Ruby class types and case statements

Question

What is the difference between

case item.class
when MyClass
  # do something here
when Array
  # do something different here
when String
  # do a third thing
end

and

case item.class
when MyClass.class
  # do something here
when Array.class
  # do something different here
when String.class
  # do a third thing
end

For some reason, the first one of these works sometimes and the second doesn't, and other times, the second one works and the first one doesn't. Why? Which one is the "proper" way to do it?

Answer 1

It depends on the nature of your item variable. If it is an instance of an object, e.g.

t = 5

then

t.class == Fixnum

but if it is a class in itself e.g

t = Array

then it will be a Class object, so

t.class == Class

EDIT: please refer to http://stackoverflow.com/questions/3801469/how-to-catch-errnoeconnreset-class-in-case-when as stated by Nakilon since my answer could be wrong.

Answer 2

You must use:

case item
    when MyClass

I had the same problem: http://stackoverflow.com/questions/3801469/how-to-catch-errnoeconnreset-class-in-case-when

Answer 3

In Ruby, a class name is a constant that refers to an object of type Class that describes a particular class. That means that saying MyClass in Ruby is equivalent to saying MyClass.class in Java.

obj.class is an object of type Class describing the class of obj. If obj.class is MyClass, then obj was created using MyClass.new (roughly speaking). MyClass is an object of type Class that describes any object created using MyClass.new.

MyClass.class is the class of the MyClass object (it's the class of the object of type Class that describes any object created using MyClass.new). In other words, MyClass.class == Class.

Answer 4

Yeah, Nakilon is correct, you must know how the threequal === operator works on the object given in the when clause. In Ruby

case item
when MyClass
...
when Array
...
when String
...

is really

if MyClass === item
...
elsif Array === item
...
elsif String === item
...

Understand that case is calling a threequal method (MyClass.===(item) for example), and that method can be defined to do whatever you want, and then you can use the case statement with precisionw