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Groups > comp.lang.c++ > #71483
| From | Tim Rentsch <tr.17687@z991.linuxsc.com> |
|---|---|
| Newsgroups | comp.lang.c++ |
| Subject | Re: Union type punning in C++ |
| Date | 2020-01-21 03:47 -0800 |
| Organization | A noiseless patient Spider |
| Message-ID | <86eevtjajp.fsf@linuxsc.com> (permalink) |
| References | (5 earlier) <qvgh2v$md0$1@gioia.aioe.org> <86v9pfguqn.fsf@linuxsc.com> <qvj1ts$4np$1@gioia.aioe.org> <86muaqgjt6.fsf@linuxsc.com> <qvm661$ieu$1@gioia.aioe.org> |
"Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> writes: > On 1/14/2020 7:05 AM, Tim Rentsch wrote: > >> "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> writes: >> >> [...] >> >>> I added in the extraneous base in the union out of some sort of >>> reflex mixed with trying to just read from o.b. >> >> Maybe you were thinking of programming in C. > > I am a C programmer at heart. [example omitted] >> In C there is often benefit to have a "stub" type that serves >> mainly to differentiate the "concrete" types. AFAICT in C++ >> there is not, except perhaps as some sort of aid to the reader. I should elaborate on this. In C, if there is a union with several structs as members, it's okay to access a member in the common initial sequence of a struct actually in a union object, __given only a pointer to the struct__ (assuming the full definition of the union is visible). In C++, I believe the "common initial sequence" rule applies only when using the union type as part of the access expression. The C++ rule is a significantly weaker guarantee than the C rule. To me it seems to rob the feature of much of its value, even not counting the incongruity with the C rule. At this point there are so many differences between C and C++ that it seems inappropriate to say C is a subset of C++, even just mostly. > Interesting points to ponder. Actually, a decent chunk of my C++ > work in the past was wrapping up a C API. A lot of times the C API > would be wrapping up some externally assembled, assembly language. > > For some reason, sometimes, when I created a pure C++ object, I > would code it up in C on my free time. Well, this was a habit wrt > interfaces that need to be exported in a shared lib. A nice clean C > API is great for this. C++ can have some issues wrt creating, say > .so, or .dlls. > > A C binding is okay with me. Be warned: the different treatments, by C and by C++, of the union "special guarantee", might mean that calling through a C binding may result in undefined behavior in C++, even if the C code is well-defined by the C standard. It isn't clear to me what the implications of C++'s rules are in this area, and in particular whether a C++ implementation may be allowed to make some assumptions that would not be allowed under the C rules. I'm sorry I don't have any better advice to offer other than it would be wise to tread cautiously.
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Union type punning in C++ Mr Flibble <flibbleREMOVETHISBIT@i42.co.uk> - 2020-01-01 19:24 +0000
Re: Union type punning in C++ boltar@nowhere.org - 2020-01-02 09:36 +0000
Re: Union type punning in C++ Öö Tiib <ootiib@hot.ee> - 2020-01-02 03:34 -0800
Re: Union type punning in C++ Bo Persson <bo@bo-persson.se> - 2020-01-02 12:49 +0100
Re: Union type punning in C++ Öö Tiib <ootiib@hot.ee> - 2020-01-02 04:52 -0800
Re: Union type punning in C++ boltar@nowhere.org - 2020-01-02 17:22 +0000
Re: Union type punning in C++ Öö Tiib <ootiib@hot.ee> - 2020-01-02 10:26 -0800
Re: Union type punning in C++ Chris Vine <chris@cvine--nospam--.freeserve.co.uk> - 2020-01-02 21:54 +0000
Re: Union type punning in C++ boltar@nowhere.org - 2020-01-03 14:49 +0000
Re: Union type punning in C++ David Brown <david.brown@hesbynett.no> - 2020-01-03 17:11 +0100
