Is an instance of a class really an instance of its superclass too?

ruby-talk
1

I understand that the answer to that question is yes.

But then a singleton class (using require 'singleton') can have two or
more instances in a Ruby program, like:

require 'singleton'

class Super
  include Singleton
end
class Sub < Super
end

puts Super.instance
puts Sub.instance

#=> <Super:0x000000024b5648>
#=> <Sub:0x000000024b55d0>

Doesn't this mean we have two instances of a class (Super) whose only
one instance should have been in the program (because the class included
'singleton')? IOW, shouldn't class Sub < Super be disallowed?

Thanks,
Kedar

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2

I think you answer your own question. The contract created by the 'singleton' feature is in regards to direct instances and not indirect instances via a subclass.

Making the contract more restrictive as you suggest, would limit the use cases for no particular reason. If you don't want a subclass, then don't create one. In general I think it is better to avoid arbitrary restrictions. In most cases, with Ruby, those restrictions don't really prevent anything because methods and classes can always be redefined.

Gary Wright

···

On Jan 24, 2011, at 8:41 PM, Kedar Mhaswade wrote:

Doesn't this mean we have two instances of a class (Super) whose only
one instance should have been in the program (because the class included
'singleton')? IOW, shouldn't class Sub < Super be disallowed?