CODEWITHSUNDEEP
rubymetaprogramming
Eric Duminil
Eric Duminil

Reputation: 54223

Retrieve a Ruby object from its singleton class?

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.

Upvotes: 7

Views: 593

Answers (4)

Marian13
Marian13

Reputation: 9238

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:

Upvotes: 2

Cary Swoveland
Cary Swoveland

Reputation: 110685

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.

Upvotes: 2

Stefan
Stefan

Reputation: 114178

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>
>

Upvotes: 7

Aleksei Matiushkin
Aleksei Matiushkin

Reputation: 121000

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]

Upvotes: 2

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