Class comparison in Ruby

by Taylor Fausak on

I recently encountered an interesting issue. It boils down to this: How do you know if an object is an instance of a class?

I had to ask this question because I help maintain ActiveInteraction, a command pattern library. It provides a way to ensure that an object is a certain class. For instance, if an interaction needs a User, it could say:

model :someone,
  class: User

During execution, it’s guaranteed that someone is in fact a User. Behind the scenes, ActiveInteraction validates that using a case statement. In this instance, it would look like this:

case someone
when User
  # It's valid.
else
  # It's invalid.
end

Turns out that’s not enough for determining if an object is an instance of a class. In particular, test mocks pretend to be something they’re not by overriding the #is_a? method. Desugaring the case statement reveals why it fails.

if User === someone
  # It's valid.
else
  # It's invalid.
end

The .=== method asks the class if an object is the right class. #is_a? does the same thing in the other direction by asking the object if it’s the right class. Since the test mock doesn’t monkey patch the class it’s mocking, the only way around this is to ask both questions.

if User === someone
  # The class says it's valid.
elsif someone.is_a?(User)
  # The object says it's valid.
else
  # It's invalid.
end

While developing a fix for ActiveInteraction, I wondered if there were other ways to do this. I did some research and discovered that there are at least 18 different ways to make this comparison.

It would be unreasonable to make all those checks. In fact, if you’re using anything other than .=== and #is_a?, you’re doing it wrong. However, I was interested in creating a class that is indistinguishable from another class. In other words, the perfect mock.

Creating the Perfect Mock

Before we create the mock, we need to create the class we’ll be mocking.

Cheese = Class.new
gouda = Cheese.new

Next up let’s create the mock. It shouldn’t have anything in common with the class it’s mocking.

FakeCheese = Class.new
american = FakeCheese.new

With those defined, we can move on to faking the comparisons.

.===

We hit a problem right out of the gate:

Cheese === american
# => false

This is an issue because it means instances of FakeCheese won’t be able to pass as Cheese in case statements. Unfortunately there’s nothing we can do about it without monkey patching Cheese. Let’s stay focused on the FakeCheese class.

FakeCheese === gouda
# => false

We can do something about this one. Let’s make FakeCheese behave like Cheese by delegating to it.

class FakeCheese
  def self.===(other)
    Cheese === other
  end
end

After making that change, we can see that the conditional returns true now.

FakeCheese === gouda
# => true

Note that we broke the default behavior:

FakeCheese === american
# => false

Instances of FakeCheese aren’t able to pass as FakeCheese in case statements anymore. We could fix that by throwing a call to super somewhere in .===, but remember that we’re trying to build the perfect mock. If Cheese === american is false, FakeCheese === american should be too. (We’ll see later that falling back to super doesn’t always make sense.)

#is_a?

Since we can’t make case statements work without monkey patching, let’s move on to something we can fix.

american.is_a?(Cheese)
# => false

We want to delegate to Cheese again, but this is an instance method. We don’t have an instance of Cheese in FakeCheese. We could make one and delegate to it, but initializing Cheese could be complicated or expensive. Let’s turn to #is_a?’s documentation for inspiration.

Returns true if class is the class of obj, or if class is one of the superclasses of obj or modules included in obj.

We need a class method that does the same thing. Looking at the documentation for .>=, it seems to fit the bill.

Returns true if mod is an ancestor of other, or the two modules are the same.

Using .>= we can essentially delegate #is_a? to Cheese without having an instance handy.

class FakeCheese
  def is_a?(klass)
    Cheese >= klass
  end
end

Let’s reevaluate our conditional to make sure it worked.

american.is_a?(Cheese)
# => true

Great! That was a little tricky, but ultimately not too bad.

#kind_of?

Even though #kind_of? and #is_a? do the same thing, they aren’t aliases.

american.kind_of?(Cheese)
# => false
class FakeCheese
  def kind_of?(klass)
    Cheese >= klass
  end
end
american.kind_of?(Cheese)
# => true

#instance_of?

Unlike #is_a? and #kind_of?, #instance_of? checks for an exact match.

american.instance_of?(Cheese)
# => false
class FakeCheese
  def instance_of?(klass)
    Cheese == klass
  end
end
american.instance_of?(Cheese)
# => true

#class

Instead of using a predicate method, we can look directly at the object’s class.

american.class
# => FakeCheese
class FakeCheese
  def class
    Cheese
  end
end
american.class
# => Cheese

This is the first method where falling back to super doesn’t make sense.

.==

We can check the classes themselves for equality.

FakeCheese == Cheese
# => false
class FakeCheese
  def self.==(other)
    Cheese == other
  end
end
FakeCheese == Cheese
# => true

.eql?

Or slightly stricter equality.

FakeCheese.eql?(Cheese)
# => false
class FakeCheese
  def self.eql?(other)
    Cheese.eql?(other)
  end
end
FakeCheese.eql?(Cheese)
# => true

.equal?

Or the strictest equality.

FakeCheese.equal?(Cheese)
# => false
class FakeCheese
  def self.equal?(other)
    Cheese.equal?(other)
  end
end
FakeCheese.equal?(Cheese)
# => true

.object_id

We can also use the object IDs to compare object equality by hand.

FakeCheese.object_id
# => 70241271125600
class FakeCheese
  def self.object_id
    Cheese.object_id
  end
end
FakeCheese.object_id
# => 70241271152880

.__id__

Ruby provides another way to get at the object IDs.

