Class << self ...end - how does this create class methods?

Ruby
1

I just ran into a seemingly weird problem until I discovered that the
method that I kept trying to call as an instance method was in fact a
class method. But of course, there isn’t anything in the signature to
indicate that to me. Apparently, if you do this:

class << self
def x
end
end

then x is a class method, not an instance method.

I’m having trouble finding a good explanation of this in the Pickaxe
book - is someone willing to give me the low down.

This appears to be the common idiom for defining class methods and I’d
like to understand so I don’t waste any more time.

Thanks,
Wes

2

Keith,

Thanks - I haven’t gotten to this section of the book yet :). It makes
some sense.

Powerful magic.

Wes

3

On 5/31/06, Wes G. [email protected] wrote:

–
Posted via http://www.ruby-forum.com/.

Here’s an excerpt from Ruby for Rails by Black that does a good job of
explaining it…

Defining class methods with class <<

By far the most frequent use of the class << notation for entering a
singleton method
class is in connection with class method definitions. You’ll see this
quite
often:
class Ticket
class << self
def most_expensive(tickets)

etc.

This code results in a class method Ticket.most_expensive. That method
could

also be defined like this (assuming this code comes at a point in the
program
where the Ticket class already exists):
class << Ticket
def most_expensive(tickets)

etc.

Because self is Ticket inside the class Ticket definition body, class <<
self
inside the body is the same as class << Ticket outside the body.
(Technically, you
could do class << Ticket even inside the body of class Ticket, but in
practice
you’ll usually see class << self whenever the object whose singleton
class
needs
opening is self.)
The fact that class << self shows up frequently in connection with the
crea-
tion of class methods sometimes leads to the false impression that the
class
<<
notation can only be used to create class methods, or that the only
expression you
can legally put on the right is self. In fact, class << self inside a
class
definition
block is just one case of the class << object notation. The technique is
general: It
puts you in a definition block for the singleton class of object,
whatever
object
may be. That, in turn, means you’re operating in a context where
whatever
you
doâ??whatever you add to the classâ??pertains only to that one object.

HTH,
Keith

4

Pickaxe2 in chapter 24 on page 362 (“Classes and Objects”) goes into
great extend explaining the “class << object” feature, with “class <<
self” in particular. With pictures and good examples. It may be a good
starting point.

Hope it helps,
Gennady.

5

On Thu, 1 Jun 2006 04:18:18 +0900, Wes G. [email protected] wrote:

then x is a class method, not an instance method.
There are several different pieces which fit together:

  1. Classes are themselves represented as objects.

class Foo
# …
end

p Foo.class # => Class

  1. Methods can be defined on a per-object basis

foo = Object.new

def foo.blah
puts “eek”
end

foo.blah # => eek

  1. you can also define per-object methods this way:

foo = Object.new

class << foo
def blah
puts “eek”
end
end

foo.blah # => eek

  1. outside of methods, within a class … end block, self refers to the
    class

class Foo
p self # => Foo
end

  1. “class methods” are simply per-object methods defined on classes

class Foo
class << self
def blah
puts “eek”
end
end
end

Foo.blah # => eek

-mental