CODEWITHSUNDEEP
ruby
gorootde
gorootde

Reputation: 4073

Ruby dynamically instantiate class

How to reference to a class within a static method? 

class Car
  def self.new_from_xml(xml)
    instance = self.class.new
    #do some stuff with xml
    instance
  end
end

class Mercedes < Car
end

class Porsche < Car
end

IRB:

Mercedes.new_from_xml(somedata) # Output is #<Class:...>, should be #<Mercedes:...>
Porsche.new_from_xml(somedata)  # Output is #<Class:...>, should be #<Porsche:...>

Upvotes: 2

Views: 680

Answers (2)

mdesantis
mdesantis

Reputation: 8507

Since you are already in a class method, you should use self.new (or simply new, as @tokland wrote) instead of self.class.new:

class Car
   def self.new_from_xml(xml)
       instance = new
       #do some stuff with xml
   end
end

class Mercedes < Car
end

class Porsche < Car
end

p Mercedes.new_from_xml(nil) #=> #<Mercedes:0x007f042d0db208>
p Porsche.new_from_xml(nil) #=> #<Porsche:0x007f042d0db118>

From a comment to this answer: Why does self.class reference to class? What's the logic here?

Inside a class block self references the class you are editing: 

class Car
  puts self #=> writes Car
end

Using def self.new_from_xml it is like if you are declaring def Car.new_from_xml, that is a method of the Car object (which is an instance of Class); so inside new_from_xml self coincides with Car.

Upvotes: 2

yagooar
yagooar

Reputation: 15979

Instead of

instance=self.class.new

just write

instance = new

Why is this?

Well, in first place, you have to understand that you are calling a class method, thus you are at a class level already. The .new method is a class method, so you can call it directly without calling self.class.new.

Why does self.class.new return Class?

Because the class of the class Car is Class (I know, sounds weird ;), because classes in Ruby are instances of Class.

This is actually a pretty deep concept, I recommend you read more about it. One nice reference I have read is the book Metaprogramming Ruby by Paolo Perrotta (ISBN-10: 1934356476) [1].

  1. http://www.amazon.com/Metaprogramming-Ruby-Program-Like-Pros/dp/1934356476

Upvotes: 4

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