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

To simplify comparison operators for objects with more than one state variable, create a 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.state == state
  end

  protected

  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

Also, if you want instances of your class to be usable as a hash key, then add:

  alias_method :eql?, :==

  def hash
    state.hash
  end

These need to be public.

Answer 2

To test all your instance variables equality at once:

def ==(other)
  other.class == self.class && other.state == self.state
end

def state
  self.instance_variables.map { |variable| self.instance_variable_get variable }
end

Answer 3

Usually with the == operator.

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

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