Feature #12374: SingletonClass

• Status: Closed
• Assignee: -
• Target version: -

[sawa (Tsuyoshi Sawada)]

I propose to have a class SingletonClass, a subclass of the class Class, to which all singleton classes belong. It should be the owner of all the properties that are specific to singleton classes. Also, the methods defined on Singleton module should be moved to this class.

Reasons are as follows:

1. I was thinking that the reason #12084 hasn't been seen positively may be because the developers do not want to define a method only on limited instances of a class. If we have SingletonClass, the method #instance proposed in #12084 could be defined as an instance method of SingletonClass.

2. The way to introduce the singleton pattern using the Singleton module (http://ruby-doc.org/stdlib-2.3.0/libdoc/singleton/rdoc/Singleton.html):

class A
   include Singleton
   # ...
end

is a bit unnatural and verbose. If we have SingletonClass, then we can define a singleton class more naturally:

A = SingletonClass.new

[sawa (Tsuyoshi Sawada)]

Sorry, for the last example, I meant:

A = SingletonClass.new

[dsferreira (Daniel Ferreira)]

Typo corrected

[dsferreira (Daniel Ferreira)]

• Description updated (diff)

[nobu (Nobuyoshi Nakada)]

• Status changed from Open to Feedback

What is SingletonClass?
A wrapper object?

def (SingletonClass = Object.new).new(*args)
  Class.new(*args) {include Singleton}
end

[sawa (Tsuyoshi Sawada)]

Nobuyoshi Nakada wrote:

What is SingletonClass?
A wrapper object?

No, it should be a subclass of Class. It would perhaps need C-level modification. For any singleton class, I want SingletonClass to be its class.

Present:

"".singleton_class.class
# => Class
"".singleton_class.class.ancestors
# => [Class, Module, Object, Kernel, BasicObject]

I want it to be:

"".singleton_class.class
# => SingletonClass
"".singleton_class.class.ancestors
# => [SingletonClass, Class, Module, Object, Kernel, BasicObject]

[nobu (Nobuyoshi Nakada)]

If a singleton class is a subclass of SingletonClass, no singleton classed inheriting other classes cannot be made.
I think it's a bad idea.

[sawa (Tsuyoshi Sawada)]

Nobuyoshi Nakada wrote:

If a singleton class is a subclass of SingletonClass, no singleton classed inheriting other classes cannot be made.
I think it's a bad idea.

Sorry, my previous example was inappropriate. I fixed it. I want a singleton class to be an instance of SingletonClass.

[trans (Thomas Sawyer)]

So...

Object..singleton_class.instance_of?(SingletonClass)  #=> true

[matz (Yukihiro Matsumoto)]

• Status changed from Feedback to Closed

I don't think the idea itself is meaningless.
But its use-case is not clear to us yet.

(For the record, Singleton module is totally different story.)

If you have any real-world use-case, please reopen the issue.

Matz.