Class and instance variables

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

Found a mistake? Have a question or improvement idea? Let me know.

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