From: Luke Kanies <luke@...>
Date: 2006-12-20T06:36:35+09:00
Subject: Re: class_eval and using a block instead of a String?

On Dec 19, 2006, at 2:47 PM, Mariano Kamp wrote:

> Hi,
>
>   given the following contrived example I am wondering how to use a  
> block with class_eval instead of using the string like in the code  
> below?!
>
>   Any ideas?
>
> Cheers,
> Mariano
>
> require 'test/unit'
> include Test::Unit::Assertions
>
> class Base
>   class << self
>     def enhance(*attributes)
>       class_eval do
>         attr_reader *attributes
>       end
>
>       initializer = "def initialize(#{attributes.join(", ")})"
>       attributes.each {|a| initializer << "@#{a}=#{a}\n"}
>       initializer << "end"
>
>      # ------
>      class_eval initializer # <-------
>      # ------
>     end
>   end
> end
> class Derived < Base; end
>
> Derived.enhance(:a, :b, :c)
> d = Derived.new("a", "b", "c")
>
> assert_equal "a", d.a
> assert_equal "b", d.b
> assert_equal "c", d.c

Something like thie following should work:

     require 'test/unit'
     include Test::Unit::Assertions

     class Base
         class << self
             def enhance(*attributes)
                     attr_accessor *attributes

                     define_method(:initialize) do |*ary|
                         attributes.each do |a|
                             send(a.to_s + "=", ary.shift)
                         end
                     end
             end
         end
     end
     class Derived < Base; end

     class Testing < Test::Unit::TestCase
         def test_initialize
             Derived.enhance(:a, :b, :c)
             d = Derived.new("a", "b", "c")

             assert_equal "a", d.a
             assert_equal "b", d.b
             assert_equal "c", d.c
         end
     end

I didn't really do what you want, because I'm not actually using  
class_eval here at all, but the assertions pass, anyway, which I  
assume is the real goal.

Note that I'm using attr_accessor here instead of attr_reader,  
because it's much easier to set the attributes this way.  You could  
just about as easily use instance_variable_set or whatever the method  
is, but I prefer this for my own code.

Note also that if you call enhance() multiple times on the same  
class, you'll recreate the initialize() method each time, which is  
probaby a bad thing; as long as this is just for testing, you should  
be fine, but I'd be wary of putting this into production anywhere.

One of the things I love about ruby is I can usually use a block  
instead of a string with the eval methods, but what I love even more  
is being able to skip the eval methods entirely.

  --
  Some people are afraid of heights.  I'm afraid of widths.
                  -- Stephen Wright
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  Luke Kanies | http://reductivelabs.com | http://madstop.com