Re: Union type punning in C++ Chris Vine <chris@cvine--nospam--.freeserve.co.uk> - 2020-01-03 17:55 +0000
Re: Union type punning in C++ boltar@nowhere.org - 2020-01-03 09:55 +0000
Re: Union type punning in C++ David Brown <david.brown@hesbynett.no> - 2020-01-03 11:20 +0100
Re: Union type punning in C++ Öö Tiib <ootiib@hot.ee> - 2020-01-03 02:21 -0800
Re: Union type punning in C++ boltar@nowhere.org - 2020-01-03 14:55 +0000
Re: Union type punning in C++ Bonita Montero <Bonita.Montero@gmail.com> - 2020-01-03 16:04 +0100
Re: Union type punning in C++ "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2020-01-03 14:45 -0800
Re: Union type punning in C++ Bonita Montero <Bonita.Montero@gmail.com> - 2020-01-04 01:26 +0100
Re: Union type punning in C++ "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2020-01-03 16:55 -0800
Re: Union type punning in C++ Melzzzzz <Melzzzzz@zzzzz.com> - 2020-01-04 01:02 +0000
Re: Union type punning in C++ "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2020-01-03 17:06 -0800
Re: Union type punning in C++ Öö Tiib <ootiib@hot.ee> - 2020-01-03 07:34 -0800
Re: Union type punning in C++ boltar@nowhere.org - 2020-01-03 16:12 +0000
Re: Union type punning in C++ Öö Tiib <ootiib@hot.ee> - 2020-01-03 08:49 -0800
Re: Union type punning in C++ Bonita Montero <Bonita.Montero@gmail.com> - 2020-01-03 19:28 +0100
Re: Union type punning in C++ Melzzzzz <Melzzzzz@zzzzz.com> - 2020-01-03 18:54 +0000
Re: Union type punning in C++ Bonita Montero <Bonita.Montero@gmail.com> - 2020-01-04 01:25 +0100
Re: Union type punning in C++ Melzzzzz <Melzzzzz@zzzzz.com> - 2020-01-04 00:28 +0000
Re: Union type punning in C++ Bonita Montero <Bonita.Montero@gmail.com> - 2020-01-04 01:41 +0100
Re: Union type punning in C++ Melzzzzz <Melzzzzz@zzzzz.com> - 2020-01-04 00:52 +0000
Re: Union type punning in C++ Bonita Montero <Bonita.Montero@gmail.com> - 2020-01-04 08:08 +0100
Re: Union type punning in C++ Bonita Montero <Bonita.Montero@gmail.com> - 2020-01-04 08:34 +0100
Re: Union type punning in C++ Bonita Montero <Bonita.Montero@gmail.com> - 2020-01-04 08:43 +0100
Re: Union type punning in C++ Melzzzzz <Melzzzzz@zzzzz.com> - 2020-01-04 08:20 +0000
Re: Union type punning in C++ Bonita Montero <Bonita.Montero@gmail.com> - 2020-01-04 09:25 +0100
Re: Union type punning in C++ Melzzzzz <Melzzzzz@zzzzz.com> - 2020-01-04 08:25 +0000
Re: Union type punning in C++ Bonita Montero <Bonita.Montero@gmail.com> - 2020-01-04 09:25 +0100
Re: Union type punning in C++ Melzzzzz <Melzzzzz@zzzzz.com> - 2020-01-04 08:27 +0000
Re: Union type punning in C++ Bonita Montero <Bonita.Montero@gmail.com> - 2020-01-04 09:36 +0100
Re: Union type punning in C++ Melzzzzz <Melzzzzz@zzzzz.com> - 2020-01-04 08:41 +0000
Re: Union type punning in C++ Bonita Montero <Bonita.Montero@gmail.com> - 2020-01-04 09:49 +0100
Re: Union type punning in C++ Melzzzzz <Melzzzzz@zzzzz.com> - 2020-01-04 09:01 +0000
Re: Union type punning in C++ Bonita Montero <Bonita.Montero@gmail.com> - 2020-01-04 10:02 +0100
Re: Union type punning in C++ Melzzzzz <Melzzzzz@zzzzz.com> - 2020-01-04 09:19 +0000
Re: Union type punning in C++ Bonita Montero <Bonita.Montero@gmail.com> - 2020-01-04 10:38 +0100
Re: Union type punning in C++ David Brown <david.brown@hesbynett.no> - 2020-01-04 12:45 +0100
Re: Union type punning in C++ Melzzzzz <Melzzzzz@zzzzz.com> - 2020-01-04 12:00 +0000
Re: Union type punning in C++ Bonita Montero <Bonita.Montero@gmail.com> - 2020-01-04 13:03 +0100
Re: Union type punning in C++ Bonita Montero <Bonita.Montero@gmail.com> - 2020-01-04 14:22 +0100
Re: Union type punning in C++ "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2020-01-03 14:46 -0800
Re: Union type punning in C++ Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2020-01-03 10:32 -0800
Re: Union type punning in C++ Öö Tiib <ootiib@hot.ee> - 2020-01-03 10:43 -0800
