From: "headius (Charles Nutter)" <headius@...>
Date: 2012-05-11T06:33:50+09:00
Subject: [ruby-core:44987] [ruby-trunk - Bug #6087] How should inherited methods deal with return values of their own subclass?


Issue #6087 has been updated by headius (Charles Nutter).


I never noticed this before, so I'm jumping in a couple months late.

Duping the original object or copying its instance vars is wrong. Instance variables are state of an individual object, and should not be carried on to a *new* object as in these messages. There's no precedent for doing that other than dup'ing, which is explicitly for making a copy of the target object.

flatten et al are not returning "copies"...they're returning new instances with a different arrangement of the same elements. Therefore, those new objects should not automatically inherit instance variables from their parents.

It would be a good idea to design a formal way by which subclasses that *want* to propagate instance vars to new instances can do so. It just shouldn't be the default.

For the pattern that keeps coming up, where A < Array...you're doing it wrong anyway. Favor composition over inheritance :)
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Bug #6087: How should inherited methods deal with return values of their own subclass? 
https://bugs.ruby-lang.org/issues/6087#change-26572

Author: marcandre (Marc-Andre Lafortune)
Status: Assigned
Priority: Normal
Assignee: matz (Yukihiro Matsumoto)
Category: core
Target version: 2.0.0
ruby -v: trunk


Just noticed that we still don't have a consistent way to handle return values:

  class A < Array
  end
  a = A.new
  a.flatten.class # => A
  a.rotate.class  # => Array
  (a * 2).class   # => A
  (a + a).class   # => Array

Some methods are even inconsistent depending on their arguments:

  a.slice!(0, 1).class # => A
  a.slice!(0..0).class # => A
  a.slice!(0, 0).class # => Array
  a.slice!(1, 0).class # => Array
  a.slice!(1..0).class # => Array

Finally, there is currently no constructor nor hook called when making these new copies, so they are never properly constructed.

Imagine this simplified class that relies on `@foo` holding a hash:

  class A < Array
    def initialize(*args)
      super
      @foo = {}
    end
  
    def initialize_copy(orig)
      super
      @foo = @foo.dup
    end
  end
  a = A.new.flatten
  a.class # => A
  a.instance_variable_get(:@foo) # => nil, should never happen

I feel this violates object orientation.


One solution is to always return the base class (Array/String/...).

Another solution is to return the current subclass. To be object oriented, I feel we must do an actual `dup` of the object, including copying the instance variables, if any, and calling `initialize_copy`. Exceptions to this would be (1) explicit documentation, e.g. Array#to_a, or (2) methods inherited from a module (like Enumerable methods for Array).

I'll be glad to fix these once there is a decision made on which way to go.



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