🚀 go-pugleaf

In article <4f7l8h$t0e@mulga.cs.mu.OZ.AU> fjh@munta.cs.mu.OZ.AU (Fergus
Henderson) writes:

> kanze@lts.sel.alcatel.de (James Kanze US/ESC 60/3/141 #40763) writes:
> 
> >dirk@becker.adviser.com (Dirk Becker) writes:
> >
> >|> In Section 9.6.1 Unions [class.union] there is a large collection of
> >|> restrictions on the union itself and on candidate member classes.
> >|> I would like to ask for reasons of several of them:
> >
> >One of the principle goals was that a union be compatible with a C
> >union, at least in use.  One of the rules is that the addresses of all
> >of the elements in the union compare equal to the address of the union
> >itself, when cast to the appropriate type.
> >
> >|> a)  "A union shall not have base classes. A union shall not be used as
> >|>      a base class."
> >
> >Without this rule, you either break the address rule, or you impose
> >some special class layout on unions.
> 
> Nevertheless, contrary to what you seem to be implying,
> that rule is not necessary for C compatibility.
> 
> C++ structs can be very different to C structs, but C compatibility
> is preserved for a certain category of C++ structs, namely POD-structs.
> In the case of unions, the same thing could have been done -
> rule (a) could have been dropped, yet C compatibility could still
> have been be supported for POD-unions.

True.  But there is one intuitive fact that remains true for all
structs: different (data) members have distinct addresses, and in fact
occupy non-overlapping memory.  The same intuitive fact for unions is
that all (non-static) data members have the same address.  (IMHO, of
course.)

There is one restriction that does seem to me totally ungrounded: not
being able to inherit from a union.  I don't think that this feature
would be enormously useful, but I cannot think of a good reason why it
should be banned.  One of the classic solutions for handling C/C++
compatibility is to put all of the data in a PODS, and inherit from it,
using the derived class in C++, but allowing a pointer to it to
automatically become a pointer to the PODS when passed to a C function
declared to take such an address.  The same argument could certainly be
used for inheriting from unions.  If I look in /usr/include/sys, I find
lots of unions which could be potentially used this way.
-- 
James Kanze           (+33) 88 14 49 00          email: kanze@gabi-soft.fr
GABI Software, Sarl., 8 rue des Francs Bourgeois, 67000 Strasbourg, France
Conseils, itudes et rialisations en logiciel orienti objet --
              -- A la recherche d'une activiti dans une region francophone
---
[ comp.std.c++ is moderated.  Submission address: std-c++@ncar.ucar.edu.
  Contact address: std-c++-request@ncar.ucar.edu.  Moderation policy:
  http://reality.sgi.com/employees/austern_mti/std-c++/policy.html. ]