Double singleton?

Ruby
1

Hello all.

Noticed this behavior:

require ‘singleton’
=> true

class A; include Singleton; end
=> A

A.instance
=> #<A:0x3703c38>

class A; include Singleton; end
=> A

A.instance
=> #<A:0x36d7a60>

A second singleton?

Cheers!
-r

2

From singleton.rb:

def included(klass)
super
klass.private_class_method :new, :allocate
klass.extend SingletonClassMethods
Singleton.init(klass)
end

and a bit earlier:

def init(klass)
klass.instance_eval { @instance = nil }
…

Perhaps that should be instead:

klass.instance_eval { @__instance__ = nil unless defined?

@instance }

Or somewhat more obscure, I think this would do:

klass.instance_eval { @__instance__ ||= nil }
3

Roger P. wrote in post #986564:

Hello all.

Noticed this behavior:

require ‘singleton’
=> true

class A; include Singleton; end
=> A

A.instance
=> #<A:0x3703c38>

I looked through the ruby docs and neither BasicObject, Object, Kernel,
Module, Class, nor Singelton have a method named ‘instance’. Where does
that come from?

4

On Mar 9, 2011, at 14:34 , 7stud – wrote:

=> #<A:0x3703c38>

I looked through the ruby docs and neither Object, Module, Class, nor
Singelton have a method named ‘instance’. Where does that come from?

9998 % ri Singleton
= Singleton

(from ruby core)

The Singleton module implements the Singleton pattern.

Usage:
class Klass
include Singleton
# …
end

  • this ensures that only one instance of Klass lets call it ``the
    instance’’
    can be created.

    a,b = Klass.instance, Klass.instance a == b # => true a.new #
    NoMethodError - new is private …

  • ``The instance’’ is created at instantiation time, in other words the
    first
    call of Klass.instance(), thus

        class OtherKlass
      include Singleton
      # ...

…