Why is it not possible to read a class's instance variable value if it was set within the class using the attr_accessor?
Is it because the the instance variable is "private?"
Setting the attribute this way works:
class Test
attr_accessor :magic
end
bob = Test.new
bob.magic = "cat" #set
print bob.magic #read
Setting the attribute this way fails:
class Test
attr_accessor :magic
@magic = "spell"
end
bob = Test.new
print bob.magic #this resolves to 'nil'
There is a significant difference between instance variables and class instance variables.
class Test
@magic = "spell" # class instance var, since the scope is class
def initialize
@magic = 42 # instance var, scope is instance
end
end
Those two live together, since they are defined on different objects:
Test.instance_variable_get(:@magic) #⇒ "spell"
Test.new.instance_variable_get(:@magic) #⇒ 42
That said, attr_accessor reads the variable from the scope, it was defined for. Yours was defined in class scope, therefore it reads the instance scoped variable.
To read the class instance variable, define attr_accessor on class’ singleton class level:
class Test
singleton_class.send :attr_accessor, :magic # reads class instance var ⇓
@magic = "spell" # class instance var, since the scope is class
attr_accessor :magic # reads instance var ⇓
def initialize
@magic = 42 # instance var, scope is instance
end
end
Test.magic
#⇒ "spell"
Test.new.magic
#⇒ 42
You can access Test's @magic by defining attr_accessor within the singleton class of Test:
class Test
@magic = "spell"
class << self
attr_accessor :magic
end
end
Test.magic #=> "spell"
You should move @magic declaration/assignment to the constructor, like this:
class Test
attr_accessor :magic
def initialize
@magic = "spell"
end
end
bob = Test.new
print bob.magic #spell
When you declare instance variable (@) outside any method (self), it becomes unreachable by the class instance.
When you declare instance variable (@) outside any method (self), it becomes unreachable by the class instance. - there is almost no unreachable things in Ruby. Having user's example with @magic = "spell": Test.new.class.instance_variable_get(:@magic) would get it :)Instance variables belong to objects (aka "instances"), that's why they are called instance variables.
In your first example, you are setting the instance variable @magic on bob to 'cat' and then you are reading the instance variable @magic on bob.
In your second example, you are setting the instance variable @magic on Test (which is a completely different object than bob) and then you are reading the instance variable @magic from bob, which hasn't been set yet, and uninitialized instance variables evaluate to nil.
Remember: classes are objects just like any other object. They can have instance variables just like any other object.
