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Bug #6087

How should inherited methods deal with return values of their own subclass?

Added by marcandre (Marc-Andre Lafortune) over 8 years ago. Updated 5 months ago.

Status:
Assigned
Priority:
Normal
Target version:
ruby -v:
trunk
Backport:
[ruby-core:42932]
Tags:

Description

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.


Related issues

Related to Ruby master - Bug #10845: Subclassing StringOpenmatz (Yukihiro Matsumoto)Actions

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