Re: Interface/Implementation (was Re: Design by Contract)

[Nick Leaton <nickle@calfp.co.uk>]

John G. Volan wrote:

> Eiffel doesn't care about the order in which things are declared, but it
> also lacks this notion of elaboration order.  So how does it handle
> intializing classwide globals?  I seem to recall something about "once"
> methods -- routines that get executed just once, the first time they are
> called.  The results are cached, and all subsequent calls just return
> the cached results.  Supposedly you can initialize classwide globals
> inside such methods.  Do I have that right?  If so, it seems to me that
> Ada's idiom for this is more straighforward than Eiffel's.  (I'm not
> saying it's better, just more straightforward for this particular
> application.)

One quick explaination first.

	x: MY_CLASS

This does no allocation

	!!x

or	!OTHER_CLASS!x

or	!!x.make -- or some other feature declared as a creation feature

The !! creates a object and allocates the memory. Prior to this, x is
Void.
The second case is interesting, OTHER_CLASS must conform to MY_CLASS.

OK, back to global objects.

	x: MY_CLASS is
	   once
	      !!Result
	   end

The first time I use x, Result will be made. The second time I use x,
the Result from the previous use will be returned. Think of it as a just
in time singleton.

So the order of creation is the order of use.

Is this a problem where you want some objects initialised first?

No, because you will want the objects initialised first in order to use
them, but the use will create them. ie a DATABASE object. You can use
this if it is a once feature anywhere, first use will create so you know
it is created. If it needs something else to be create first, the client
of DATABASE shouldn't be worried, DATABASE itself should create the
appropiate objects, probably using a once feature.

Are there any issues with once features?

Yes, they never get 'deleted', once created they exist for the lifetime
of the program. They could be large structures only used during the
initialisation of the program, and therefore with never be garbage
collected. However with a generational garbage collector, they will be
compacted. (Once features a roots as far as GC is concerned)

There is a solution to this problem if needed, just add a layer of
indirection.

-- 

Nick

Eiffel - Possibly the best language in the world - unless proven
otherwise.