From: Ryan Davis <ryand-ruby@...>
Date: 2006-10-17T14:37:24+09:00
Subject: Re: dynamically assigning instance variables (redux)


On Oct 16, 2006, at 8:45 PM, Phrogz wrote:

> Bil Kleb wrote:
>>   Is there a way to dynamically assign instance variables?
> [...snip...]
>> Actually, I'd like the assignment and make them attr_readers too.
>
> class Foo
>   def initialize( hash={} )
>     hash.each{ |key,value|
>       var_name = "@#{key}"
>       instance_variable_set( var_name, value )
>       singleton_class = class << self; self; end
>       singleton_class.class_eval{
>         define_method( key ){
>           instance_variable_get( var_name )
>         }
>         define_method( "#{key}=" ){ |new_value|
>           instance_variable_set( var_name, new_value )
>         }
>       }
>     }
>   end
> end

I find this much too complex with no real need for using  
singleton_classes (ever, really). Try:

class Foo
   def initialize( hash={} )
     hash.each do |ivar, val|
       self.class.send :attr_accessor, ivar unless respond_to? ivar
       send "#{ivar}=", val
     end
   end
end

The primary benefit of writing code like this is that you can know  
what it does in half a glance with no extra thinking. I should also  
point out that calling a define_method method is about 2x slower than  
an attr_* method. (not a big deal in this case but take a look at  
rails' primary_key and think about that impact)

> f = Foo.new :name=>'whee', :age=>12
> p f, f.age
> #=> #<Foo:0x32b900 @age=12, @name="whee">
> #=> 12
> f.age=32
> p f
> #=> #<Foo:0x32b900 @age=32, @name="whee">