[ruby-core:113187] [Ruby master Feature#19573] Add Class#singleton_inherited

Issue #19573 has been updated by mame (Yusuke Endoh).


@jeremyevans0 Could you explain the use case?

> This could be used to warn or raise on singleton class creation

Why do you want to warn such a case? (To avoid some performance degeneration?)

> or modify the instance to change behavior, such as allow optimizations when a singleton class does not exist, but allow fallbacks if it does exist.

AFAIK, it is not visible for a Ruby programmer if a singleton class is "generated" or not. What optimization do you have in mind?

My understanding of the Ruby object model is that every object has a singleton class from the beginning. `class << obj` just retrieves it.
Because a naive implementation that actually makes every object have a singleton class will be slow, it is lazily generated as needed. But I think that it is just a kind of internal optimization, which should be invisible to a Ruby programmer.

----------------------------------------
Feature #19573: Add Class#singleton_inherited
https://bugs.ruby-lang.org/issues/19573#change-102726

* Author: jeremyevans0 (Jeremy Evans)
* Status: Open
* Priority: Normal
----------------------------------------
This would be similar to `Class#inherited`, but would be called with singleton classes of instances instead of subclasses. This could be used to warn or raise on singleton class creation, or modify the instance to change behavior, such as allow optimizations when a singleton class does not exist, but allow fallbacks if it does exist.

```ruby
c = Class.new do
  def self.inherited(subclass)
    p :inherited
  end
  def self.singleton_inherited(singleton_class)
    # could use singleton_class.attached_object for modifying related object
    p :singleton_inherited
  end
end

Class.new(c)
# prints :inherited

c.new.singleton_class
# prints :singleton_inherited
```

This could potentially be an instance method (e.g. `Kernel#singleton_class_created` or `BasicObject#singleton_class_created`) instead of a class method.  However, that would not grant any additional flexibility, since per-object behavior first requires creation of a singleton class.

If this is accepted, should the method be called for singleton classes created by `Kernel#clone` if the receiver has a singleton class?  I think it should, as `Class#inherited` is called for `Class#clone`.



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