From: Paul Lutus <nospam@...>
Date: 2006-09-21T05:55:12+09:00
Subject: Re: Message passing between objects

Tod wrote:

/ ...

> To me that looks like two objects communicating directly with one
> another.

It is, and IMHO it should be avoided to the degree possible. Again IMHO, the
best data structure is a pure object-oriented one, in which all the data
objects are related by a class hierarchy, rather than by ad hoc
message-passing.

In the above example, the "registrar" class would possess the data available
for each student and allow by direct access what would otherwise require a
message protocol.

Example:

-------------------------------------

#!/usr/bin/ruby -w

class Registrar

   Names = [ "Tom","Dick","Harry","Moe","Larry","Curly" ]

   Classes = [ { :name => "Home economics", :gpa => 2.0 },
      { :name => "Post-modernism", :gpa => 1.5 },
      { :name => "Literature", :gpa => 3.2 },
      { :name => "Comparative blogging", :gpa => 3.0 },
   { :name => "Quantum physics", :gpa => 3.5 } ]

   def initialize
      @student_list = []
      1000.times do |n|
         @student_list << Student.new(n,Names[n % Names.size],2.0 +
rand(100) / 50.0)
      end
   end

   def get_student_data(n)
      return @student_list[n]
   end

   def handle_application(student,class_data)
      if student.gpa < class_data[:gpa]
         puts "\tNo, forget it, #{student.name}, you aren't smart enough for
the #{class_data[:name]} class."
      else
         student.class_list << class_data
         puts "\t#{student.name}, you have been accepted for the
#{class_data[:name]} class."
      end
   end
end

class Student < Registrar
   attr_reader :name
   attr_reader :id
   attr_reader :gpa
   attr_accessor :class_list

   def initialize(id,name,gpa)
      @name = name
      @id = id
      @gpa = gpa
      @class_list = []
   end

   def list_classes
      list = []
      class_list.each do |item|
         list << item[:name]
      end
      return list.join(",")
   end

   def to_s
      return "ID: #{id} Name: #{name} GPA: #{gpa} Classes: #{list_classes}"
   end

   def apply_for_class(class_data)
      handle_application(self,class_data)
   end
end


registrar = Registrar.new

100.upto(104) do |n|

   student = registrar.get_student_data(n)

   puts "Student #{student.name} applying for classes ..."

   Registrar::Classes.each do |class_data|
      student.apply_for_class(class_data)
   end

   puts "Student data:"
   puts "\t#{student}\n\n"

end

------------------------------------

Try running this program, and examine how it works. It shows very simply how
a class hierarchy can handle communication between objects (and it should
be obvious how one student object could communicate to any other student
object by way of the registrar class).

-- 
Paul Lutus
http://www.arachnoid.com