Ruby Language Classes Class and instance variables
Example
There are several special variable types that a class can use for more easily sharing data.
Instance variables, preceded by @. They are useful if you want to use the same variable in different methods.
class Person
def initialize(name, age)
my_age = age # local variable, will be destroyed at end of constructor
@name = name # instance variable, is only destroyed when the object is
end
def some_method
puts "My name is #{@name}." # we can use @name with no problem
end
def another_method
puts "My age is #{my_age}." # this will not work!
end
end
mhmd = Person.new("Mark", 23)
mhmd.some_method #=> My name is Mark.
mhmd.another_method #=> throws an error
Class variable, preceded by @@. They contain the same values across all instances of a class.
class Person
@@persons_created = 0 # class variable, available to all objects of this class
def initialize(name)
@name = name
# modification of class variable persists across all objects of this class
@@persons_created += 1
end
def how_many_persons
puts "persons created so far: #{@@persons_created}"
end
end
mark = Person.new("Mark")
mark.how_many_persons #=> persons created so far: 1
helen = Person.new("Helen")
mark.how_many_persons #=> persons created so far: 2
helen.how_many_persons #=> persons created so far: 2
# you could either ask mark or helen
Global Variables, preceded by $. These are available anywhere to the program, so make sure to use them wisely.
$total_animals = 0
class Cat
def initialize
$total_animals += 1
end
end
class Dog
def initialize
$total_animals += 1
end
end
bob = Cat.new()
puts $total_animals #=> 1
fred = Dog.new()
puts $total_animals #=> 2
