Question

class A
  def set(v)
    @@v = v
  end
  def put
    puts @@v
  end
end

class B < A
end
class C < A
end

B.new.set 'b'
B.new.put # => b
C.new.set 'c'
C.new.put # => c
B.new.put # => c

Why? And how should I write this to have ‘b’ in last B.new.put?

Answer 1

Here is a nice article on the subject – Class and Instance Variables In Ruby.

Basically, what you can do is:

class A
  class << self
    attr_accessor :class_var
  end

  def set_class_var(value)
    self.class.class_var = value
  end

  def get_class_var
    self.class.class_var
  end
end

class B < A; end

A.class_var = 'a'
B.class_var = 'b'
puts A.class_var # => a
puts B.class_var # => b

A.new.set_class_var 'aa'
B.new.set_class_var 'bb'
puts A.new.get_class_var # => aa
puts B.new.get_class_var # => bb

To understand it you should think about A as an instance of Class class (and that’s how it is in Ruby). But every object in Ruby has its own singleton class that stores object-specific stuff like methods defined on object itself:

a = A.new
def a.foo
  puts 'foo'
end

In that case foo is method defined only for a object and not for every instance of A class. And another way to define method in object’s singleton class is like that:

class << a # open a's singleton class
  def bar  # define method that will be available only on 'a' object
    puts 'bar'
  end
end

In the first code snippet we use that approach to define class_var attribute accessor in the context of singleton class of our A class (it’s a bit tricky, so you need to think about it). As the result class itself has class_var variable as well as its descendant class B. The difference is that every one of them has its own class_var variable that do not interfere.