NetNews Offline 2/NetNews Offline Volume 2.iso/news/comp/lang/c++-part1/9144

Internet Message Format | 1996-08-05 | 2.7 KB

Path: watnews.watson.ibm.com!ncohen
From: [e-mail] (Norman H. Cohen)
Newsgroups: comp.lang.ada,comp.lang.c++
Subject: Re: on OO differnces between Ada95 and C++
Date: 28 Feb 1996 20:59:22 GMT
Organization: IBM T.J. Watson Research Center
Distribution: world
Message-ID: <4h2fna$[e-mail]>
References: <4gbq7q$[e-mail]> <4gdidj$[e-mail]> <4gveoa$[e-mail]>
Reply-To: [e-mail]
NNTP-Posting-Host: rios8.watson.ibm.com

In article <4gveoa$[e-mail]>,
[e-mail] (Adam Morris) writes:

|> In article <4gdidj$[e-mail]>,
|>    [e-mail] (Norman H. Cohen) wrote:
...
|>                                                           In C and C++
|> >importing is transitive--if B #includes A and C #includes B, then C has
|> >effectively #included A--while in Ada it is not--if B has a with clause for
|> >A and C has a with clause for B, then A is not visible in C unless C has
|> >its own explicit with clause for A.
|>
|> Only if you do what you shouldn't do...  the .h file should have forward
|> declarations of all classes it uses except in certain cases.  And what are
|> those cases, if A inherits from B then (and only then as far as I know) should
|> you #include B.h in A.h  otherwise you have a declaration saying
|> class B{};

Yes, but A.H must include B.H even if A inherits from B as a PRIVATE base
class.  In that case the interface of class B should not in any way be
considered part of the interface of class A, but any file that #includes
A gets gratuitous access to the entities declared in B.H.  This does not
happen in Ada.  The Ada equivalent is

   with B_Package;
   package A_Package is
      type A_Type is tagged private;
      ...
   private
      type A_Type is new B_Package.B_Type with ...;
      ...
   end A_Package;

A package that has a with clause for A_Package, but not for B_Package,
does not get access to B_Package.

The issue arises not only for inheritance, but also for composition.
If A has a private member of class B, A.H must #include B.H.  Again, the
interface of class B is not logically part of the interface for class A,
but you can't #include A.H without also #including B.H.  The Ada
equivalent is

   with B_Package;
   package A_Package is
      type A_Type is tagged private;
      ...
   private
      type A_Type is tagged
         record
            ...
            Some_Component: B_Package.B_Type;
            ...
         end record;
      ...
   end A_Package;

and once again, a package that has a with clause for A_Package, but not
for B_Package, does not get access to B_Package.

--
Norman H. Cohen    [e-mail]