From: David Alan Black <dblack@...>
Date: 2001-10-27T22:17:08+09:00
Subject: [ruby-talk:23621] Re: sleeping, calling methods inside class defs

Hello --

On Sat, 27 Oct 2001, Tobias Reif wrote:

> ruby 1.6.3 (2001-03-19) [i386-cygwin]
>
> Hi,
>
> 1. Why is the following sleeping?
>
> It just sleeps forever; how can I achieve it that it uses the method
> above, not the sleep method?.
>
> -----
> class Tobi
>
>    def sleep
>      print "rzzzzz..\n"
>    end
>
>    sleep
>
> end
> -----


Because the bare call to sleep is a call to Kernel#sleep.  If you want
to call the instance method defined in class Tobi, you need an
instance of class Tobi :-)


> 2. I know this is basic, but I don't get it:
> -----
> class Tobi
>
>    def hi
>      print "hi!\n"
>    end
>
>    hi
>
> end
> -----
> gives NameError: :7: undefined local variable or method `hi' for Tobi:Class


Same idea.  Executed in the context of the class definition, "hi" doesn't refer to
anything.

Compare this:

  irb 1> def hi; puts "hi"; end
     ==>nil
  irb 2> class Tobi; hi; end
  hi
Here, the hi method is visible from inside the class definition, because it's in
an outer scope.


> -----
> class Tobi
>
>    def hi
>      print "hi!\n"
>    end
>
>    self.hi
>
> end
> -----
> gives 7: undefined method `hi' for Tobi:Class (NameError)


Same principle.  The "self." doesn't change anything.


> -----
> class Tobi
>
>    def hi
>      print "hi!\n"
>    end
>
> end
>
> Tobi.new.hi
> -----
> works; but how can I use the method inside the class?


Make it a class method:

  class Tobi
    def Tobi.hi
      puts "hi"
    end
    hi
  end

or:

  class Tobi
    class << self
      def hi
        puts "hi"
      end
    end
    hi
  end


Note that there's a consistent underlying principle to all of this:
Given object o and method m, if you call m in the context of o (when
self is o), Ruby looks in o's class (and that class's ancestry, etc.)
for an instance method called m.  So if o is the Class object Tobi,
then the search is for an instance method called hi in the class of o,
which is the class Class.  There is no instance method called "hi" in
class Class.  (There is one in class Tobi, but that isn't relevant
unless "hi" is called by a Tobi object.)


> -----
> class Tobi
>
>    def hey
>      print "hey!\n"
>    end
>
>    def hey_ho
>      hey
>      print "ho!\n"
>    end
>
> end
>
> Tobi.new.hey_ho
> -----
> here I can call the first method inside the second, when calling the
> second method from outside the class def.


That's because hey_ho is executing in the context of the new Tobi
object, which also has knowledge of its other instance methods (such
as hey).

> How to call a method inside the class def?
>
> -----
> class Tobi
>
>    def hey
>      print "hey!\n"
>    end
>
>    def hey_ho
>      hey
>      print "ho!\n"
>    end
>
>    @hey_ho = self.hey_ho
>
> end
> -----
> doesn't work.


See above.  Here, self is the Class object "Tobi".  It has no instance
method called hey_ho.  That is: Tobi defines an instance method hey_ho
for *its* instances -- but class Tobi is an instance of class Class,
not of class Tobi.


David

-- 
David Alan Black
home: dblack@candle.superlink.net
work: blackdav@shu.edu
Web:  http://pirate.shu.edu/~blackdav