From: dblack@...
Date: 2003-01-10T07:14:24+09:00
Subject: Re: attr_reader/writer vs. attr_accessor w/ access control

Hi --

On Fri, 10 Jan 2003, Tom Sawyer wrote:

> it strikes me as curious....
>
> i've spent quite a bit of time wrestling with some special attr_* module
> methods for use with CGI and i was quite concerned with honoring the full
> bredth of reader, writer, and accessor even though it meant more work; that
> is to say if i just used accessor it would be a lot easier. and that's when
> it hit me: if i don't want the external world reading and/or writing to my
> object's instance variables, why wouldn't i just set the access control to
> private (or protected)?
>
> class AttrDemo
> 	attr_accessor :readme
> 	protected :readme=
> end
>
> granted this adds in a touch more code, but that's not the point i'm making.
> i'm only pointing out that in effect this serves the same purpose as:
>
> class AttrDemo
> 	attr_reader :readme
> end
>
> the only differenece being that in the first example i have a setter method
> available to the class itself. fine with me.
>
> except for the fact that instance variables can spring into existence
> dynamically, i wonder why they aren't simply attr_accessor by default and
> access control the sole means by which we make them visible or invisible to
> the outside.

This would mean, for one thing, that every object would respond to
every message; there would no longer be a distinction between those
messages an object understood and those it didn't.  If you said:

  s = "abc"
  s.keys

then you'd get the value of the @keys instance variable of s, instead
of an indication that s doesn't respond to #keys.  And so forth....
Rather more i.v.'s in the landscape than one wants :-)

> so, in realizing this, i modified my attr_* methods to return their effected
> methods so that i could pass them on to an access control method.
> unfortuantely the access control methods will not take arrays. :-( it would
> be nice if they used #flatten so they could.

You can use the unary * to un-array your arguments (hey, I never
noticed the similarity of 'unary' and 'unarray' :-)

  class A
    def a; end
    def b; end
    c = [:a, :b]
    private(*c)

  end

  A.new.b      # error: private method 'b' called


David

-- 
David Alan Black
home: dblack@candle.superlink.net
work: blackdav@shu.edu
Web:  http://pirate.shu.edu/~blackdav