From: Marc-Andre Lafortune <ruby-core@...>
Date: 2012-03-02T11:22:39+09:00
Subject: [ruby-core:43043] [ruby-trunk - Bug #6087] How should inherited methods deal with return values of their own subclass?


Issue #6087 has been updated by Marc-Andre Lafortune.


I apparently forgot to mention that I prefer the second approach, i.e. the equivalent of calling `dup` on the receiver.

I believe Aaron Patterson seconds this in [ruby-core:43030]

If this approach is accepted, the last remaining question is what of cases of instances of Array/String/... in which instance variables where set using `instance_variable_set`. Should the instance variables copied over?

  b = []
  b.instance_variable_set(:@foo, 42)
  b.flatten.instance_variable_get(:@foo) # => nil or 42?

I think that to be consistent, they should be copied (again, assuming we decide to return an instance of subclasses). In the discussion of #4136, Charles Nutter thinks it could hinder performance to do so, but I feel that cases where such objects happen to have instance variables set should be extremely rare, so I don't think it would have much effect in practice.
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Marc-Andr��
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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

Author: Marc-Andre Lafortune
Status: Open
Priority: Normal
Assignee: 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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