{attr_name}=(value)
                @#{attr_name} = value
            end

            def greetings
                #{new_var_name} = ["hi", "everyone"] unless #{new_var_name}
                return #{new_var_name}
            end
        /)
    end
end

class Foo
    super_accessor_wow(:bar)
end

foo1 = Foo.new()
foo1.super_bar = 1000

puts foo1.super_bar
puts foo1.greetings.inspect

Outputs

1000
["hi", "everyone"]