How can I add new attributes to a particular instance?

Question

How can I add new attributes to a particular instance ?

For example here I want to add attr_acessors methods to attributes "m1","m2" in object e1 and "m4".."m6" to e2

e1=Element.new("e1",["m1","m2"])
e2=Element.new("e2",["m4","m5","m6"])
e1.m1 = "try"
puts e2.m4

To allow this :

class Element
  attr_accessor :name
  def initialize name,meths=[]
    @name=name
    meths.each do |m|
      #?? 
    end
  end
end

Gregory Mostizky · Answer 1 · 2011-03-10T16:04:17.887

2

Why not use a simple OpenStruct instead?

require 'ostruct'
e1 = OpenStruct.new
e1.m1 = 'try'

Alternatively, you can add attribute to any object using:

a.instance_eval('def m2; @m2; end; def m2=(x); @m2=x;end')

If you want to add attributes to all instances of specific class you can also:

a.class.instance_eval('attr_accessor :mmm')

edited Mar 10 '11 at 16:04

answered Mar 10 '11 at 15:54

Gregory Mostizky

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ostuct force me to modify my object inheritance. Thx anyway ! – JCLL Mar 10 '11 at 21:22

score 1 · Accepted Answer · answered Mar 10 '11 at 16:19

1

Try this:

meths.each do |m|
  singleton_class().class_eval do
    attr_accessor m
  end
end

where the singleton_class() method is defined as:

def singleton_class
  class << self
    self
  end
end

(you probably want to make it private).

This will create the accessors only on the specific instance rather than on the Element class.

answered Mar 10 '11 at 16:19

Jonathan

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Thx Jonathan. That is my favorite solution ! I was struggling with something similar this afternoon. – JCLL Mar 10 '11 at 16:27

score 1 · Answer 3 · answered Mar 12 '11 at 20:04

Here is a simpler solution:

methods.each do |method|
 class << self
  attr_accessor method
 end
end

This way, you get rid of the extra method definition and class_eval because class << self already puts you into the scope of the eigenclass, where you add singleton methods.

How can I add new attributes to a particular instance?

3 Answers3