Calling super from a class method

Question

When redefining a class method I want to be able to call super, just as I would in a instance method.

For example, I have a class hi with a class method Hi.hi

class Hi
  def self.hi
    puts "hi"
  end
end

Hi.hi #=> "hi"

Now, Lets say i want to redefine self.hi

class Hi
  def self.hi
    super
    puts "Oh!"
  end
end

Hi.hi #=> NoMethodError: super: no superclass method `hi' for Hi:Class

Why doesn't this work?

I know I can get the same functionality using alias (as below), it just seems rather unnecessary.

class Hi
  class << self
    def new_hi
      old_hi
      puts "Oh!"
    end
    alias :old_hi :hi
    alias :hi :new_hi 
  end
end

Hi.hi #=> "hi\n Oh!"

Is there a better way to do this?

Your new code with inheritance works fine for me (it puts _hi_ and _Oh_). What do you expect? if you want only _Oh!_ (you talked about redefine), then just don't call _super_. — knut, Jul 19 '11 at 20:58
This question is a duplicate of [When monkey patching a method, can you call the overridden method from the new implementation](http://StackOverflow.Com/q/4470108/#4471202). — Jörg W Mittag, Jul 19 '11 at 22:39

score 4 · Accepted Answer · edited Jul 20 '16 at 18:47

4

Super works for child classes inherited from a parent class.

In your case alias is the only way. Or you can use alias_method:

alias_method :old_hi, :hi

def self.hi
  old_hi
  puts "Oh!"
end

edited Jul 20 '16 at 18:47

kylewelsby

4,031
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answered Jul 19 '11 at 20:46

Vadim Golub

1,655
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score 2 · Answer 2 · answered Jul 19 '11 at 20:56

In the context of your code "super" refers to the Object class, since Object doesn't have a "hi" class level method it fails. "Hi" is an object of type "Class".

You don't have a super class defined so it defaults to object. You can see this at the console by typing "Hi.superclass"

The better way is to use traditional OOP to do this:

class Hi
  def self.hi
    puts "hi"
  end
end

class Ho < Hi
  def self.hi
    super
    puts "Oh!"
  end
end

The "class << self" syntax ends up creating an invisible intermediate class which does the inheritance chain.

It shouldn't matter that you created a new class since the new class extends the old one, they are interchangeable, except for the different output of self.hi()

I'm avoiding traditional OO because I'm trying to inject functionality into a gem. — diedthreetimes, Jul 19 '11 at 21:00

score 0 · Answer 3 · answered Jul 19 '11 at 20:43

0

It does not work.Because the ancestors for your class Hi don't have a method hi.

answered Jul 19 '11 at 20:43

This makes since, since the ancestors of Hi (or any class) is only ever class and object. I guess in the broader sense, why doesn't ruby have this functionality? Is it just an implementation challenge? – diedthreetimes Jul 19 '11 at 20:47
See knut's answer.He adds the method Hi to the object class(The base class for every class in ruby) – Jul 19 '11 at 20:51

score 0 · Answer 4 · answered Jul 19 '11 at 20:49

I don't understand your problem.

If you really redefine self.hi (Saying: super has a hi), then it works:

class Hi_super
  def self.hi
    puts "Oh - super!"
  end
end


class Hi < Hi_super
  def self.hi
    super
    puts "Oh!"
  end
end

Hi.hi

Or in another version:

class Object
  def self.hi
    puts "Oh - super!"
  end
end


class Hi 
  def self.hi
    super
    puts "Oh!"
  end
end

Hi.hi

Both version s results in

Oh - super!
Oh!

Calling super from a class method

4 Answers4