CODEWITHSUNDEEP
rubyhash
pierallard
pierallard

Reputation: 3371

What equality is used by Hash methods?

I have class Foo, and I overload two of its methods == and eql?:

class Foo

  def initialize(bar)
    @bar = bar
  end

  def bar
    @bar
  end

  def ==(o)
    self.bar == o.bar
  end

  def .eql?(o)
    return ==(o)
  end

end

I test that f1 and f2 below are equal with respect to the two methods:

u = User.find(12345)
f1 = Foo.new(u)
f2 = Foo.new(u)

f1 == f2    # => true
f1.eql?(f2) # => true

But Hash#has_key? does not render them equal:

{f1 => true}.has_key?(f2) # => false

What is the equality method used in Hash#has_key??

Upvotes: 1

Views: 110

Answers (2)

Danil Speransky
Danil Speransky

Reputation: 30453

It uses hash method. You may concatinate properties of your objects there or something like that. In your case you want hash value to be the same for 2 objects if and only if they are equal.

Upvotes: 2

Ry-
Ry-

Reputation: 224922

Most implementations of a hash type, Ruby’s included, rely on a hash first (for speed!) and then equality checks. To verify that it works, first, you can just add

def hash
    1
end

After that, you should work on providing as many possible distinct return values for hash that will still be equal if the objects are considered equal (as long as it’s fast, of course).

Upvotes: 2

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