Retrieve a Ruby object from its singleton class?

Question

It is possible to access a singleton class from a Ruby object with:

some_object.singleton_class

Is it possible to do the reverse operation : access the original object when inside the singleton class?

class << some_object
  # how to reference some_object without actually typing some_object?
end

I wanted to DRY this method:

class Example
  PARENTS = []
  class << PARENTS
    FATHER = :father
    MOTHER = :mother
    PARENTS.push(FATHER, MOTHER)
  end
end

and tried to replace PARENTS inside the class with something more generic.

There's also [`singleton_class?`](http://ruby-doc.org/core-2.6.1/Module.html#method-i-singleton_class-3F) so a singleton class is quite aware of its special status. — Stefan, Feb 05 '19 at 09:36
_"without actually typing some_object"_ – it's even worse: if `some_object` is a local variable, it's not defined in the `class << some_object` block. Attempting to reference it within the block results in a `NameError`. — Stefan, Feb 05 '19 at 10:06

Stefan · Answer 1 · 2019-02-05T11:26:50.013

I'm not aware of any built-in method or keyword but you could write a method that adds a (singleton) method to an object's singleton class, returning the object itself:

class Object
  def define_instance_accessor(method_name = :instance)
    singleton_class.define_singleton_method(method_name, &method(:itself))
  end
end

Usage:

obj = Object.new              #=> #<Object:0x00007ff58e8742f0>
obj.define_instance_accessor
obj.singleton_class.instance  #=> #<Object:0x00007ff58e8742f0>

In your code:

class Example
  PARENTS = []
  PARENTS.define_instance_accessor
  class << PARENTS
    FATHER = :father
    MOTHER = :mother
    instance.push(FATHER, MOTHER)
  end
end

Internally, YARV stores the object in an instance variable called __attached__. The instance variable doesn't have the usual @ prefix, so it isn't visible or accessible from within Ruby.

Here's a little C extension to expose it:

#include <ruby.h>

static VALUE
instance_accessor(VALUE klass)
{
    return rb_ivar_get(klass, rb_intern("__attached__"));
}

void Init_instance_accessor()
{
    rb_define_method(rb_cClass, "instance", instance_accessor, 0);
}

Usage:

irb -r ./instance_accessor
> obj = Object.new
#=> #<Object:0x00007f94a11e1260>
> obj.singleton_class.instance
#=> #<Object:0x00007f94a11e1260>
>

Excellent work with the C extension. I'm still hoping there's a a clean-ish plain Ruby solution, though. — Eric Duminil, Feb 05 '19 at 13:39

score 2 · Answer 2 · answered Feb 05 '19 at 10:20

2

Just out of curiosity (please don’t use at home or school)

object = []
class << object
  type, id = to_s[/(?<=:#<).*?(?=>)/].split(':')
  ObjectSpace.each_object(Kernel.const_get(type)).find do |e|
    e.__id__ == id.to_i(16) >> 1
  end << :father
end   
#⇒ [:father]

answered Feb 05 '19 at 10:20

Aleksei Matiushkin

119,336
10
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160

3

@MarcinKołodziej yeah, it’s “how to turn all your teammates into violent enemies in 24 hours” :) – Aleksei Matiushkin Feb 05 '19 at 10:34

Cary Swoveland · Answer 3 · 2019-02-12T19:32:40.033

2

We can do that as follows.

def singleton_class_to_object(sc)
  ObjectSpace.each_object(Object).find { |o|
    (o.singleton_class == sc) rescue false }
end

o = Object.new
  #=> #<Object:0x00005b52e502d030> 
singleton_class_to_object(o.singleton_class)
  #=> #<Object:0x00005b52e502d030> 

class C; end
singleton_class_to_object(C.singleton_class)
  #=> C

The in-line rescue is to deal with objects o that are immediate objects, having no singleton classes.

In MRI v2.7.0,

ObjectSpace.each_object(Object).to_a.size
  #=> 35362

a mere pittance.

edited Feb 12 '19 at 19:32

answered Feb 12 '19 at 19:13

Cary Swoveland

106,649
6
63
100

Good idea. It's still not completely straightforward but it goes in the right direction, and without needing C. – Eric Duminil Feb 12 '19 at 20:19
It works fine with the original example : https://pastebin.com/DqJh9JuB Do you have any better idea for method name? – Eric Duminil Feb 12 '19 at 20:31
For the name, how about `ssalc_notelgnis`? I'm not sure you want to check if the argument is a singleton class within the method, but if you do perhaps raise an exception if it isn't. (Consider a guard clause rather than `if`: `raise ... unless singleton_class`.) Also, shouldn't that be defined in `Class` rather than `Module`? – Cary Swoveland Feb 12 '19 at 20:58
Another idea is `singleton_class_of_what?`. – Cary Swoveland Feb 12 '19 at 21:14

score 2 · Accepted Answer · answered May 24 '23 at 18:56

Class#attached_object (Ruby 3.2+)

Starting from Ruby 3.2, there is a Class#attached_object method:

Returns the object for which the receiver is the singleton class.

Raises an TypeError if the class is not a singleton class.

For example:

class Foo; end

Foo.singleton_class.attached_object     #=> Foo
Foo.attached_object                     #=> TypeError: `Foo' is not a singleton class
Foo.new.singleton_class.attached_object #=> #<Foo:0x000000010491a370>
TrueClass.attached_object               #=> TypeError: `TrueClass' is not a singleton class
NilClass.attached_object                #=> TypeError: `NilClass' is not a singleton class

Sources:

Excellent, thanks! I just tested it. It seems to work fine with `attached_object.push(FATHER, MOTHER)`. — Eric Duminil, May 24 '23 at 20:15

Retrieve a Ruby object from its singleton class?

4 Answers4

Class#attached_object (Ruby 3.2+)

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