Re: Ada202X: Easy to use "UML private"-like components

["Randy Brukardt" <randy@rrsoftware.com>]

"Robert A Duff" <bobduff@shell01.TheWorld.com> wrote in message 
news:wccli5zqat1.fsf@shell01.TheWorld.com...
> Niklas Holsti <niklas.holsti@tidorum.invalid> writes:
>
>> That seems likely to create program-portability problems - on a 64-bit
>> machine, a 64-bit record could be opaque, but it could not be opaque on
>> a 32-bit machine (unless all pointers are artificially made 64 bits
>> wide, too).
>
> Yes.  I was appalled by that feature for exactly that reason.
> I'm a big fan of portability.
>
> But given the time frame of Modula-2, I think your example should be
> 16-bit/32-bit not 32/64.  ;-)
>
> In any case, in plain English, "opaque" doesn't have all these
> pointer/heap implications, so I suggest saying something like
> "Modula-2-style opaque types" rather than just "opaque types"
> if that's what you mean.
>
>> The main difference is that in Ada, the unknown underlying type is named
>> -- what you call the stt-incomplete type. This type is of course
>> invisible to the users (clients) of the private type. Off hand, it does
>> not seem to me that such named-but-incomplete types should cause hard
>> problems for the compiler, but it appears that you have experienced such
>> problems. Perhaps they did not exist in Ada 83, but came about because
>> of other extensions such as controlled types?
>
> The implementation problems existed in Ada 83.  I'm not sure I remember
> all the issues, but the most obvious is that there's an implicit
> assumption that you can tell the size/representation of a pointer
> without knowing what it points at.  There are various reasons that might
> not be true.  In fact, it's not true in GNAT.
>
> Also, I think it was legal to do "X.all" on those things (in the package
> spec) in Ada 83.  It would always raise Constraint_Error, but the
> "natural" implementation wants to know the size of X.all.  I fixed that
> in Ada 95 by changing the freezing rules to make it illegal, IIRC.

I think there was also an issue with constraints in Ada 83 that was fixed in 
Ada 95.

Note that stt-incomplete types were the basis for the "limited with" feature 
in Ada 2007, so it's not like the language designers have neglected the 
feature. (We thought that if stt-incomplete already worked, implementers 
couldn't complain about limited with - that turned out not to be quite 
true.)

                     Randy.