Question

How can I add an instance variable to a defined class at runtime, and later get and set its value from outside of the class?

I’m looking for a metaprogramming solution that allows me to modify the class instance at runtime instead of modifying the source code that originally defined the class. A few of the solutions explain how to declare instance variables in the class definitions, but that is not what I am asking about.

Answer 1

You can use attribute accessors:

class Array   attr_accessor :var end 

Now you can access it via:

array = [] array.var = 123 puts array.var 

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Note that you can also use attr_reader or attr_writer to define just getters or setters or you can define them manually as such:

class Array   attr_reader :getter_only_method   attr_writer :setter_only_method    # Manual definitions equivalent to using attr_reader/writer/accessor   def var     @var   end    def var=(value)     @var = value   end end 

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You can also use singleton methods if you just want it defined on a single instance:

array = []  def array.var   @var end  def array.var=(value)   @var = value end  array.var = 123 puts array.var 

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FYI, in response to the comment on this answer, the singleton method works fine, and the following is proof:

irb(main):001:0> class A irb(main):002:1>   attr_accessor :b irb(main):003:1> end => nil irb(main):004:0> a = A.new => #<A:0x7fbb4b0efe58> irb(main):005:0> a.b = 1 => 1 irb(main):006:0> a.b => 1 irb(main):007:0> def a.setit=(value) irb(main):008:1>   @b = value irb(main):009:1> end => nil irb(main):010:0> a.setit = 2 => 2 irb(main):011:0> a.b => 2 irb(main):012:0>  

As you can see, the singleton method setit will set the same field, @b, as the one defined using the attr_accessor… so a singleton method is a perfectly valid approach to this question.