How do I get the class of a BasicObject instance?

Asked 2012-02-08T16:38:49.200

I have a script that iterates using ObjectSpace#each_object with no args. Then it prints how many instances exist for each class.

I realized that some classes redefine the #class instance method, so I had to find another way to get the actual class; Let's say it's stored in variable "klass", and klass === object is true.

In Ruby 1.8 I could do this, assuming Object wasn't monkeypatched:

Object.instance_method(:class).bind(object).call

This worked for ActiveSupport::Duration instances:

# Ruby 1.8
# (tries to trick us)
20.seconds.class
=> Fixnum
# don't try to trick us, we can tell
Object.instance_method(:class).bind(20.seconds).call
=> ActiveSupport::Duration

But, in Ruby 1.9 this no longer works:

# Ruby 1.9
# we are not smart...
Object.instance_method(:class).bind(20.seconds).call
TypeError: bind argument must be an instance of Object
  from (irb):53:in `bind'
  from (irb):53
  from /Users/user/.rvm/rubies/ruby-1.9.2-p0/bin/irb:17:in `<main>'

It turns out that ActiveSupport::Duration subclasses ActiveSupport::BasicObject. The latter is made to subclass ::BasicObject in Ruby 1.9, so Object is excluded from the inheritance chain. This doesn't, and can't, happen in Ruby 1.8, so ActiveSupport::BasicObject is a subclass of Object.

I haven't found any way to detect the actual class of a Ruby 1.9 object that isn't an instance of Object. BasicObject in 1.9 is really bare-bones:

BasicObject.instance_methods
=> [:==, :equal?, :!, :!=, :instance_eval, :instance_exec, :__send__]

Ideas?

UPDATE:

Since ruby 1.9 reached end-of-life, I'm changing my accept to @indirect's answer. The mentions of ruby 1.9 above are

ruby ruby-1.9

Edit Report

2 Answers

The following solution refers to the superclass of the eigenclass. As a consequence, it has the side effect of allocating the eigenclass (detectable by ObjectSpace.count_objects[:T_CLASS] in MRI). But since BasicObject#class is only invoked on blank slate objects (i.e. objects that are not kind-of Object, i.e. that are not Objects) the side effect also applies just for blank slate objects. For Objects, the standard Kernel#class is invoked.

class BasicObject
  def class
    (class << self; self end).superclass
  end
end

# tests:
puts RUBY_VERSION               # 1.9.2
class B < BasicObject; end
class X;               end
p BasicObject.new.class             # BasicObject
p B          .new.class             # B
p X          .new.class             # X
p               6.class             # Fixnum
p B.instance_method(:class).owner   # BasicObject
p X.instance_method(:class).owner   # Kernel
p          6.method(:class).owner   # Kernel

Edit - Note: Indeed, there is an issue with ActiveSupport::Duration. This class uses interception (method_missing) for redirecting messages to the :value attribute. As a consequence, it provides false introspection for its instances. To preserve this falsity, it is necessary to use another name for the class map, e.g. the proposed __realclass__. Thus, the modified solution might look like this:

class BasicObject
  def __realclass__; (class << self; self end).superclass end
end
class Object; alias __realclass__ class end

Another way of not invoking class << self on Objects is via Module#===, as suggested by Kelvin on this page.

answered 2012-04-18T19:51:27.600

fguillen's link made me think of this way.

Pros:

It doesn't need external libraries.

Cons:

It must be executed before loading any classes that subclass BasicObject.
It adds a method to every new class

class BasicObject
  def self.inherited(klass)
    klass.send(:define_method, :__realclass__) { klass }
  end
  def __realclass__
    BasicObject
  end
end

# ensures that every Object will also have this method
class Object
  def __realclass__
    Object.instance_method(:class).bind(self).call
  end
end

require 'active_support/core_ext'

20.seconds.__realclass__  # => ActiveSupport::Duration

# this doesn't raise errors, so it looks like all objects respond to our method
ObjectSpace.each_object{|e| e.__realclass__ }

answered 2012-02-08T19:00:04.547

How do I get the class of a BasicObject instance?

2 Answers

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