From: Thomas Sawyer <transfire@...>
Date: 2012-03-02T19:35:59+09:00
Subject: [ruby-core:43052] [ruby-trunk - Bug #6087] How should inherited methods deal with return values of their own subclass?


Issue #6087 has been updated by Thomas Sawyer.


> This has two problems:
> 1) It imposes an API on the constructor of subclasses (i.e. that they accept one parameter which would be an instance of the base class)
> 2) The builtin classes constructors doesn't even respect that, i.e.
>   Hash.new({1 => 2}).has_key?(1) # => false

You took me a bit too literally. I only meant it should be _equivalent_ too calling `self.class.new`. In other words, it should return an instance of the subclass, not the base class. I did not mean to imply the necessary use of the constructor in this way --which (perhaps unfortunately) is not possible in some notable cases, as you point out.

----------------------------------------
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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