Re: Union type punning in C++ Melzzzzz <Melzzzzz@zzzzz.com> - 2020-01-03 18:55 +0000
Re: Union type punning in C++ Daniel <danielaparker@gmail.com> - 2020-01-08 09:40 -0800
Re: Union type punning in C++ Öö Tiib <ootiib@hot.ee> - 2020-01-08 13:23 -0800
Re: Union type punning in C++ Daniel <danielaparker@gmail.com> - 2020-01-08 22:41 -0800
Re: Union type punning in C++ Daniel <danielaparker@gmail.com> - 2020-01-08 22:52 -0800
Re: Union type punning in C++ Öö Tiib <ootiib@hot.ee> - 2020-01-09 12:59 -0800
Re: Union type punning in C++ Daniel <danielaparker@gmail.com> - 2020-01-09 13:32 -0800
Re: Union type punning in C++ Mister2U@honorific.org - 2020-01-10 12:28 +0000
Re: Union type punning in C++ David Brown <david.brown@hesbynett.no> - 2020-01-10 14:02 +0100
Re: Union type punning in C++ Mister2U@honorific.org - 2020-01-10 16:21 +0000
Re: Union type punning in C++ David Brown <david.brown@hesbynett.no> - 2020-01-10 19:18 +0100
Re: Union type punning in C++ Daniel <danielaparker@gmail.com> - 2020-01-10 15:41 -0800
Re: Union type punning in C++ David Brown <david.brown@hesbynett.no> - 2020-01-11 12:01 +0100
Re: Union type punning in C++ Daniel <danielaparker@gmail.com> - 2020-01-11 07:05 -0800
Re: Union type punning in C++ David Brown <david.brown@hesbynett.no> - 2020-01-11 23:53 +0100
Re: Union type punning in C++ Mr Flibble <flibble@i42.removethisbit.co.uk> - 2020-01-12 01:41 +0000
Re: Union type punning in C++ Daniel <danielaparker@gmail.com> - 2020-01-11 19:00 -0800
Re: Union type punning in C++ Mr Flibble <flibble@i42.removethisbit.co.uk> - 2020-01-13 17:32 +0000
Re: Union type punning in C++ Mister2U@honorific.org - 2020-01-11 11:53 +0000
Re: Union type punning in C++ David Brown <david.brown@hesbynett.no> - 2020-01-11 13:20 +0100
Re: Union type punning in C++ Mister2U@honorific.org - 2020-01-11 12:33 +0000
Re: Union type punning in C++ David Brown <david.brown@hesbynett.no> - 2020-01-11 14:28 +0100
Re: Union type punning in C++ Jorgen Grahn <grahn+nntp@snipabacken.se> - 2020-01-11 10:48 +0000
Re: Union type punning in C++ Jorgen Grahn <grahn+nntp@snipabacken.se> - 2020-01-11 11:05 +0000
Re: Union type punning in C++ Mister2U@honorific.org - 2020-01-11 11:55 +0000
Re: Union type punning in C++ Bo Persson <bo@bo-persson.se> - 2020-01-02 21:12 +0100
Re: Union type punning in C++ boltar@nowhere.org - 2020-01-03 14:46 +0000
Re: Union type punning in C++ Bo Persson <bo@bo-persson.se> - 2020-01-03 20:01 +0100
Re: Union type punning in C++ David Brown <david.brown@hesbynett.no> - 2020-01-02 23:23 +0100
Re: Union type punning in C++ peter koch <peter.koch.larsen@gmail.com> - 2020-01-02 12:39 -0800
Re: Union type punning in C++ "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2020-01-02 15:39 -0800
Re: Union type punning in C++ Öö Tiib <ootiib@hot.ee> - 2020-01-02 16:58 -0800
Re: Union type punning in C++ "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2020-01-02 17:55 -0800
Re: Union type punning in C++ Öö Tiib <ootiib@hot.ee> - 2020-01-03 00:40 -0800
Re: Union type punning in C++ Mr Flibble <flibbleREMOVETHISBIT@i42.co.uk> - 2020-01-03 14:04 +0000
Re: Union type punning in C++ David Brown <david.brown@hesbynett.no> - 2020-01-03 15:24 +0100
Re: Union type punning in C++ "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2020-01-03 14:34 -0800
Re: Union type punning in C++ Melzzzzz <Melzzzzz@zzzzz.com> - 2020-01-03 22:59 +0000
Re: Union type punning in C++ Mr Flibble <flibbleREMOVETHISBIT@i42.co.uk> - 2020-01-04 00:11 +0000
Re: Union type punning in C++ Melzzzzz <Melzzzzz@zzzzz.com> - 2020-01-04 00:21 +0000
Re: Union type punning in C++ Mr Flibble <flibbleREMOVETHISBIT@i42.co.uk> - 2020-01-04 00:26 +0000
Re: Union type punning in C++ "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2020-01-04 16:51 -0800
Re: Union type punning in C++ "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2020-01-03 18:47 -0800
