From: Jean-Hugues ROBERT <jean_hugues_robert@...>
Date: 2002-05-04T23:12:33+09:00
Subject: Re: dynamically assigning instance variables

At 20:35 04/05/2002 +0900, David Alan Black wrote:
>Hello --
>
>On Sat, 4 May 2002, Jean-Hugues ROBERT wrote:
>
> > Question: What the preferred idiom for creating objects in Ruby ?
> > The C++ idioms ('new' & overloading) seems rather unpractical to me, due
> > to the inability to overload new (or any other method, for that matter)
> > based on the parameters.
>
>I'm not sure what you mean.  Isn't new() the usual way to create
>objects?
>
>
>David

Let me clarify. The way you create objects using MyClass.new() is similar 
to the way you do it in C++. With one major limitation: There can be only 
one initialize() method. Whereas in C++ you can have multiple ones. The 
difference being what type/number of parameters each one gets (in C++ the 
"initialize()" method is not called "initialize()", it is called after the 
name of the class).

So, in C++, based on what parameters you create a new object, you define 
multiple "constructors".

The way to do it in Ruby is not natural/direct. Because you have only one 
initialize() method, you need to determine yourself what the caller 
intention was, based on the supplied parameters.

def initialize( param = nil )
   case param
   when nil
     # no parameter constructor
     ...
   when self.class
     # Copy constructor
   when xxx
     # Whatever other constructor
end

Some other languages will create objects based on prototypes:

def initialize()
   proto = @@proto
   proto.instance_variables.each do | var |
     instance_eval( "#{var} = #{proto.instance_eval( #{var})}")
   end
end

Some other languages will have no "constructor" or "initialize()" specific, 
language defined, construct:
def MyClass.create()
   obj = Object.new()
   obj.class = MyClass  # Not ruby: Smalltalk's style .becomes() missing
   obj.body = { :@myVar1 => "Hello", :@myVar2 => "world" } # Not ruby
end


Some other languages just create objects, without having the method that 
create objects beeing named in any specific way:

def MyClass.createFromInt( ii )
   obj = MyClass.new()
   obj.foo = ii
   return obj
end
def doubleIt()
   obj = MyClass.new()
   obj.foo = self.foo * 2
end

.. an so on ..

The basic OO equation is: an_object is a_id + a_value + a_behaviour.

There are many idioms to create objects. Appararently the Ruby one is to 
use SomeClass.new( some_param) that leads to: A) the creation of the object 
ii with no initial value and the behaviour of its class, followed by B) a 
call to ii.initialize( some_param).

Hope this is clearer.

Yours,

Jean-Hugues


-------------------------------------------------------------------------
Web:  http://hdl.handle.net/1030.37/1.1
Phone: +33 (0) 4 92 27 74 17