From: Robert Klemme <bob.news@...>
Date: 2005-09-27T00:51:43+09:00
Subject: Re: class_eval

Derek Chesterfield wrote:
> Stealing someone else's recent example code, what is the difference
> between this:
>
>> class A
>>      def meth2
>>           puts "I'm meth2. I try to access something I ain't
>> allowed: #{static_var}"
>>      end
>>      static_var = 3
>>      define_method(:meth3) do
>>           puts "I'm meth3. I'm allowed to access #{static_var}"
>>      end
>> end
>> a = A.new
>> a.meth3
>> a.meth2
>
> and this, which seems to do the same thing, but without the
> class_eval:
>
>> class B
>>      def meth2
>>           puts "I'm meth2. I try to access something I ain't
>> allowed: #{static_var}"
>>      end
>>      class_eval do
>>           static_var = 3
>>           define_method(:meth3) do
>>                puts "I'm meth3. I'm allowed to access #{static_var}"
>>           end
>>      end
>> end
>> b = B.new
>> b.meth3
>> b.meth2
>
> In other words, what does class_eval do that I'm not understanding?!

In this case it's not really interesting to use class_eval.  It's more
interesting for cases like these

>> class Foo;end
=> nil
>> Foo.class_eval do
?> def bar() end
>> end
=> nil
>> Foo.new.bar
=> nil
>> Foo.class_eval "def bar() \"bar\" end"
=> nil
>> Foo.new.bar
=> "bar"
>> x=10
=> 10
>> Foo.class_eval do
?>   define_method(:baz) { puts x }
>> end
=> #<Proc:0x1016a660@(irb):14>
>> Foo.new.baz
10
=> nil

And note the difference to instance_eval:

>> Foo.instance_eval do
?> def ban() "ban" end
>> end
=> nil
>> Foo.new.ban
NoMethodError: undefined method `ban' for #<Foo:0x100c3d58>
        from (irb):20
>> Foo.ban
=> "ban">> Foo.instance_eval "def borg() 'org' end"
=> nil
>> Foo.new.borg
NoMethodError: undefined method `borg' for #<Foo:0x101b4e50>
        from (irb):23
        from :0
>> Foo.borg
=> "org"

HTH

Kind regards

    robert