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From: Robert A Duff <bobduff@TheWorld.com>
Newsgroups: comp.lang.ada
Subject: Re: Ada Annoyances
Date: Thu, 29 Jun 2017 17:12:20 -0400
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Xref: news.eternal-september.org comp.lang.ada:47192
Date: 2017-06-29T17:12:20-04:00
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Niklas Holsti <niklas.holsti@tidorum.invalid> writes:

> There are two reasons why tagged types hamper such analysis:

As Randy pointed out, it is more correct to say that class-wide types do
that.  Tagged types by themselves do not cause these problems.
Tagged types without class-wide types are not super useful,
but they are somewhat useful.

> a) dispatching calls (as you said), where the actual callee is
> determined by run-time values (tags) which are hard to predict by static
> analysis
>
> b) the non-static size of class-wide objects (of type T'Class), which
> means that the compiler and/or the programmer must use dynamic
> allocation (usually heap or secondary stack) for such objects.
...
> For some time, I have had in mind a possible Ada extension to solve
> point (b): an attribute/aspect that would let the programmer set a
> static upper bound on the size of any object in T'Class. If we call this
> aspect Maximum_Size (or perhaps Maximum_Size'Class), the programmer
> could use it like this:
>
>    type Root is tagged record ... end record
>    with Maximum_Size => 128;

Something like that was considered and rejected for Ada 9X.
Part of the problem is that it seems so low level to be talking
about sizes.  It's not even portable.  And not maintainable -- if you
delete a big type, or make it smaller, you're now wasting space.

It would be better to have the compiler compute the maximum
size needed.  That  would require the compiler to do a fairly
global analysis, which is something Ada compilers are not
set up to do.

>    type Object_List is array (List_Index) of Root'Class;
>
>    type Object_Pair is record
>       A, B : Root'Class;
>    end record;

Would you allow:

    X : Object_List (1..10);
    Y : Object_Pair;

?  If so, what is the 'Tag of the various components?
"Undefined" is not a very satisfying answer.

These things are analogous to records with defaulted discriminants.
The language makes some (unsuccessful!) attempt to prevent
uninitialized discriminants.

- Bob