From: Geoff Youngs <g@...>
Date: 2003-08-01T17:33:02+09:00
Subject: Re: Extracting a parent class

On Fri, Aug 01, 2003 at 07:12:19AM +0900, Daniel Carrera wrote:
> Oh, sorry.  In that case I don't imagine it's possible by default
> because in general there might not be an "obvious" way to turn class B
> into class A.

> You should make a B#to_A method.

Unless you were to use the Struct class instead of a normal class, in
which case you can tell what should stay and what should go.

e.g.

class Struct
	def cast_as(klass)
		raise "Not a Class" unless klass.kind_of?(Class)
		raise "Not a Struct" unless klass.ancestors.include?(Struct)
		obj = klass.new()
		members.each do |name|
			obj[name] = self[name] if obj.members.include?(name)
		end
		obj
	end
end

StructA = Struct.new('StructA',:alpha, :beta)
StructB = Struct.new('StructB',:alpha, :beta, :gamma)
foo = Struct::StructB.new(1,2,3)
bar = foo.cast_as(Struct::StructA)
p foo,bar

With a bit more effort you could try a reverse castable object - ie.
one which would preserve members unused by the target class and copy all
instance variables, but I don't think that that is what is required
here.

HTH,


Geoff