Re: Trouble with overriding and class-wide parameters

[Yves Bailly <kafka.fr@laposte.net>]

Jeffrey R. Carter wrote:
> Note that Sub_Type is not a subtype.

Yes I know ;-) a better name would have been "Derived_Type" I guess.
 
> Sub_Pkg defines 2 procedures named Setup:
> 
> 1. Implicitly, from the primitive Setup for Some_Type, with "not null
> access Sub_Type" and "access Some_Type'Class". The implementation of
> this is identical to the implementation of Pkg.Setup.
> 
> 2. Explicitly, "not null access Sub_Type" and "access Sub_Type'Class",
> since it's "not overriding".

OK, now it's clearer for me. I hadn't perceived the "implicitiness" of
such packages/types structures.
 
> I would expect this to be ambiguous, since it matches the profile for
> both of the Setup procedures. If I'm correct, this would be a compiler
> error.

Actually, if I replace "st.Setup(null)" by "Sub_Pkg.Setup(st'Access, null)"
then I received an error, telling me it's ambiguous... which seems
sensible to me now and matches your comments. Maybe it's a compiler bug if
the error does not arise when using dotted notation, I'll send a message 
to GNAT developers.
 
> In the C/++ mind set, it's common to have visible pointers and pointer
> parameters all over the place. In the Ada mind set, access types,
> parameters, and values are avoided whenever possible, and hidden when
> not. This isolates memory management, making memory-management errors
> easier to find, and usually makes the package easier for the client to
> use. It's rare, in a well designed Ada system, to have access
> types/values/parameters in package specifications. There's probably a
> way to design your system to avoid having access parameters in the specs.

I understand this, several people already told this to me. So I guess
I'll review everything to hide pointers as much as possible. Thanks all
for your advices, though they imply more work than expected ;-)

> As an Ada beginner, you will probably find it valuable to avoid "use"
> clauses. This may seem like more work, but you will learn useful things
> that way. In this case, you have no need for Pkg at all, and so should
> not even "with" it

That's true, in fact it's just a remain of experiments.

> ; typing "Sub_Pkg." in the declaration of St is 
> actually less work than typing " use Sub_Pkg;".

Well, I'm not really a truely beginner in Ada, however my experience is
back to Ada83... There's a huge gap between Ada83 and Ada2005, but with
all your nice helps I'm sure I'll succeed in learning.

Thanks a lot all again for your help.

-- 
(o< | Yves Bailly  : http://kafka-fr.net   | -o)
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