Test whether a Ruby class is a subclass of another class

202
votes

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

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.

2
rubyinheritancesubclasssuperclass
A.class #=> Class. This is why B.is_a? A returns false. - Wen
what about kind_of? - akostadinov
kind_of? tests whether an object is an instance of a class. Not whether the object inherits from a class. - Confusion
kind_of? is an alias of is_a? - coreyward

2 Answers

378
votes

Just use the < operator

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

or use the <= operator

B <= A # => true
A <= A # => true
64
votes

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.