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What equality test does Ruby Hash use when comparing keys?

I have a wrapper class around some objects that I want to use as keys in Hash. Wrapped and unfolded objects must map to the same key.

A simple example would be the following:

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 tried ==, ===, eq? and the hash is all to no avail.

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2 answers

Does Hash use 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 eql? and hash methods. The hash method must have the property, what 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
+4
source

eql? hash.

hash , .

+4

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