Is there a way to create methods just for the instance of a Ruby class from inside that instance?

Question

Let there be class Example defined as:

class Example   def initialize(test='hey')     self.class.send(:define_method, :say_hello, lambda { test })   end end 

On calling Example.new; Example.new I get a warning: method redefined; discarding old say_hello. This, I conclude, must be because it defines a method in the actual class (which makes sense, from the syntax). And that, of course, would prove disastrous should there be multiple instances of Example with different values in their methods.

Is there a way to create methods just for the instance of a class from inside that instance?

Answer 1

You need to grab a reference to the instance's singleton class, the class that holds all the instance specific stuff, and define the method on it. In ruby 1.8, it looks a little messy. (if you find a cleaner solution let me know!)

Ruby 1.8

class Example   def initialize(test='hey')     singleton = class << self; self end     singleton.send :define_method, :say_hello, lambda { test }   end end 

Ruby 1.9 however, provides a much easier way in.

Ruby 1.9

class Example   def initialize(test='hey')     define_singleton_method :say_hello, lambda { test }   end end