Question

If I create two String instances with the same content separately they are identical. This is not the case with custom classes by default (see example below).

If I have my own class (Test below) and I have a variable (@v below) which is unique, ie. two Test instances with the same @v should be treated as identical, then how would I go about telling Ruby this is the case?

Consider this example:

class Test
  def initialize(v)
    @v = v
  end
end

a = {Test.new('a') => 1, Test.new('b') => 2}

a.delete(Test.new('a'))

p a
# # Desired output:
# => {#<Test:0x100124ef8 @v="b">=>2}

Answer 1

I found a different way to tackle this, by keeping track of all the instances of Test internally I can return the premade instance rather than making a new one and telling ruby they’re equivalent:

class Test
  def self.new(v)
    begin
      return @@instances[v] if @@instances[v]
    rescue
    end

    new_test = self.allocate
    new_test.instance_variable_set(:@v,v)
    (@@instances ||= {})[v] = new_test
  end
end

Now Test.new('a') == Test.new('a') and Test.new('a') === Test.new('a') 🙂