I'm trying to find a way to declare class variables in a module that can be mixed into class methods and retain scope for only that particular class instead of being defacto module-variables.
Here's a simplified example (albeit somewhat silly) of what i'm trying to do:
module Foo
module Bar
#...some instance methods
def self.included(klass)
klass.extend(BarClassMethods)
end
end
module BarClassMethods
def greeting
@@greeting ||= self.determine_greeting
end
def determine_greeting
hello
end
end
end
class A
include Foo::Bar
def self.hello
"hello from A"
end
end
class B
include Foo::Bar
def self.hello
"hello from B"
end
end
puts A.greeting
#=> 'hello from A'
puts B.greeting
#=> 'hello from A'
The result i would hope for is "hello from A" and "hello from B" respectively. But since @@greeting is really scoped module-wide it is only intialized once. How can i force the class variable to pertain only to the singelton class that mixes it in?
uuh-ooh. i guess i could use an instance variable for the Singleton class instead
def greeting @greeting ||= self.determine_greeting
end
It seems to have the exact same effect than a class variable defined in the class. Is this correct?
···
On 2007-01-24 18:51:46 -0600, Sebastian <shfriedrich@gmail.com> said:
I'm trying to find a way to declare class variables in a module that can be mixed into class methods and retain scope for only that particular class instead of being defacto module-variables.
Here's a simplified example (albeit somewhat silly) of what i'm trying to do:
module Foo
module Bar
#...some instance methods
def self.included(klass)
klass.extend(BarClassMethods)
end
end
module BarClassMethods
def greeting
@@greeting ||= self.determine_greeting
end
def determine_greeting
hello
end
end
end
class A
include Foo::Bar
def self.hello
"hello from A"
end
end
class B
include Foo::Bar
def self.hello
"hello from B"
end
end
puts A.greeting
#=> 'hello from A'
puts B.greeting
#=> 'hello from A'
The result i would hope for is "hello from A" and "hello from B" respectively. But since @@greeting is really scoped module-wide it is only intialized once. How can i force the class variable to pertain only to the singelton class that mixes it in?
uuh-ooh. i guess i could use an instance variable for the Singleton class instead
def greeting @greeting ||= self.determine_greeting
end
That isn't an instance variable for the singleton class. It is an
instance variable associated with the class itself. The singleton
class for an object acts as a container for method definitions but
it is the object itself that has the instance variables.
It seems to have the exact same effect than a class variable defined in the class. Is this correct?
There are various differences but the biggest is that a class
instance variable, just like all instance variables, is only
accessible via the individual class object and not through any
subclasses as would be the case with a class variable.
Most Ruby veterans seem to shun Ruby class variables in favor of
class instance variables. Despite the name, Ruby class variables
are not like class variables in other object oriented languages.
Almost always a 'class instance variable' is a better choice.
I'm trying to find a way to declare class variables in a module that
can be mixed into class methods and retain scope for only that
particular class instead of being defacto module-variables.
