Re: Entity and Identity

On Jul 21, 9:39 pm, Brian <br..._at_selzer-software.com> wrote:
> On Jul 21, 5:04 am, David BL <davi..._at_iinet.net.au> wrote:
>
> > On Jul 21, 1:47 am, Brian <br..._at_selzer-software.com> wrote:
>
> > > First, it doesn't matter if objects can conceal part of their states
> > > provided that the references to those objects can be used to
> > > distinguish between them, and second, if two objects are identical in
> > > state, then they cannot differ in location, for that would constitute
> > > a difference in state.
>
> > When you say 'object' do you mean in the OO sense? Usually the OO
> > community use 'object' to mean an identifiable state machine located
> > at some address and don't regard the location to be part of its state.
> > Furthermore usually the identity of an object is determined *only* by
> > its location and has nothing at all to do with its current state.
>
> I disagree with your use of the terms 'location' and 'identity.' In
> the OO world, objects are instances of reference types. The location
> of an object can change over its lifetime, but what is used to
> reference each object, the object identifier, doesn't.

You're assuming "location" means physical address in memory.

In all OO systems there is a concept of an address space for object references. Using the street address metaphor we might say that the objects are located at these addresses. That is what I meant by "location".

> The identity of an object is determined
> (functionally) by its object identifier but can also be determined by
> its current state in the same way that a relation schema can have more
> than one key.

Agreed, but only assuming "can" is qualified with "sometimes" (not "always").

In your original statement you implied that location was part of an object's state. That was the part I disagreed with. Received on Wed Jul 22 2009 - 05:15:49 CEST