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

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.