From: Robert Klemme <bob.news@...>
Date: 2005-01-05T01:11:39+09:00
Subject: Re: Composition or Module


"Tomoyuki Kosimizu" <greentea@fa2.so-net.ne.jp> schrieb im Newsbeitrag 
news:20050104.233447.97297001.greentea@fa2.so-net.ne.jp...
> Hi,
>
> Let me hear your opinions, composition or module.
>
> Composition:
>
>  class Representation
>    def to_string(obj)
>      return sprintf('%s=%s', obj.class, obj.value)
>    end
>  end
>
>  class Domain
>    def initialize
>      @value = 10
>    end
>    attr_reader :value
>
>    def to_s
>      Representation.new.to_string(self)

Not efficient because you create a stateless instance every time you need 
the conversion to string.  You could make Representation a singleton though.

>    end
>  end
>
>  puts(Domain.new)
>
> or Module:
>
>  module Representation
>    def to_s
>      return sprintf('%s=%s', self.class, @value)

Rather do this as it is more flexible:

     return sprintf('%s=%s', self.class, self.value)

Also, you might want to add

   def value() nil end

to avoid errors for classes that don't implement "value()".

>    end
>  end
>
>  class Domain
>    include Representation
>
>    def initialize
>      @value = 10
>    end
>  end
>
>  puts(Domain.new)

Why do you want to separate this if you override Domain#to_s (i.e. a 
standard method) anyway?

You could as well

 - put the implementation into Domain#to_s and leave out Representation

 - Make the conversion an instance method of the module so you don't need to 
create new instances of the module all the time (for composition).

 - Take the second approach (i.e. the one with the mixin) but make 
Representation a base class.

Maybe there are other options but that depends on your design goal.  If you 
tell a bit more maybe we can come up with a more appropriate solution.

Kind regards

    robert