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There are several types of variables in Ruby that differ in that, who owns them - to an object, a class, or all descendants at once. Let's deal with examples simply.
@ - instance variable
This is a normal variable that belongs to a specific object. Everybody instance of the class (object) has its own copy of such a variable.
class User
def initialize(name)
@name = name
end
def name
@name
end
end
u1 = User.new("Oleh")
u2 = User.new("Ira")
u1.name # => "Oleh"
u2.name # => "Ira"
Each user has their own name. @name does not share between objects - that's their personal data.
@@ - class variable
Variable with two dogs (@@) common between all instances and subclasses. It lives inside the entire class hierarchy.
class Animal
@@count = 0
def initialize
@@count += 1
end
def self.count
@@count
end
end
class Dog < Animal; end
a1 = Animal.new
d1 = Dog.new
Animal.count # => 2
Dog.count # => 2
Both Animal and Dog see the same variable @@count. If a subclass changes it, it affects the parent class. Because of this, @@ is considered a dangerous and outdated practice - it is better to avoid its use.
Class instance variable - a safe alternative to @@
You can create a variable at class level, but that it should belong only to this class, not to the heirs. This is a normal @variable, but declared at the level of the class itself (and not inside initialize).
class User
@count = 0
def self.add_user
@count += 1
end
def self.count
@count
end
end
class Admin < User; end
User.add_user
User.count # => 1
Admin.count # => nil
In this example, User has its own @count and Admin has a separate one (nil first until it is defined). This makes the code safe and predictable.
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