From: Robert Klemme <bob.news@...>
Date: 2004-03-30T21:54:25+09:00
Subject: Re: Attr_constructor


"Daniel Sheppard" <daniels@pronto.com.au> schrieb im Newsbeitrag
news:EE0A2C5DE8D7CA4792891604D4FED52E016749EB@mlbmail.pronto.com.au...
> I've been writing a couple of very struct-like classes, and got annoyed
> at having to write:
>
> class MyStructLikeThing
>   attr_accessor :one, :two, :three, :four, :five
>   def initialize (one, two, three, four, five)
>     @one, @two, @three, @four, @five = one, two, three, four, five
>   end
> end
>
> And how ugly that looked in the code, so I decided to try my hand at
> whipping up an attr_constructor statement.
>
> class Module
>   private
>   def attr_constructor(*args)
>     self.class_eval %Q{
>       def initialize(#{args.join(', ')})
>         #{args.collect{ |setting|
> "instance_variable_set(\"@#{setting}\", #{setting})" }}
>       end
>     }
>   end
> end
>
> class MyStructLikeThing
>   attr_accessor :one, :two, :three, :four, :five
>   attr_constructor :one, :two, :three, :four, :five
> end
>
> It strikes me as a little cludgy. Is there a better way of doing this?

Well, you could at least save the attr_accessor line for those fields
contained in the constructor:

class Module
  private
  def attr_constructor(*args)
    self.class_eval %Q{
      def initialize(#{args.join(', ')})
        #{args.collect{ |setting| "@#{setting} = #{setting}\n" }}
      end
    }

    args.each do |sym|
      attr_accessor sym
    end
  end
end

class MyStructLikeThing
  attr_constructor :one, :two, :three, :four, :five
end

mst = MyStructLikeThing.new 1,2,3,4,5

> Is there a way to get the args variable so that it can be read from
> within a module_eval block?
>
> (Note also, that I know about the Struct module, this is just a
> simplified example)

Why don't you use Struct then?  You can add arbitrary methods after
creating the struct like this:

MyStruct = Struct.new( :one, :two, :three, :four, :five )

class MyStruct
  def test
    p "test"
  end
end

Regards

    robert