CODEWITHSUNDEEP
rubyhashequalsequality
alexloh
alexloh

Reputation: 1616

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

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 be this:

class A
  attr_reader :x
  def initialize(inner)
    @inner=inner
  end
  def x; @inner.x; end
  def ==(other)
    @inner.x==other.x
  end
end
a = A.new(o)  #o is just any object that allows o.x
b = A.new(o)
h = {a=>5}
p h[a] #5
p h[b] #nil, should be 5
p h[o] #nil, should be 5

I've tried ==, ===, eq? and hash all to no avail.

Upvotes: 7

Views: 4310

Answers (2)

Marc-André Lafortune
Marc-André Lafortune

Reputation: 79562

You must define eql? and hash.

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

Upvotes: 5

aromero
aromero

Reputation: 25761

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

Upvotes: 6

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