From: 7stud -- <bbxx789_05ss@...>
Date: 2011-03-08T07:07:51+09:00
Subject: Re: basic programming question, help please

Kaye Ng wrote in post #985934:
> class Square
>   def initialize
>     if defined?(@@number_of_squares)
>       @@number_of_squares = @@number_of_squares + 1
>     else
>       @@number_of_squares = 1
>     end
>   end
>   def self.count
>     @@number_of_squares
>   end
> end
>
> a = Square.new    #would return 1
> b = Square.new    #would return 2
> puts Square.count #2 would be printed on the screen
>
> --------------------------------------------------------------------
>
> My question is: why is the "self" necessary in "def self.count".
> I don't understand the logic behind it.
>

It isn't necessary:

class Square

  def initialize
    if defined?(@@number_of_squares)
      @@number_of_squares = @@number_of_squares + 1
    else
      @@number_of_squares = 1
    end
  end

  def count
    @@number_of_squares
  end

end

a = Square.new
b = Square.new
puts a.count  #2


...but the way in which you define the method determines how you can 
call the method.  You can choose to define a "class method" or an 
"instance method".  A class method is called using the class, and an 
instance method is called using an object/instance of the class.  You 
have to decide how you want to call the method: with the class or an 
instance.

In your case, I would suggest that Square.count makes more sense in 
English where if you ask for a.count, it is not really clear what that 
means.

> (I know that I can also type "def Square.count" instead of "def
> self.count")
>

You can also use a third syntax:

class Square

  def Square.class_meth1
    puts 'class_meth1'
  end

  def self.class_meth2
    puts 'class_meth2'
  end

  class << self
    def class_meth3
      puts 'class_meth3'
    end
  end

end

--output:--
class_meth1
class_meth2
class_meth3

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