Overriding the == operator in Ruby

Question

According to the docs, Array.include? uses the == comparison on objects. I come from Java where such things are (usually) done with .equals() which is easy to override for a particular object.

How can I override == in Ruby to allow me to specify the behaviour of Array.include? for my particular object?

See also https://stackoverflow.com/questions/7156955/whats-the-difference-between-equal-eql-and — David Moles, Feb 17 '21 at 01:17

score 82 · Accepted Answer · answered Jun 25 '12 at 09:43

82

In Ruby == is just a method (with some syntax sugar on top allowing you to write foo == bar instead of foo.==(bar)) and you override == just like you would any other method:

class MyClass
  def ==(other_object)
    # return true if self is equal to other_object, false otherwise
  end
end

answered Jun 25 '12 at 09:43

sepp2k

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6

Would you need to also overwrite the hash and eql? methods? – Vitalij Jul 11 '16 at 17:00
2

@Vitalij eql? is usually just an alias for ==, and you do need to override hash if you intend to use it in hashes and sets. – bluehallu Dec 06 '16 at 17:34

Kai Wren · Answer 2 · 2022-10-02T10:41:31.527

It's not significantly different from overriding equality in java, and unfortunately not followed anywhere as rigorously as it should be in the Ruby ecosystem. It's not unusual to find libraries that break equality and behave unpredictably when used in collections, for example.

When overriding == please also override hash; this is easier in practice to implement in Ruby because you can delegate to Array or Set to compute the hash: [field1, field2].hash
Please ensure eql? is also overridden correctly to ensure that instances behave correctly when used as keys in a Hash
The overridden equality must by reflexive i.e a == a must be true; you should use equal? to test if it is the exact same object and then short-circuit the equality check if so for a substantial performance boost.
The overridden equality must be symmetric i.e if a == b then b == a must also be true
The overridden equality must be transitive i.e if a == b and b == c then a == c must also be true
The overridden equality must guarantee either true or false as a return value. It can never throw an Exception.
A simple thing to keep an eye on is the number of specs you write every time you override ==; you will need to add at least 5 to 7 specs outside those you might add to cover boundary conditions for the equality logic.
As I said at the start, beware of overriding == when dealing with libraries and frameworks. A common case is equality of ActiveRecord instances, where you can break Rails if you aren't thoughtful.

score 1 · Answer 3 · edited Jul 10 '21 at 23:02

As described above, == is a method in Ruby and can be overridden. You code a custom equality condition in the method. For example:

class Person
  attr_reader :name, :gender, :social_id
  attr_accessor :age

  def initialize(name, age, gender, social_id)
    self.name = name
    self.age = age.to_i
    self.gender = gender
    self.social_id = social_id
  end

  def ==(other)
    social_id == other.social_id
  end
end

You don't need to override other methods any more.

Overriding the == operator in Ruby

3 Answers3

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