Re: Inherited Methods and such

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

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

> On Fri, 28 Sep 2007 13:44:32 -0700, Maciej Sobczak wrote:
>
>> On 28 Wrz, 21:42, Robert A Duff <bobd...@shell01.TheWorld.com> wrote:
>> 
>>> Here's an example.  What do you think?
>> 
>> Perfect! I wasn't aware of the possibility to use return to achieve
>> this "progressive" construction process.
>
> It is not return, the job is done by extension aggregates, these are Ada 95
> by the way. C++'s
>
>    Derived () : Base (), Component () {}
>
> is equivalent to Ada's
>
>    Derived'(Base with Component);
>
> What Ada 2005 return adds, is a non-empty body of, i.e. the stuff between
> {...} is do ... end return;

And a name for the result.

> Then Ada 2005 allowed aggregates for limited types.

Yes, aggregates and functions.

>> I knew this form of return, but not the gory details of how tags are
>> assigned.
>
> The tag is set to reflect the object's type, what did you expect? (:-)) But
> note, nothing is mutating in the example.

At the semantic level, that's right.  Internally, the Tag of the parent
is stored at the same memory location as the Tag of the child object.
So at the implementation level, the Tag is in fact changing.

>>> Do you revise your opinion that
>>> Ada doesn't properly support constructors?
>> 
>> Yes, this example is convincing.
>
> No, it is not. Because aggregate is not a replacement for user-defined
> constructors.
>
> 1. It breaks encapsulation as you should make the record nature of type
> public.

I don't understand this objection.  In my example, all types are
private (or "new ... with private").

> 2. The base type cannot enforce vital construction code, when records are
> exposed.
>
> 3. Nothing can be done for destructors.
>
> 4. Dispatch upon construction completion and before destruction beginning
> is still impossible.

My example dispatched at the end of each "return".
It's not foolproof, but it seems to get the job done.

- Bob