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Ruby - is_a? v instance_of? - what is the difference?

Archive - Originally posted on "The Horse's Mouth" - 2010-01-27 18:14:20 - Graham Ellis

In Ruby, have you ever wanted to know if an object is of a particular type?

The instance_of? method will return a true value if an object is of the type given as a parameter. However, instance_of? will return a false if we use it to check whether an object inherits from another class. So it's an exact check on the class name.

The is_a? method will return a true value if an object is a of the type given as a parameter OR if it inherits from the type given as a parameter. So in effect, you can use it to ask "is there going to be a method from a class which I can run on this object".


Some sample code:

glug.each do |current|
   puts current.bright, current.sides, current.area
   puts current.class
   puts current.instance_of?(Shape)
   puts current.is_a?(Shape)
   end

If the objects in the array glug are of type "Box" and "Circle" (which they were on today's Ruby Course), and boxes and circles inherit from Shapes, then the instance_of? calls will all return false, and the is_a? calls will return true.