FakeCheese.__id__
# => 70241271125600
class FakeCheese
  def self.__id__
    Cheese.__id__
  end
end
FakeCheese.__id__
# => 70241271152880

.<=>

Now that we’ve faked all of the ways to check equality, let’s move on to inequality. The obvious place to start is with the spaceship operator.

FakeCheese <=> Cheese
# => nil
class FakeCheese
  def self.<=>(other)
    Cheese <=> other
  end
end
FakeCheese <=> Cheese
# => 0

Even though Class implements .<=>, it doesn’t include Comparable. So we have to manually override all of the associated methods.

.<

FakeCheese < Cheese
# => nil
class FakeCheese
  def self.<(other)
    Cheese < other
  end
end
FakeCheese < Cheese
# => false

.>

FakeCheese > Cheese
# => nil
class FakeCheese
  def self.>(other)
    Cheese > other
  end
end
FakeCheese > Cheese
# => false

.<=

FakeCheese <= Cheese
# => nil
class FakeCheese
  def self.<=(other)
    Cheese <= other
  end
end
FakeCheese <= Cheese
# => true

.>=

FakeCheese >= Cheese
# => nil
class FakeCheese
  def self.>=(other)
    Cheese >= other
  end
end
FakeCheese >= Cheese
# => true

.ancestors

Another way to see if two classes are the same is to see if they have the same ancestors. Let’s make FakeCheese pretend like it has the same family tree as Cheese.

FakeCheese.ancestors
# => [FakeCheese, Object, PP::ObjectMixin, Kernel, BasicObject]
class FakeCheese
  def self.ancestors
    Cheese.ancestors
  end
end
FakeCheese.ancestors
# => [Cheese, Object, PP::ObjectMixin, Kernel, BasicObject]

.to_s

Having exhausted all of the somewhat reasonable ways to compare classes, let’s move on to comparing their string representations.

FakeCheese.to_s
# => "FakeCheese"
class FakeCheese
  def self.to_s
    Cheese.to_s
  end
end
FakeCheese.to_s
# => "Cheese"

.inspect

By default, .to_s and .inspect do the same thing, but they aren’t aliased.

FakeCheese.inspect
# => "FakeCheese"
class FakeCheese
  def self.inspect
    Cheese.inspect
  end
end
FakeCheese.inspect
# => "Cheese"

.name

.name is just like .to_s and .inspect. It’s not aliased either.

FakeCheese.name
# => "FakeCheese"
class FakeCheese
  def self.name
    Cheese.name
  end
end
FakeCheese.name
# => "Cheese"

#to_s

Instances of classes in Ruby don’t use their class’s string representation in their own string representation.

american.to_s
# => "#<FakeCheese:0x007fa3e09ccd00>"

Even though we overrode .to_s, .inspect, and .name, the instance somehow uses the class’s real name. So we have to provide a custom #to_s implementation that mimics the default behavior. Other than shifting the object ID, this is pretty easy.

class FakeCheese
  def to_s
    "#<#{Cheese}:0x#{'%x' % (object_id << 1)}>"
  end
end
american.to_s
# => "#<Cheese:0x7fa3e09ccd00>"

#inspect

Unsurprisingly, this is not an alias.

american.inspect
# => "#<FakeCheese:0x007fa3e09ccd00>"
class FakeCheese
  def inspect
    "#<#{Cheese}:0x#{'%x' % (object_id << 1)}>"
  end
end
american.inspect
# => "#<Cheese:0x7fa3e09ccd00>"

Shorter & More Generic

We created the perfect mock, but it took a lot of code and we repeated ourselves quite a bit. We can make it a lot simpler. Let’s write a function that takes a class and returns a perfect mock of that class.

require 'forwardable'

def fake(klass)
  Class.new(BasicObject) do
    eigenclass = class << self; self end
    eigenclass.extend Forwardable
    eigenclass.def_delegators klass, *%i[
      <
      <=
      <=>
      ==
      ===
      >
      >=
      __id__
      ancestors
      eql?
      equal?
      inspect
      name
      object_id
      to_s
    ]

    define_method :class do
      klass
    end

    define_method :inspect do
      "#<#{klass.name}:0x#{'%x' % (__id__ << 1)}>"
    end
    alias_method :to_s, :inspect

    define_method :instance_of? do |other|
      klass == other
    end

    define_method :is_a? do |other|
      klass >= other
    end
    alias_method :kind_of?, :is_a?
  end
end

We can replace all our work above with just one function call.

FakeCheese = fake(Cheese)
# => Cheese
american = FakeCheese.new
# => #<Cheese:0x7fd8e1e5ba48>

And it passes all the checks!

[
  american.is_a?(Cheese),
  american.kind_of?(Cheese),
  american.instance_of?(Cheese),
  american.class == Cheese,
  FakeCheese == Cheese,
  FakeCheese.eql?(Cheese),
  FakeCheese.equal?(Cheese),
  FakeCheese.object_id == Cheese.object_id,
  FakeCheese.__id__ == Cheese.__id__,
  (FakeCheese <=> Cheese) == 0,
  FakeCheese <= Cheese && FakeCheese >= Cheese,
  FakeCheese.ancestors == Cheese.ancestors,
  FakeCheese.to_s == Cheese.to_s,
  FakeCheese.inspect == Cheese.inspect,
  FakeCheese.name == Cheese.name,
  american.to_s =~ /#{Cheese}/,
  american.inspect =~ /#{Cheese}/
].all?
# => true
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