Which equality test does Ruby's Hash use when comparing keys?

Question

I have a wrapper class around some objects that I want to use as keys in a Hash. The wrapped and unwrapper objects should map to the same key.

A simple example will

Answer 1

Hash uses key.eql? to test keys for equality. If you need to use instances of your own classes as keys in a Hash, it is recommended that you define both the eql? and hash methods. The hash method must have the property that a.eql?(b) implies a.hash == b.hash.

So...

class A
  attr_reader :x
  def initialize(inner)
    @inner=inner
  end
  def x; @inner.x; end
  def ==(other)
    @inner.x==other.x
  end

  def eql?(other)
    self == other
  end

  def hash
    x.hash
  end
end

Answer 2

You must define eql? and hash.

The documentation of Hash was lacking but has been improved recently.