Re: on OO differnces between Ada95 and C++

[je@bton.ac.uk (John English)]

Nasser Abbasi (nabbasi@qualcomm.com) wrote:
: In article <Dn4J2F.uI@bton.ac.uk>, je@bton.ac.uk (John English) says:
: >If the spec of Saving_Account has "with Account" at the beginning,
: >"with Saving_Account" will imply "with Account". 

: I do not think this is true. That is the main reason I asked the 
: question in the first place. (I did not like having to "with" 
: a package that was allready with'ed by a package I am "withing" ).

Hmm.  I've been having another look at the visibility rules in the LRM and,
as usual, they've made my head go round and round :-)

Certainly GNAT allows you to reference Account.X if you have "with
Saving_Account" (but I'm all too aware that GNAT /= Ada95 :-), although
the same is not true for "use" clauses ("use Saving_Account" does not
imply "use Account").  Remember that I'm talking about *with clauses in
package specifications* and not use clauses *or* package bodies (in case
there is any confusion about this).

: If what you say was the case, then types defined in package "A"
: will be seen by clients to a package "B" where "B" has with'ed A. But
: it is not so. Clients of "B" must also 'with' "A" to see types defined
: in "A" even though "B: has allready  with'ed "A".

If you're right this would be a real pain.  Clients of a package would have
to know (recursively) what other packages the spec(s) reference; if the
spec for X "withs" a package Y so it can use type Y.T as a procedure
parameter then you wouldn't be able to use X without Y (i.e. "with X"
on its own would be useless; you'd have to have "with X, Y" and probably
other things as well if Y has any "with" clauses in its specification.

This is such a horrible concept that I'm inclined to believe GNAT, but
if anyone who can understand all the subleties of the visibility rules
can give us all a definitive explanation in words of sufficiently few
syllables that Bears of Very Little Brain like me can understand, I for
one would be profoundly grateful.

-- 
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John English <je@brighton.ac.uk>, Dept. of Computing, University of Brighton
  "Disks are divided into sex and tractors..."
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