Use method arguments inside class definition (Ruby)

Question

I am attempting to add a method to all objects which allows adding variables to instances on the fly. However, I have run into the issue that when I begin the class block, the method arguments are out of scope and thus no longer available.

How might I access the method arguments from within the class block? Or is there a better way to do this?

class Object
    def addvar(name, value = nil)
        puts name # => "test"
        class << self
            puts name # => returns nil!
            attr_accessor name # => nil is not a symbol nor a string (TypeError)
        end
        self.instance_variable_set("@" + name.to_s, value)
    end
end

x = Object.new
x.addvar("test", 3)

It's also possible to use eval with string substitution, but that's something I would rather avoid for security reasons:

class Object
    def addvar(name, value = nil)
        eval("class << self
                  attr_accessor :#{name}
             end
             self.#{name} = #{value}")
    end
end

x = Object.new
x.addvar(:test, 3)
puts x.test

Why do you want to add variables on the fly? What do you try to achieve? — spickermann
– spickermann, Commented Oct 9, 2017 at 6:18
The use I thought of would be for mods in games; rather than having to mess with the underlying classes (possibly breaking inter-mod compatibility), modders can instead directly add properties to instances. — TristanBomb
– TristanBomb, Commented Oct 9, 2017 at 6:20

Igor Drozdov · Accepted Answer · 2017-10-09 06:41:57Z

1

You might need the instance_variable_set, instance_variable_get, and other methods for metaprogramming in Ruby. Honestly, this is what attr_accessor uses under the hood.

The following code adds a singleton method to the object you call the add_var method on by using define_singleton_method:

class Object
  def addvar(name, value = nil)
    instance_name = "@#{name}"

    instance_variable_set(instance_name, value)

    define_singleton_method(name) do
      instance_variable_get(instance_name)
    end

    define_singleton_method("#{name}=") do |new_value|
      instance_variable_set(instance_name, new_value)
    end
  end
end

x = Object.new
x.addvar("test1", 3)
p x.test1

p Object.new.test1

=> 3
=> 1.rb:20:in `<main>': undefined method `test1' for #<Object:0x007fc98b161e10> (NoMethodError)

If you want to declare the method for any object of the class, you can use define_method on the class:

class Object
  def addvar(name, value = nil)
    instance_name = "@#{name}"

    instance_variable_set(instance_name, value)

    self.class.send(:define_method, name) do
      instance_variable_get(instance_name)
    end

    self.class.send(:define_method, "#{name}=") do |new_value|
      instance_variable_set(instance_name, new_value)
    end
  end
end

x = Object.new
x.addvar("test1", 3)
p x.test1

p Object.new.test1

=> 3
=> nil

By the way, you still can use attr_accessor, you can just call it on the class object:

class Object
  def addvar(name, value = nil)
    instance_variable_set("@#{name}", value)

    self.class.send(:attr_accessor, name)
  end
end

x = Object.new
x.addvar("test1", 3)
p x.test1

x.test1 = 5
p x.test1

p Object.new.test1

edited Oct 9, 2017 at 6:41

answered Oct 9, 2017 at 6:23

Igor Drozdov

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3 Comments

TristanBomb Over a year ago

This adds a get method, but what about a set method? Ideally, one would only need to use addvar once, and then can use the syntax x.test1 = 5.

TristanBomb Over a year ago

Great! I also noticed that you can also use self.singleton_class instead of self.class in your last example, and it will not add the property to all objects.

Max Over a year ago

In general, class << self is not often needed. Always better to use the singleton methods if possible

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