Guaranteed way to compare objects by identity in Ruby

Question

What is a guaranteed way to compare two objects by their identity in Ruby? Given two variables, I want to return true if the variables point to the exact same object in memory.

For most Ruby objects, the equal? method compares by identity:

f = g = Object.new
p f.equal? g  # => true

However, this doesn't work for all objects. For example:

class F
  def ==(obj) false end
  def ===(obj) false end
  def eql?(obj) false end
  def equal?(obj) false end
  def object_id; self end
end

f = g = F.new
p f == g       # => false
p f === g      # => false
p f.eql? g     # => false
p f.equal? g   # => false
p f.object_id == g.object_id  # => false

What is a foolproof/guaranteed way of comparing two objects by identity which can't be defeated?

This is a purely intellectual question. The answer to any question that begins with "why" will probably be "Because I am curious."

Answer 1

You could grab an unbound version of Object#object_id, bind it to the object in question, and see what it says. Given your F class with one addition:

class F
  # ...
  def inspect; 'pancakes!' end # Just so we can tell what we have later.
end

Then:

>> f = F.new
>> f.object_id
=> pancakes!
>> unbound_object_id = Object.instance_method(:object_id)
>> unbound_object_id.bind(f).call
=> 2153000340
>> ObjectSpace._id2ref(2153000340).inspect
=> "pancakes!"

Of course, if someone opens up Object and replaces object_id then you're out of luck but this will be the least of your problems if someone does that. If you can grab your unbound_object_id UnboundMethod before anything else is loaded, then it won't matter if someone changes Object#object_id as your unbound_object_id will still be the original correct one.

So this round-about hack gives you a reliable object_id for any object (subject to the caveats above). Now you can grab and compare the object ids to get your reliable comparison.