Re: Ada OS based on Minix3

[Robert A Duff <bobduff@shell01.TheWorld.com>]

"Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> writes:

> On Tue, 11 Nov 2008 17:09:54 -0500, Robert A Duff wrote:
>> Initialization: Use function calls.  They work for all types.
>> I think we've discussed this idea before, but I don't remember
>> any fundamental problems.
>
> The fundamental problem is that a constructor cannot be decomposed into
> functions. In the function body you have to construct the object before you
> return it. Who does it and what?

In Ada 2005, a constructor function creates the object via another
function call, or via an aggregate, or via an uninitialized
variable (to which it assigns components).

> Under initialization I do not mean replacing a "default" value with
> something else. I mean creation a new object (at some memory location)
> initialized by a value. It had no other value before.

That's exactly what a function call does:

    X : T := F(...);

F creates a new object.  In the limited case, that object _is_ X.
In the nonlimited case, it gets copied into X.  Either way,
X has been turned from "raw bits" into an object of type T --
as you say, "it had no other value before."

> Doing this by a function is semantically wrong.

I agree with a lot of the rest of what you wrote,
which I snipped.

But I'm interested in discussing this particular point.
It seems to me the whole point of functions is to create
new objects.  Why is this "semantically wrong"?

I understand that there are tricky issues when constructing an
object of a specific type and then returning it as a class-wide
thing, and possibly storing pointer-to-classwide into global
data structures.  But I don't get what's fundamentally wrong
with using functions as constructors.

- Bob