From: Mark Probert <probertm@...>
Date: 2002-02-23T04:52:14+09:00
Subject: Re: dispatching and class references



At 04:24 AM 2/23/2002 +0900, Paul B. wrote:
> >
> > It seems that ruby returns object values by reference.
> > Is there any way to return the value?  I would like the
> > pattern, stored in Sym::val to be treated as a template
> > and retain its value ('xxx') over each dispatch call.
>
>Returning by value is almost the same as returning a reference to a
>copy.
>
>To make a smallow copy:
>   obj.dup
>
>To make a deep copy:
>   Marshal::load(Marshall::dump(obj))
>
>Returning copies will have a performance impact.  If you want to return
>a reference but still not allow the user to change the returned object,
>you can return a frozen object.

Now I -am- confused! :-)

I have added 'freeze' to the Sym.new() call, and ruby says the object
is frozen.  However, I can still modify the returned object val. The
same happens if I .dup() the dispatch method.

The manual states: "[freeze] Prevents further modifications to obj.
A TypeError will be raised if modification is attempted."  I don't
really know what is going here -- I would have thought that freeze
would stop modifications to the object, or modification to the contents
of the object.  I guess that it doesn't!


     class Sym
         attr_reader :sym, :val
         def initialize(sym, val)
             @sym = sym
             @val = val
         end
         def to_s
             "sym = #{@sym} val=#{@val}"
         end
     end

     class Bob

         @@dtbl = {
             'a' => Sym.new(:do_a, 'this is a string: xxx').freeze,
             'b' => Sym.new(:do_b, 'so is this: xxx').freeze
         }

         def do_a(msg=nil)
             puts "a --> #{msg}"
         end

         def do_b(msg=nil)
             puts "b --> #{msg}"
         end

         def dispatch(ref=nil)
             if @@dtbl[ref]
                 obj = @@dtbl[ref]
                 s = obj.val
                 s.sub!(/xxx/, '123')
                 send obj.sym, obj.val
             else
                 puts "--> unknown ref (#{ref})"
             end
         end

         def prntbl
             @@dtbl.keys.each do
                |k| puts @@dtbl[k]
                puts " --> frozen" if @@dtbl[k].frozen?
             end
         end
     end

     irb(main):001:0> b = Bob.new
     #<Bob:0xa0b8a38>

     irb(main):002:0> b.prntbl
     sym = do_a val=this is a string: xxx
      --> frozen
     sym = do_b val=so is this: xxx
      --> frozen
     ["a", "b"]

     irb(main):003:0> b.dispatch('a')
     a --> this is a string: 123
     nil

     irb(main):004:0> b.prntbl
     sym = do_a val=this is a string: 123
      --> frozen
     sym = do_b val=so is this: xxx
      --> frozen
     ["a", "b"]






>Paul


-mark

-----
Mark Probert
GNPS - Optera Metro 3000
Tel (613) 768-1082 (ESN: 398-1082)
Mob (613) 715-2045
email: probertm@nortelnetworks.com