Object Oriented Programming

Class and Instance Variables in Ruby

Used declare variables within a class. There are two main types: class variables, which have the same value across all class instances (i.e. static variables), and instance variables, which have different values for each object instance.


Syntax
$variableName #global scope
@@classVariable #class scope, static across all class instances
@instanceVariable #class object scope, value specific to object instance

#class variable getter, called on the class itself (hence self keyword)
def self.getClassVar
    @@classVariable
end

#Ruby has shortcuts for instance variable getters and setters

#both getter and setters
attr_accessor :instanceVariable
#getter
attr_reader :instanceVariable
#setter
attr_writer :instanceVariable

Notes

Class and instance variables are private , so getters and setters are necessary to access the variables outside the class.

Instance variables are not inherited.


Example
class Car
    @make
    @@wheels = 4

    def initialize(make)
        @make = make
    end

    def self.wheels
        @@wheels
    end
    attr_accessor :make
end

#outside the class
civic = Car.new("Honda")

#instance variable, called on the object
>>civic.make
=>"Honda"
#class variable, called on the class itself
>>Car.wheels
=>4


See Also
Related

Variable Declaration
Methods

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