Re: Union type punning in C++ "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2020-01-03 19:07 -0800
Re: Union type punning in C++ Öö Tiib <ootiib@hot.ee> - 2020-01-03 06:47 -0800
Re: Union type punning in C++ Mr Flibble <flibbleREMOVETHISBIT@i42.co.uk> - 2020-01-03 14:57 +0000
Re: Union type punning in C++ Öö Tiib <ootiib@hot.ee> - 2020-01-03 07:38 -0800
Re: Union type punning in C++ Mr Flibble <flibbleREMOVETHISBIT@i42.co.uk> - 2020-01-03 17:06 +0000
Re: Union type punning in C++ Öö Tiib <ootiib@hot.ee> - 2020-01-03 11:05 -0800
Re: Union type punning in C++ "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2020-01-03 14:38 -0800
Re: Union type punning in C++ Öö Tiib <ootiib@hot.ee> - 2020-01-03 22:37 -0800
Re: Union type punning in C++ Tim Rentsch <tr.17687@z991.linuxsc.com> - 2020-01-11 07:10 -0800
Re: Union type punning in C++ Öö Tiib <ootiib@hot.ee> - 2020-01-11 10:23 -0800
Re: Union type punning in C++ Tim Rentsch <tr.17687@z991.linuxsc.com> - 2020-01-13 09:01 -0800
Re: Union type punning in C++ Öö Tiib <ootiib@hot.ee> - 2020-01-13 11:03 -0800
Re: Union type punning in C++ James Kuyper <jameskuyper@alumni.caltech.edu> - 2020-01-13 12:48 -0800
Re: Union type punning in C++ Öö Tiib <ootiib@hot.ee> - 2020-01-13 14:15 -0800
Re: Union type punning in C++ James Kuyper <jameskuyper@alumni.caltech.edu> - 2020-01-13 21:47 -0500
Re: Union type punning in C++ Öö Tiib <ootiib@hot.ee> - 2020-01-13 23:44 -0800
Re: Union type punning in C++ James Kuyper <jameskuyper@alumni.caltech.edu> - 2020-01-14 08:26 -0500
Re: Union type punning in C++ Tim Rentsch <tr.17687@z991.linuxsc.com> - 2020-01-14 07:30 -0800
Re: Union type punning in C++ Öö Tiib <ootiib@hot.ee> - 2020-01-14 09:11 -0800
Re: Union type punning in C++ James Kuyper <jameskuyper@alumni.caltech.edu> - 2020-01-14 21:52 -0500
Re: Union type punning in C++ Öö Tiib <ootiib@hot.ee> - 2020-01-14 23:15 -0800
Re: Union type punning in C++ Tim Rentsch <tr.17687@z991.linuxsc.com> - 2020-01-21 02:03 -0800
Re: Union type punning in C++ Tim Rentsch <tr.17687@z991.linuxsc.com> - 2020-01-11 07:28 -0800
Re: Union type punning in C++ "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2020-01-11 20:40 -0800
Re: Union type punning in C++ James Kuyper <jameskuyper@alumni.caltech.edu> - 2020-01-12 20:16 -0500
Re: Union type punning in C++ "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2020-01-12 17:28 -0800
Re: Union type punning in C++ James Kuyper <jameskuyper@alumni.caltech.edu> - 2020-01-12 21:53 -0500
Re: Union type punning in C++ Manfred <noname@add.invalid> - 2020-01-13 18:50 +0100
Re: Union type punning in C++ "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2020-01-13 16:21 -0800
Re: Union type punning in C++ Tim Rentsch <tr.17687@z991.linuxsc.com> - 2020-01-13 08:57 -0800
Re: Union type punning in C++ "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2020-01-13 16:27 -0800
Re: Union type punning in C++ Tim Rentsch <tr.17687@z991.linuxsc.com> - 2020-01-14 07:05 -0800
Re: Union type punning in C++ "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2020-01-14 20:58 -0800
Re: Union type punning in C++ Tim Rentsch <tr.17687@z991.linuxsc.com> - 2020-01-21 03:47 -0800
Re: Union type punning in C++ Öö Tiib <ootiib@hot.ee> - 2020-01-15 01:22 -0800
Re: Union type punning in C++ James Kuyper <jameskuyper@alumni.caltech.edu> - 2020-01-15 08:04 -0500
Re: Union type punning in C++ Öö Tiib <ootiib@hot.ee> - 2020-01-15 09:38 -0800
Re: Union type punning in C++ James Kuyper <jameskuyper@alumni.caltech.edu> - 2020-01-15 11:01 -0800
Re: Union type punning in C++ Tim Rentsch <tr.17687@z991.linuxsc.com> - 2020-01-13 08:37 -0800
Re: Union type punning in C++ "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2020-01-13 16:29 -0800
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