Hello,
This is my first post at ruby-talk
I'm a some year exprience of rubyist without deep understanding 
Now I'm trying to understand more about meta programming.
I browsed other people's sources and have a question.
What does (class << self; self; end) means ?
I did an experience like this.
#!/usr/bin/ruby
class Cat
聽聽聽def self.mew
聽聽聽聽聽p self
聽聽聽聽聽p (class << self; self; end)
聽聽聽end
end
Cat.mew
The output was
> Cat
> #<Class:Cat>
I think the latter( #<Class:Cat> ) means Class singleton object.
Moreover, from my understanding I've been thinking that the former also means Class singleton object.
Could you please tell the difference between self and (class << self; self; end) in the context ?
Thank you,
Toshi Umehara now @Tokyo, soon @Atlanta
class << X
opens the singleton class of object X, whatever that is. X could be a
normal object or a class or whatever. Inside that scope, "self" means
the object that is the singleton class of X.
So,
class Cat
def self.mew
class << self
opens the scope of the singleton class of the object that happens to
be self at that point. As you are in a class instance method, self is
the object Cat (the class object, remember that classes are objects
too). So,
class Cat
def self.mew # this creates a class instance method
p self # here, self is the Cat class object (classes are objects too)
class << self
self # this self is the singleton class of the previous line
self, which is the Cat class, so: singleton class of Cat
end
end
end
The interesting thing is that when you define a class instance method,
it's actually defined in the singleton class of Cat. So, mew is an
instance method of #<Class:Cat>
2.0.0p195 :010 > class << Cat; p instance_methods.grep(/mew/); end
[:mew]
Jesus.
路路路
On Wed, Jun 18, 2014 at 4:41 PM, Toshi Umehara <toshi@niceume.com> wrote:
Hello,
This is my first post at ruby-talk
I'm a some year exprience of rubyist without deep understanding 
Now I'm trying to understand more about meta programming.
I browsed other people's sources and have a question.
What does (class << self; self; end) means ?
I did an experience like this.
#!/usr/bin/ruby
class Cat
def self.mew
p self
p (class << self; self; end)
end
end
Cat.mew
The output was
Cat
#<Class:Cat>
I think the latter( #<Class:Cat> ) means Class singleton object.
Moreover, from my understanding I've been thinking that the former also
means Class singleton object.
Could you please tell the difference between self and (class << self; self;
end) in the context ?
Thank you , Jes煤s Gabriel y Gal谩n
> class << X
>
> opens the singleton class of object X, whatever that is. X could be a
> normal object or a class or whatever.
This explains all !
I read the book, "Metaprogramming Ruby" again and I could understand your words.
Thanks.
路路路
On 06/19/2014 12:41 AM, Jes煤s Gabriel y Gal谩n wrote:
On Wed, Jun 18, 2014 at 4:41 PM, Toshi Umehara <toshi@niceume.com> wrote:
Hello,
This is my first post at ruby-talk
I'm a some year exprience of rubyist without deep understanding
Now I'm trying to understand more about meta programming.
I browsed other people's sources and have a question.
What does (class << self; self; end) means ?
I did an experience like this.
#!/usr/bin/ruby
class Cat
def self.mew
p self
p (class << self; self; end)
end
end
Cat.mew
The output was
Cat
#<Class:Cat>
I think the latter( #<Class:Cat> ) means Class singleton object.
Moreover, from my understanding I've been thinking that the former also
means Class singleton object.
Could you please tell the difference between self and (class << self; self;
end) in the context ?
class << X
opens the singleton class of object X, whatever that is. X could be a
normal object or a class or whatever. Inside that scope, "self" means
the object that is the singleton class of X.
So,
class Cat
def self.mew
class << self
opens the scope of the singleton class of the object that happens to
be self at that point. As you are in a class instance method, self is
the object Cat (the class object, remember that classes are objects
too). So,
class Cat
def self.mew # this creates a class instance method
p self # here, self is the Cat class object (classes are objects too)
class << self
self # this self is the singleton class of the previous line
self, which is the Cat class, so: singleton class of Cat
end
end
end
The interesting thing is that when you define a class instance method,
it's actually defined in the singleton class of Cat. So, mew is an
instance method of #<Class:Cat>
2.0.0p195 :010 > class << Cat; p instance_methods.grep(/mew/); end
[:mew]
Jesus.