Class method is accessing instance variable - ruby

Question

I'd like to understand this code. Why is it returning Hello instead of Howdy! ?
class Speaker
#message = "Hello!"
class << self
#message = "Howdy!"
def speak
#message
end
end
end
puts Speaker.speak

Answer 1

First off, your message #message is not an instance variable, or rather not the type of instance variable you may be thinking about: it's a class-level instance var, so an instance variable of Speaker itself, which as an object is an instance of class Class.
Here's a version of the code that does what you're trying to do with a local variable and a closure:
class Speaker
#message = "Hello!"
class << self
message = "Howdy!"
define_method(:speak) { message }
end
end
Speaker.speak
#=> "Howdy!"
And here's some code that illustrates the difference between the class-level instance variable and a "normal" instance variable:
class Speaker
#message = 'Howdy!' # class-level instance variable
def initialize
#message = 'Hello!' # instance variable of Speaker's instances
end
def speak
#message
end
class << self
def speak
#message
end
end
end
Speaker.speak
#=> "Howdy!"
Speaker.new.speak
#=> "Hello!"

Answer 2

Here is your code, except I've defined the class method in the usual way (def self.speak...). As a class method is nothing more than an instance method defined on the class' singleton class, this change is merely a different way of creating the same class method. (If you doubt that, run the code below both ways.) I made that change because I thought it would make my explanation of what is happening clearer. I also added a puts statement.
class Speaker
#message = "Hello!"
def self.speak
puts "self=#{self}"
#message
end
class << self
#message = "Howdy!"
end
end
The first line of the class definition creates a class instance variable #message:
Speaker.instance_variables
#=> [:#message]
Speaker.instance_variable_get(:#message)
#=> "Hello!"
By constrast,
#message = "Howdy!"
creates an instance variable on Speaker's singleton class:
Speaker.singleton_class.instance_variables
#=> [:#message]
Speaker.singleton_class.instance_variable_get(:#message)
#=> "Howdy!"
Now invoke speak on Speaker:
Speaker.speak
# self=Speaker
#=> "Hello!"
As self #=> Speaker, speak is obviously returning the value of the class instance variable.
For speak to return the value of the instance variable defined on Speaker's singleton class we can write the following:
class Speaker
#message = "Hello!"
def self.speak
puts "self=#{self}"
puts "singleton_class = #{singleton_class}"
singleton_class.instance_variable_get :#message
end
class << self
#message = "Howdy!"
end
end
puts Speaker.speak
# self=Speaker
# singleton_class = #<Class:Speaker>
# Howdy!
In the last expression, because self equals Speaker and self is the implied receiver when there is no explicit receiver, "singleton_class is equivalent to Speaker.singleton_class.

Answer 3

The reason this code returns 'Hello' is that it is attempting to change an instance variable in a class << self block.
Class methods are for anything that does not deal with an individual instance of that class - instance variables are tied to individual instances of a class, and we can't change instance variables at a class level.
Instead of using an instance variable in the speak method, we should use a class variable (denoted by ##).
As an example, the following code will return 'Howdy!' -
class Speaker
##message = "Hello!"
class << self
##message = "Howdy!"
def speak
##message
end
end
end
puts Speaker.speak