Subclassing self

[John]

X-No-archive: yes

I'm a new ruby user. I have the Ruby Way book.

My question is, in what case would you use

class TheClass
class << self
def foo

Couldn't you just say

class TheClass
def foo

?

 

[Gavin Sinclair]

X-No-archive: yes

I'm a new ruby user. I have the Ruby Way book.

My question is, in what case would you use

class TheClass
class << self
def foo

Couldn't you just say

class TheClass
def foo

Take a look at http://www.rubygarden.org/ruby?ClassMethods

Gavin

 

[Fredrik Jagenheim]

Instead of using the class << self notation, you could do this:

class TheClass
def TheClass::foo

But that can get tedious when defining multiple class methods,
plus you run the risk of missing a change if you rename the class
later.

I was just shown:

class TheClass
def self.foo

This way you don't risk the missing a change when you rename the
class. You still get the tedious typing though.

However, what does 'class << self' really mean? Why is this one
of the 'obvious' ways to define class methods?

No, I'm not being sarcastic, just realizing that I don't know the ruby
language as well as I would want to. To me it looks like we're
shifting the current class into an anonymous class, and how that can
spell 'class methods' I really can't see.

//F

 

[Mauricio Fernández]

I was just shown:

class TheClass
def self.foo

This way you don't risk the missing a change when you rename the
class. You still get the tedious typing though.

However, what does 'class << self' really mean? Why is this one
of the 'obvious' ways to define class methods?

class << obj opens the singleton class of obj. In a class context,
self points to the Class object, therefore in

class A
self # this is A
class << self # same as class << A but needs not be changed if
# if A is renamed
# we're in A's singleton class
end
end

Class methods are in fact class singleton methods (ie. singleton methods
of the object of class Class). You sometimes want to use the class << self
idiom to create attribute accessors for the class, etc:

class A
class << self
attr_accessor :foo
end
end

A.foo = 1
A.foo # => 1

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-- Oliver Elphick

 

[Gavin Sinclair]

I was just shown:

class TheClass
def self.foo

This way you don't risk the missing a change when you rename the
class. You still get the tedious typing though.

However, what does 'class << self' really mean? Why is this one
of the 'obvious' ways to define class methods?

It's not necessarily obvious; it's an idiom, but it makes perfect
sense once you know, and it involves core Ruby concepts, so it is
worth knowing.

s = "Hi"
class << s
def foo # Singleton method on object s
5
end
end
s.foo # -> 5

s = String
class << s
def foo # Singleton method on object String
5
end
end
s.foo # -> 5
String.foo # -> 5

See http://www.rubygarden.org/ruby?ClassMethods for more info.

Gavin