The answers are partially correct because @@ is actually a class variable which is per class hierarchy meaning it is shared by a class, its instances and its descendant classes and their instances.
class Person
@@people = []
def initialize
@@people << self
end
def self.people
@@people
end
end
class Student < Person
end
class Graduate < Student
end
Person.new
Student.new
puts Graduate.people
This will output
#<Person:0x007fa70fa24870>
#<Student:0x007fa70fa24848>
So there is only one same @@variable for Person, Student and Graduate classes and all class and instance methods of these classes refer to the same variable.
There is another way of defining a class variable which is defined on a class object (Remember that each class is actually an instance of something which is actually the Class class but it is another story). You use @ notation instead of @@ but you can't access these variables from instance methods. You need to have class method wrappers.
class Person
def initialize
self.class.add_person self
end
def self.people
@people
end
def self.add_person instance
@people ||= []
@people << instance
end
end
class Student < Person
end
class Graduate < Student
end
Person.new
Person.new
Student.new
Student.new
Graduate.new
Graduate.new
puts Student.people.join(",")
puts Person.people.join(",")
puts Graduate.people.join(",")
Here, @people is single per class instead of class hierarchy because it is actually a variable stored on each class instance. This is the output:
#<Student:0x007f8e9d2267e8>,#<Student:0x007f8e9d21ff38>
#<Person:0x007f8e9d226158>,#<Person:0x007f8e9d226608>
#<Graduate:0x007f8e9d21fec0>,#<Graduate:0x007f8e9d21fdf8>
One important difference is that, you cannot access these class variables (or class instance variables you can say) directly from instance methods because @people in an instance method would refer to an instance variable of that specific instance of the Person or Student or Graduate classes.
So while other answers correctly state that @myvariable (with single @ notation) is always an instance variable, it doesn't necessarily mean that it is not a single shared variable for all instances of that class.