class Counter
def self.counted_new
@count = 0 if @count.nil?
@count += 1
new
end
def self.count
@count
end
end
In format, @count looks like a instance variable, but when I load this in "irb"(interface ruby) and type four command code
Counter.counted_new
Counter.count
Counter.counted_new
Counter.count
@count finally became 2! Acting like a class variable
@count is always an instance variable, but it can be an instance variable on a class if it is declared in that context.
In this case, @count is a class instance variable. In other words, by writing @count inside of a class level method you are assigning a variable to that class.
By writing @count inside of a instance method, you are assigning an instance variable that is available in that particular instance only.
If you declare a variable with @@count you get a class variable.
THe main difference between class variables and class instance variables is that class variables are retained in inheritance.
class Foo
@@klass = "class level variable"
@klass_instance = "class instance level variable"
end
class Bar < Foo
end
puts Foo.instance_variables.inspect # => [:@klass_instance]
puts Foo.class_variables.inspect # => [:@@klass]
puts Foo.instance_variable_get(:@klass_instance)
# => "klass instance level variable"
puts Foo.class_variable_get(:@@klass)
# => "class level variable"
# The class variable is inherited, but the class instance variable is not
puts Bar.instance_variables.inspect # => []
puts Bar.class_variables.inspect # => [:@@klass]
# The @@klass variable is shared between all classes in the downward inheritance chain
# So for example:
Foo.class_variable_set(:@@klass, "foo")
puts Bar.class_variable_get(:@@klass) # => "foo"
@blah is a instance variable
Foo, then the @count instance in Foo doesn't share the same data field with the @count instance in class. Is it right?The reason why this works, is because Classes themselves are instances. So the reason your variable behaves like a class variable, is because it's an instance variable of your class.
This is rather confusing for people new to Ruby, and it's also a question whether this is the right behaviour you're programming there. Normally class instances should be declared with double @.
The person here gives a good detailed explanation: Using Instance Variables in Class Methods - Ruby
