An instance variable is a variable that exists as long as the object instance it is scoped to exists. We use instance variable to tie data to objects.
Note however that will they are initialized in a constructor, instance variables do not disapear after the constructor is run. They are scoped to the object instance and lives on with it.
In Ruby, instance variable are written with an at sign (
@) inside the
initialize method definition (the constructor):
class Flamingo def initialize @color = 'pink' end end chilean_flamingo = Flamingo.new
Above we saw that instance variable capture information related to the specific instance of classes. Class variables are to classes what instance variables are to instance. Class variables are used to keep track of informations that relates to the class and not the instances. One example would be to track the number of time a new object is instantiated.
class Flamingo @@flock = 0 def initialize @@flock += 1 end def self.number_of_flamingos @@flock end end puts Flamingo.number_of_flamingos # => 0 chilean_flamingo = Flamingo.new greater_flamingo = Flamingo.new puts Flamingo.number_of_flamingos # => 2
Constants are variables that never change. In Ruby, the first letter should be capitalized, although most Rubyists uppercase the whole word.
CORONATION_OF_NAPOELON_I = 1804