Maintain instance reference

Question

i've been battling for two hours with this but can't solve it. Imagine i have this class:

class SimpleExpression
attr_writer :value

def initialize(value)
    @value = value
end

def evaluate
    @value
end

And then i want to instance it like:

exp1 = SimpleExpression.new(true)
exp3 = exp1.not
p exp3.evaluate # This should give me false
exp1.value = false
p exp3.evaluate # This should give me true now

I can't really figure out how to maintain the reference for exp1 on exp3, i've tried making a method at SimpleExpression that negates the value and returns self but that clearly does not work since i'd be changing both instances. Thanks. PS: not should be a method in SimpleExpression, i did not write it since i really do not know how to.

Answer 1

If it was simple "disconnected" negation, the implementation would've been simple:

def not
  SimpleExpression.new(!value)
end

Here, changing exp1 does not affect exp3.

exp1 = SimpleExpression.new(true)
exp3 = exp1.not
p exp3.evaluate # => false
exp1.value = false
p exp3.evaluate # => false

But making them "live" requires something a little bit more advanced. I'm thinking, some wrapper objects. Like this, for example:

class SimpleExpression
  attr_accessor :value

  def initialize(value)
    @value = value
  end

  def evaluate
    @value
  end

  def not
    NegationWrapper.new(self)
  end

  private

  class NegationWrapper < DelegateClass(self)
    def evaluate
      !__getobj__.evaluate
    end
  end
end

exp1 = SimpleExpression.new(true)
exp3 = exp1.not
p exp3.evaluate # => false
exp1.value = false
p exp3.evaluate # => true