Making a chainable subclass of ::Array?

Question

I'm trying to make an Array-like class with chainable methods, but I seem to be screwed because Ruby's Array methods call ::Array.new instead of self.class.new:

class MyArray < Array; end
x = MyArray.new
y = MyArray.new
(x+y).class     # Array, expected MyArray!

I understand that I could go through and re-implement all of Array's methods which call its constructor, but that seems really bogus.

Interestingly, Set DOES work 'correctly':

class MySet < Set; end
x = MySet.new([1,2,3])
y = MySet.new([4,5,6])
(x+y).class     # MySet -- yeah!

What am I missing? Is there a Ruby idiom for subclassing Array in this way? Thanks!

yeah, things are kind of inconsistent implementation with regard to stuff like this in the stdlib, I've found too. Sorry, I don't think there's any simple magic answer. — jrochkind
– jrochkind, Commented Sep 28, 2015 at 16:23

sawa · Accepted Answer · 2015-09-28 08:51:23Z

2

The underlying implementation of + method is different between Array and Set. We can read the source code to find the root cause:

For Array, it uses a new variable to store the generated array:

rb_ary_plus(VALUE x, VALUE y)
{
    VALUE z; 
    long len, xlen, ylen;
    ......
    return z;
}

but for Set, + is alias for: |; it uses dup, which produces a shallow copy of the original Set object, it doesn't generate a new Set object to store the result.

def |(enum)
  dup.merge(enum)
end

edited Sep 28, 2015 at 8:51

sawa

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answered Sep 28, 2015 at 1:45

pangpang

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1 Comment

Eric Over a year ago

Indeed, Array is (rightly) concerned with efficiency so it seems to alloc the space for xlen+ylen elements and then memcpy's the items together. Makes total sense and I'm happy to see the speed. OTOH, it seems very anti-social for an OO implementation. Might have been better for Array to use its own new(x) implementation for the allocation. Oh well, c'est la vie!

jrochkind · Accepted Answer · 2015-09-28 16:24:35Z

0

yeah, things are kind of inconsistent implementation with regard to stuff like this in the stdlib, I've found too. Sorry, I don't think there's any simple magic answer. There isn't really a ruby idiom, ruby doesn't really want you to do it, and some people have posted advocating for never sub-classing Array or Hash. http://words.steveklabnik.com/beware-subclassing-ruby-core-classes

answered Sep 28, 2015 at 16:24

jrochkind

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1 Comment

Eric Over a year ago

Excellent article - I wish I had seen it before posting here - thanks so much. I've implemented my class with delegation to Array and wrapping the result, I guess that's where this will end up after all. Thanks for helping out. Much appreciated.

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