Access the attributes of a class instance from within the same class definition in Ruby

Question

Suppose that I have a class "Facility" and I have two instances 'Myhome' and 'Central'. How can I access/modify the attributes of the instance 'Central', whenever an instance of the class is created. Example:

class Facility  
    attr_accessor :name, :address  

    def initialize(name,adrs)
        @name = name
        @address = adrs
    end

    def some_process
        puts "My address #{@address}"
        Central.@address = @address + "something" 
        # I know that this is incorrect usage but 
        # this is the operation that I would like to perform
    end
end
Myhome = Facility.new("Fosters", "B-12 Kensington Farms, Dublin")
Central = Facility.new("Directory", "Not Important")
Myhome.some_process
puts Central.address # This should display the address + "something"  

My objective is to access the attribute 'address' of the instance 'Central' for every other instance of the class Facility, without creating any new classes.

Answer 1

Passing the instance, as Andrey stated, is the recommended way to do it. But another option would be to create a class variable (you do that with two at signs like so: @@class_variable_name ). That would be updated across all instances of the class (kind of like a global variable).

Answer 2

There are two approaches that could be taken here:

• save an instance within a class instance variable
• save an instance outside the class and pass it as an argument to the class' instance methods.

Since a particular instance must be saved somewhere, it makes sense to store it within the class (option #1 above). That both confines its scope to the class and simplifies the code.

@baron816 mentions the same approach except he/she advocates the use of a class variable. The problem with class variables is that they are accessible in subclasses. That may not be a problem here, but there are no advantages to using a class variable instead of a class instance variable.

Here's how this approach might be implemented.

class Facility  
  attr_accessor :name, :address  

  def initialize(name, address)
    @name = name
    @address = address
  end

  def saved_instance_set
    self.class.instance_variable_set(:@saved_instance, self)
  end

  def saved_instance_get
    self.class.instance_variable_get(:@saved_instance)
  end

  def addresses
    puts "Address for #{ name } is #{ address }"
    puts "Saved instance address is #{ saved_instance_get.address }"
  end
end

This class might be used as follows.

my_home = Facility.new("Fosters", "B-12 Kensington Farms, Dublin")
  #=> #<Facility:0x...8 @name="Fosters", @address="B-12 Kensington Farms, Dublin">

Now make this the saved instance.

Facility.instance_variables
  #=> [] (confirm there are no class instance variables initially)
my_home.saved_instance_set
  #=> #<Facility:0x...8 @name="Fosters", @address="B-12 Kensington Farms, Dublin"> 
Facility.instance_variables
  #=> [:@saved_instance] (class instance variable has been created)

Now for Bob.

bobs_home = Facility.new("Rikers Island", "Room 253, New York")
  #=> #<Facility:0x...8 @name="Rikers Island", @address="Room 253, New York"> 

Execute addresses for Bob.

bobs_home.addresses

prints

Address for Rikers Island is Room 253, New York
Saved instance address is B-12 Kensington Farms, Dublin