Ruby class types and case statements

问题

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?

回答1:

You must use:

case item
when MyClass
...

I had the same problem: How to catch Errno::ECONNRESET class in "case when"?

回答2:

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

回答3:

You can use:

case item.class.to_s
    when 'MyClass'

...when the following twist is not possible:

case item
    when MyClass

The reason for this is that case uses ===, and the relationship the === operator describes is not commutative. For example, 5 is an Integer, but is Integer a 5? This is how you should think of case/when.

回答4:

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.

回答5:

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 How to catch Errno::ECONNRESET class in "case when"? as stated by Nakilon since my answer could be wrong.

来源：https://stackoverflow.com/questions/3908380/ruby-class-types-and-case-statements

标签

ruby

duck-typing

case-statement