From: Bira <u.alberton@...>
Date: 2006-05-07T04:29:07+09:00
Subject: Re: Creating sub-classes using Struct

On 5/6/06, Eli Bendersky <eliben@gmail.com> wrote:
>
> I have a flat-file full of records. The way records are stored can be
> customized via a separate configuration file. I want to have a class
> Database which is a container of Record classes. The Record is best
> implemented as a Struct, IMHO, and I want to create the class
> dynamically after reading the configuration (which says which fields
> there are in a Record). I think it would be convenient for a Record to
> be a subclass of Database, since there is no point having Records
> without a Database.

I think that what you mean by "subclass" is something different than
the traditional meaning of the term.

Subclassing establishes a "is a" relation between the two classes. To
do it in Ruby you'd write:

class Database
  #code goes here
end

class Record < Database
  #more code goes here
end

I don't think this is completely correct, however, since a Record is
not a Database. The relationship you want to establish is "contains":
a Database contains Records. To do this, you'd have to declare a
collection of Records as an attribute of Database, doing something
like this:

class Database
  def initialize()
    @records = []
  end

  def add_record(record)
    @records << record
  end

  #et cetera

end

Defining Record within Database, as in your first example, doesn't
necessarily do any of those things, and is more commonly known as
creating a "nested class".

If you want to define Record dinamically, you could maybe do something
like this:

class Record
  def initialize(*attribs)
    @internal_class = Struct.new(*attribs)
    @internal_object = @internal_class.new
  end

  def method_missing(name, *args)
    if @internal_object.respond_to? name
      @internal_object.send(name, *args)
    else super(name, *args)
    end
  end

  def inspect
    @internal_object.inspect
  end

end

This way, everytime you create a new Record object, you'll need to
define what attributes it has (and these attributes would probably
come from you configuration files). You can place the code for the
Record class inside the Database class to make it a nested class, and
you can create an attribute for Database that is an Array or Hash of
Records.



--
Bira
http://compexplicita.blogspot.com
http://sinfoniaferida.blogspot.com