How to figure out what class a method is in for a *class method*?

ruby-talk
1

Hi all -

Given the following bit of code, I'd like it to spit out "Cat" followed by "Dog", but what I get now is "Object" twice.

···

---------------------------------------------------------------
class Animal
     def self.what_am_i
         self.class
     end
end

class Cat < Animal
end

class Dog < Animal
end

puts Cat.what_am_i
puts Dog.what_am_i
---------------------------------------------------------------

The closest I can get is self.object_id which prints out different ID's, but I'd really like the name if I can get it...

Thanks!

-philip

2

Hi all -

Given the following bit of code, I'd like it to spit out "Cat" followed by "Dog", but what I get now is "Object" twice.

---------------------------------------------------------------
class Animal
   def self.what_am_i
       self.class
   end
end

class Cat < Animal
end

class Dog < Animal
end

puts Cat.what_am_i
puts Dog.what_am_i
---------------------------------------------------------------

The closest I can get is self.object_id which prints out different ID's, but I'd really like the name if I can get it...

Naturally I find it right after posting...

self.name does the trick.

-philip

3

Try
def self.what_am_i
  self.name
end

Because self is an instance of Class, you want to ask it what its name is.

···

On 4/20/06, Philip Hallstrom <ruby@philip.pjkh.com> wrote:

Hi all -

Given the following bit of code, I'd like it to spit out "Cat" followed by
"Dog", but what I get now is "Object" twice.

---------------------------------------------------------------
class Animal
     def self.what_am_i
         self.class
     end
end

4

I have a related question - a similar problem to the one just given,
but that solution doesn't fit here because I don't just want the class
name - I want an attribute associated with the class, which subclasses
can override.

I've tried doing this using class variables, but it doesn't work:

···

--

class Animal
  @@sound = "nothing"
  def to_s
    "I say " + @@sound
  end
end

class Cat < Animal
  @@sound = "miaow"
end

class Dog < Animal
  @@sound = "woof"
end

fudge = Cat.new()
puts fudge

tip = Dog.new()
puts tip

--

This is an abstraction of my real situation but it captures the
essense: each subclass has some attribute which needs to be
manipulated at some point; the code to manipulate the attribute is
common across all subclasses, so it should live in the superclass.
(In fact, ideally, Animal is "abstract" and I don't want to declare a
@@sound there at all - but if I do that, to_s complains about
"uninitialized class variable".)

Alas:

[gimbo@lotus learn] ruby -v subclass_names.rb
ruby 1.8.4 (2005-12-24) [i386-freebsd6]
subclass_names.rb:9: warning: already initialized class variable @@sound
subclass_names.rb:13: warning: already initialized class variable @@sound
I say woof
I say woof

So I guess I'm misusing class variables.

I also tried it with constants (which would be even better since the
value won't ever change) but got "I say nothing" twice instead.

If anyone can help me understand what's going on and how this _should_
be done, I'd be grateful. I'm still thinking in python a bit too
much, I think.

Thanks,

-Andy

--
Andy Gimblett
Computer Science Department
University of Wales Swansea
http://www.cs.swan.ac.uk/~csandy/

5

Let me get this straight: you have a class object and you want a
method that returns this class object? The code that does what you
want is this:

def self.what_am_i?
  self
end

But this seems a rather weird thing to do...

Did you actually try this?

puts Cat, Dog

You can even use the name, but the output won't differ

puts Cat.name, Dog.name

If your problem is something else, please let us know.

Kind regards

robert

···

2006/4/21, Philip Hallstrom <ruby@philip.pjkh.com>:

Hi all -

Given the following bit of code, I'd like it to spit out "Cat" followed by
"Dog", but what I get now is "Object" twice.

---------------------------------------------------------------
class Animal
     def self.what_am_i
         self.class
     end
end

class Cat < Animal
end

class Dog < Animal
end

puts Cat.what_am_i
puts Dog.what_am_i
---------------------------------------------------------------

The closest I can get is self.object_id which prints out different ID's,
but I'd really like the name if I can get it...

--
Have a look: Robert K. | Flickr

6

I have a related question - a similar problem to the one just given,
but that solution doesn't fit here because I don't just want the class
name - I want an attribute associated with the class, which subclasses
can override.

I've tried doing this using class variables, but it doesn't work:

--

class Animal
  @@sound = "nothing"
  def to_s
    "I say " + @@sound
  end
end

class Cat < Animal
  @@sound = "miaow"
end

class Dog < Animal
  @@sound = "woof"
end

fudge = Cat.new()
puts fudge

tip = Dog.new()
puts tip

--

class Animal
@sound = "nothing"
class << self
    attr_accessor :sound
end
def to_s
   "I say " + self.class.sound
end
end

class Cat < Animal
self.sound = "miaow"
end

class Dog < Animal
self.sound = "woof"
end

fudge = Cat.new()
puts fudge

tip = Dog.new()
puts tip

That's how ruby treats it, there are some who argue that @@ should go away,
maybe they are right.

Hope that helps
Robert

···

On 4/21/06, Andy Gimblett <A.M.Gimblett@swansea.ac.uk> wrote:

--
Andy Gimblett
Computer Science Department
University of Wales Swansea
http://www.cs.swan.ac.uk/~csandy/

--
Deux choses sont infinies : l'univers et la bêtise humaine ; en ce qui
concerne l'univers, je n'en ai pas acquis la certitude absolue.

- Albert Einstein

7

:slight_smile: This is just the simplest example I could think of. The actual method in the parent class is responsible for storing "stuff" in memcache and I want to dynamically set the key used to store it to the class name and the primary key. That way I won't have to worry about collisions.

For the archives, seems that

def self.what_am_i
   put self
   put self.name
end

will do exactly what I want (return the name of the calling class)

Thanks all!

-philip

···

2006/4/21, Philip Hallstrom <ruby@philip.pjkh.com>:

Hi all -

Given the following bit of code, I'd like it to spit out "Cat" followed by
"Dog", but what I get now is "Object" twice.

---------------------------------------------------------------
class Animal
     def self.what_am_i
         self.class
     end
end

class Cat < Animal
end

class Dog < Animal
end

puts Cat.what_am_i
puts Dog.what_am_i
---------------------------------------------------------------

The closest I can get is self.object_id which prints out different ID's,
but I'd really like the name if I can get it...

Let me get this straight: you have a class object and you want a
method that returns this class object? The code that does what you
want is this:

def self.what_am_i?
self
end

But this seems a rather weird thing to do...