object.md

#is_a? vs #kind_of? vs #instance_of?

Given the following example:

module ParentModule; end
module ChildModule;  end

class Parent;
  include ParentModule
end

class Child < Parent
  include ChildModule
end

child  = Child.new
parent = Parent.new

The methods behave as below:

• obj.kind_of?( klass )
• true if klass is the class or a superclass of obj
• true if klass is a module included in obj

child.kind_of? Parent       == true
child.kind_of? ParentModule == true # Child include ParentModule through its superclass Parent
child.kind_of? Child        == true
child.kind_of? ChildModule  == true

parent.kind_of? Parent       == true
parent.kind_of? ParentModule == true
parent.kind_of? Child        == false
parent.kind_of? ChildModule  == false

• obj.instance_of?( klass )
• returns true only if klass is the class of obj

child.instance_of? Parent       == false
child.instance_of? ParentModule == false
child.instance_of? Child        == true
child.instance_of? ChildModule  == false

parent.instance_of? Parent       == true
parent.instance_of? ParentModule == false
parent.instance_of? Child        == false
parent.instance_of? ChildModule  == false

• #is_a? is an alias of #kind_of?
• both point to the same implementation (see ruby/object.c)