Test whether a Ruby class is a subclass of another class

Question

I would like to test whether a class inherits from another class, but there doesn't seem to exist a method for that.

class A
end

class B < A
end

B.is_a? A 
=> false

B.superclass == A
=> true

(N.B. the is_a? test above does not make any sense, as the first comment correctly points out B.new.is_a?(A) would make sense, but is not generally applicable, as not every class has a #initialize method that accepts 0 arguments)

A trivial implementation of what I want would be:

class Class
  def is_subclass_of?(clazz)
    return true if superclass == clazz
    return false if self == Object
    superclass.is_subclass_of?(clazz)
  end
end

but I would expect this to exist already.

`kind_of?` tests whether an object is an _instance_ of a class. Not whether the object inherits from a class. — Confusion, Jul 29 '14 at 06:18

score 399 · Accepted Answer · edited Apr 17 '13 at 12:28

399

Just use the < operator

B < A # => true
A < A # => false

or use the <= operator

B <= A # => true
A <= A # => true

edited Apr 17 '13 at 12:28

webwurst

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answered Dec 28 '10 at 11:11

Marcel Jackwerth

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This works...curious if anyone knows why `is_a?` doesn't though? Thanks! – Brian Armstrong Oct 24 '12 at 04:46
14

@Brian Because `is_a?` translates to *is instance of*. `B` isn't an instance of `A`, `B.new` is though (`B.new.is_a? A # => true`). – Marcel Jackwerth Oct 24 '12 at 07:44
5

Hmm, strange syntax (wouldn't have been my first guess), but thanks for the clarification! – Brian Armstrong Oct 26 '12 at 01:31
2

For documentation see [Core API / Module / #<](http://www.ruby-doc.org/core-1.9.3/Module.html#method-i-3C). – webwurst Feb 15 '13 at 15:22
2

My love/hate relationship with Ruby continues… Why provide a different function for an operator used for declaring class relationships AND provide two different ways of doing it? – Ben Gotow Apr 16 '13 at 15:08
5

@BenGotow - 1. Because < is not an operator, it's a method. 2. Because < only checks for a subclass, and <= also includes self. – superluminary Nov 28 '13 at 12:03
Haha, I don't know it so simple like that, I thought it'd be more complicated. Thank you so much! – Blue Smith Jun 11 '14 at 01:53
this is the correct answer, use `thing.class < X` instead of `thing.is_a? X`, because the second version includes modules – Rivenfall Oct 27 '16 at 21:04
`module#<` *does* include modules as well – Michael Apr 19 '17 at 17:16

score 68 · Answer 2 · answered Dec 28 '10 at 14:26

68

Also available:

B.ancestors.include? A

This differs slightly from the (shorter) answer of B < A because B is included in B.ancestors:

B.ancestors
#=> [B, A, Object, Kernel, BasicObject]

B < B
#=> false

B.ancestors.include? B
#=> true

Whether or not this is desirable depends on your use case.

answered Dec 28 '10 at 14:26

Phrogz

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More readable: `B <= B` (same result as `B.ancestors.include? B`). – Marcel Jackwerth Dec 28 '10 at 14:49
1

Update: The immediately preceding solution works with 1.9+ whereas there is no "ancestors?" in 1.9. – Nov 14 '11 at 06:40
@Barry I do not see `ancestors?` mentioned in any answer on this page, including this one. To what are you referring? – Phrogz Nov 15 '11 at 16:30
Update^2: The immediately preceding solution works with 1.9+ whereas I don't see "ancestors" in 1.9. --- Ambiguity corrected. My bad, I thought I was asking a question while using the overformal "Are these double-quotes sexy?" syntax. – Nov 18 '11 at 22:54
9

This will not confuse people not familiar with the '<' syntax, and for that reason I prefer it. – Asfand Qazi Nov 18 '13 at 14:42
I note that this answer iterates through all ancestors, which might take a while. Is `B < A` meaningfully faster, or do they do the same thing behind the scenes? – Simon Lepkin Jul 10 '15 at 22:18
2

@SimonLepkin Probably not "a while", unless you can experience microseconds ticking by. ;) Yes, behind the scenes the `include?` and `<` methods [loop through the ancestor chain](https://github.com/ruby/ruby/blob/trunk/class.c#L1023). It does this in C, so surely faster than looping through the Ruby array...but practically the two should be indistinguishable. – Phrogz Jul 10 '15 at 22:36
@Phrogz Could you please explain me why `B.ancestors` returns itself `B` also? Should not it be `[A, Object, Kernel, BasicObject]` ? – Junan Chakma May 05 '18 at 05:45
2

@JunanChakma Based on how the English word "ancestors" is defined, I agree that the return value should not include `B`. But it does. The [method documentation](http://ruby-doc.org/core-2.5.0/Module.html#method-i-ancestors) says, _"Returns a list of modules included/prepended in mod (**including mod itself**)."_ (emphasis mine). I'm guessing it includes its own class for convenience when using `.include?`, but that's just speculation on my part. – Phrogz May 07 '18 at 02:59
@Phrogz Thanks! – Junan Chakma May 11 '18 at 10:55

Test whether a Ruby class is a subclass of another class

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