What's the right way to implement equality in ruby

Question

For a simple struct-like class:

class Tiger
  attr_accessor :name, :num_stripes
end

what is the correct way to implement equality correctly, to ensure that ==, ===, eql?, etc work, and so that instances of the class play nicely in sets, hashes, etc.

EDIT

Also, what's a nice way to implement equality when you want to compare based on state that's not exposed outside the class? For example:

class Lady
  attr_accessor :name

  def initialize(age)
    @age = age
  end
end

here I'd like my equality method to take @age into account, but the Lady doesn't expose her age to clients. Would I have to use instance_variable_get in this situation?

Answer 1

Usually with the == operator.

def == (other)
  if other.class == self.class
    @name == other.name && @num_stripes == other.num_stripes
  else
    false
  end
end

Answer 2

This is a nice writeup comparing the ins and outs of defining object equality

Answer 3

To simplify comparison operators for objects with more than one state variable, create a private method that returns all of the object's state as an array. Then just compare the two states:

class Thing

  def initialize(a, b, c)
    @a = a
    @b = b
    @c = c
  end

  def ==(o)
    o.class == self.class && o.send(:state) == state
  end
  alias_method :eql?, :==

  private

  def state
    [@a, @b, @c]
  end

end

p Thing.new(1, 2, 3) == Thing.new(1, 2, 3)    # => true
p Thing.new(1, 2, 3) == Thing.new(1, 2, 4)    # => false