Re: Inherited Methods and such

["Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>]

On Mon, 24 Sep 2007 20:59:53 -0500, Randy Brukardt wrote:

> "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> wrote in message
> news:9ukf2wtqjs0q$.iuijmal4x56b$.dlg@40tude.net...
> ...
>>> Right. Ada doesn't have *any* constructors. :-)
>>
>> Surely it has. The point is that in NO typed language a constructor can be
>> defined solely in the language terms. Constructors have to be generated
>> "per magic." But the language shall provide hooks for user-defined
>> insertions in the generated constructors at the points, where the type
>> contracts are satisfied. Isn't it obvious?
>>
>> Ada keeps on trying to use only magic. Returning limited types, new in Ada
>> 2005, is just another step in this direction.
> 
> Ada has no choice. You agree that "magic" is needed to do the memory
> allocation, layout, etc. for the underlying object. (To say otherwise would
> be to require all objects to be fixed in size.) But that is not divisible
> from the initialization of the value in Ada (and the values of the
> components), because the memory allocation depends on the value of the
> discriminants, and even the components that exist depends on those values.

It can be made divisible:

For each type T we define a plain built-in record type T'Constraints such
that it has all the constraints (discriminants, bounds, tags) of T. Prior
allocation of T a user hook is called to return T'Constrants. It gets the
parameters of the constructor. Then the memory is allocated. After that the
implementation type of T is constructed (the constraints are set, the
components are constructed). Then we proceed to the construction of T. Here
the parent types are constructed first and finally a user hook equivalent
to Initialize is called on T. For class-wide types there is an additional
step where the class-wide user hook is called.

> Note that this is very similar to the reasons that Ada cannot have "real"
> user-defined assignment; it's not possible to divide an assignment into
> components managed separately.

Same as above. Moreover it would be probably possible to make the old LHS
visible in the hook computing the constraints.

>> And also, it is inconsistent with Ada's own stance on types
>> matching by-name rather than by-structure.
> 
> This I don't see at all. Since constructors are named (and I agree that
> Initialize is not a constructor in any sense, its more of an initial
> mutator) and are tied to the return types, they fit well into the Ada type
> model. The best constructors are dispatching, and those surely are
> one-to-one with the types that they construct (Ada 95 required those to be
> overridden for each new type; the Amendment changed that over my strong
> objections, but even so you surely are allowed to override them).

That was a mistake, but overriding is a wrong model for constructors
anyway. Constructors are extended but overridden. Further I don't think
that constructing functions can be called constructors.

We don't need constructors, we need insertions in the compiler-generated
ones.

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de