Question

I have the following defined:

#!/usr/bin/env ruby

class Something
  def self._attr_accessor key, value, type
    (class << self; self; end).send( :attr_accessor, key.to_sym)
    instance_variable_set "@#{key}", value
  end
end

class Client < Something
  _attr_accessor 'foo_bar', 'json', String
end

my_something = Client.new
puts my_something.foo_bar

But I recieve the following error:

/test_inheritance.rb:18:in `<class:Client>': undefined method `foo_bar' for Client:Class (NoMethodError)
    from ./test_inheritance.rb:14:in `<main>'

The bit of metaprograming I am doing works:

#!/usr/bin/env ruby
class Something
  def self._attr_accessor key, value, type
    (class << self; self; end).send( :attr_accessor, key.to_sym)
    instance_variable_set "@#{key}", value
  end
end

class Client < Something
  _attr_accessor 'foo_bar', 'json', String

  puts self.foo_bar
end

my_something = Client.new

#puts my_something.foo_bar

As it outputs the proper result. But how do I define the _attr_accessor methods such that I am able to access it methods publicly?

Answer 1

For one I think you’re tripping over the fact that format is a reserved method on Class and it’s conflicting with your attr_accessor attempt.

Secondly there’s a better way to do this. I’ve made a fairly robust “accessor” utility class for a project I’m working on. It allows you to define class-level defaults and still override instance definitions.

The implementation looks like this:

module OptionAccessor
  # Given a list of names, this declares an option accessor which works like
  # a combination of cattr_accessor and attr_accessor, except that defaults
  # defined for a class will propagate down to the instances and subclasses,
  # but these defaults can be over-ridden in subclasses and instances
  # without interference. Optional hash at end of list can be used to set:
  #  * :default => Assigns a default value which is otherwise nil
  #  * :boolean => If true, creates an additional name? method and will
  #                convert all assigned values to a boolean true/false.
  def option_accessor(*args)
    option_reader(*args)
    option_writer(*args)
  end

  # Given a list of names, this declares an option reader which works like
  # a combination of cattr_reader and attr_reader, except that defaults
  # defined for a class will propagate down to the instances and subclasses,
  # but these defaults can be over-ridden in subclasses and instances
  # without interference. Optional hash at end of list can be used to set:
  #  * :default => Assigns a default value which is otherwise nil
  #  * :boolean => If true, creates an additional name? method and will
  #                convert all assigned values to a boolean true/false.
  def option_reader(*names)
    names = [ names ].flatten.compact
    options = names.last.is_a?(Hash) ? names.pop : { }

    names.each do |name|
      iv = :"@#{name}"

      (class << self; self; end).class_eval do
        if (options[:boolean])
          define_method(:"#{name}?") do
            iv_value = instance_variable_get(iv)

            !!(iv_value.nil? ? (self.superclass.respond_to?(name) ? self.superclass.send(name) : nil) : iv_value)
          end
        end

        define_method(name) do
          iv_value = instance_variable_get(iv)

          iv_value.nil? ? (self.superclass.respond_to?(name) ? self.superclass.send(name) : nil) : iv_value
        end
      end

      define_method(name) do
        iv_value = instance_variable_get(iv)

        iv_value.nil? ? self.class.send(name) : iv_value
      end

      if (options[:boolean])
        define_method(:"#{name}?") do
          iv_value = instance_variable_get(iv)

          !!(iv_value.nil? ? self.class.send(name) : iv_value)
        end
      end

      instance_variable_set(iv, options[:default])
    end
  end

  # Given a list of names, this declares an option writer which works like
  # a combination of cattr_writer and attr_writer, except that defaults
  # defined for a class will propagate down to the instances and subclasses,
  # but these defaults can be over-ridden in subclasses and instances
  # without interference. Options can be specified:
  #  * :boolean => If true, converts all supplied values to true or false
  #                unless nil, in which case nil is preserved.
  def option_writer(*names)
    names = [ names ].flatten.compact
    options = names.last.is_a?(Hash) ? names.pop : { }

    names.each do |name|
      iv = :"@#{name}"

      (class << self; self; end).class_eval do
        if (options[:boolean])
          define_method(:"#{name}=") do |value|
            instance_variable_set(iv, value.nil? ? nil : !!value)
          end
        else
          define_method(:"#{name}=") do |value|
            instance_variable_set(iv, value)
          end
        end
      end

      if (options[:boolean])
        define_method(:"#{name}=") do |value|
          instance_variable_set(iv, value.nil? ? nil : !!value)
        end
      else
        define_method(:"#{name}=") do |value|
          instance_variable_set(iv, value)
        end
      end
    end
  end
end