Question

I’m reading this book “The Well-Grounded Rubyist” and this random question came to me. I know in Ruby that it’s possible to re-open a class and overwrite the method.

Example:

class A
  def x
    "First definition of x"
  end

  def x
    "Second definition of x"
  end
end

test = A.new
test.x #returns "Second definition of x"

Based on the results above, I was curious if it was possible to overwrite the class method attr_accessor with my own (random) definition. Here’s what I’m thinking:

class Dummy
  attr_accessor :test

  def self.attr_accessor(method_name)
    puts "Overwrite the default functionality of attr_accessor by printing this text instead."
  end
end

d = Dummy.new
d.test #not sure why this returns nil instead of putting my message
Dummy.attr_accessor(test) #fails because of ArgumentError: wrong number of arguments (0 for 2..3)

For the two examples above, I’m hoping to understand Ruby better by tinkering around and asking questions to get your insight.

Answer 1

Yes, it is possible, and you just did it!

d.test #not sure why this returns nil instead of putting my message

returns nil because you used attr_accessor :test before redefining it below, as such, Ruby performed attr_accessor‘s default behavior and created a member and accessors in the Dummy class. It returns nil because the member’s value is unset…nil.

Dummy.attr_accessor(test) #fails because of ArgumentError: wrong number of arguments (0 for 2..3)

fails not for the reason you think. This call works:

Dummy.attr_accessor("method_name") 

The problem is you are calling a method named test and not providing all of it’s expected values. See the docs for Kernel.test() http://www.ruby-doc.org/core-1.9.3/Kernel.html#method-i-test.

The error message you are seeing is because you are calling the test method incorrectly, NOT because of a mistake in your redefinition of attr_accessor.