I have the following piece of code:
class Fish
# @message = "I can swim"
class << self
@message = "I can jump!"
define_method(:action) { @message }
end
end
Fish.action => nil
As soon as I uncomment the above @message variable, Fish.action returns I can swim. Why in both cases it is ignoring the I can jump message. Why is that? Why is Fish class being binded to the @message defined at the start but not inside the singleton class?
It's because class << self opens the class' singleton class context:
class Foo
p self # Foo
class << self
p self # #<Class:Foo>
define_method(:bar) { p self } # Foo
end
end
Foo.bar
You can verify that by:
Fish.singleton_class.instance_variable_get(:@action) # => "I can jump!"
class << self changes self to Foo's singleton class"? It seems more precise to me. I know "open a class" is often used, but nowhere (as far as I know) does Ruby provide a definition for "opening" a class.
self changes, but the current class also changes (think of it like instance_eval vs class_eval).self is changed to Foo.singleton_class between class << self and its end.class << self changes self to Foo's singleton class" basically means class << self; _something_; end and singleton_class.instance_eval { _something_ } are equivalent (they are not). "Opens Foo's ' singleton class context" is the equivalent of singleton_class.class_eval { _something_ }class Bar; p self; def baz; p self; end; end, the self outside the method will be Bar, but the self inside the method will be whatever instance you call it on. Methods are defined in classes, but act on the instances of said classes.
actionoutside the singleton class and define it (equivalently)def self.action; @message; end, the answers to the earlier question would apply to your question as well.