Re: Union type punning in C++

Subject	Re: Union type punning in C++
Newsgroups	comp.lang.c++
References	<kl6PF.88068$CSS.31003@fx01.dcg> <quluvv$10pp$1@gioia.aioe.org> <490dd5d6-fb8a-482b-a88d-550b8a4defd0@googlegroups.com> <qum6v6$3aj$1@gioia.aioe.org> <42c28e06-0493-4d9c-a9f1-21e4318d2fdd@googlegroups.com>
From	Mr Flibble <flibbleREMOVETHISBIT@i42.co.uk>
Organization	Jupiter Mining Corporation
Message-ID	<yRHPF.180536$3sa.135707@fx02.dcg> (permalink)
Date	2020-01-03 14:04 +0000

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On 03/01/2020 08:40, Öö Tiib wrote:
> On Friday, 3 January 2020 03:56:03 UTC+2, Chris M. Thomasson  wrote:
>> On 1/2/2020 4:58 PM, Öö Tiib wrote:
>>> On Friday, 3 January 2020 01:39:54 UTC+2, Chris M. Thomasson  wrote:
>>>> On 1/1/2020 11:24 AM, Mr Flibble wrote:
>>>>> Undefined behaviour aside is C++ union type punning an acceptable hack
>>>>> or not, given that it is legal in C?
>>>>> If it works on all the compilers I care about does it really matter that
>>>>> it is *currently* classed as undefined behaviour?
>>>>> I would say that everyone has "secretly" fucking done it some time or
>>>>> other so the C++ Standards Committee should get there act together and
>>>>> legalize it like it has been in C.
>>>>> Can anyone provide a straight answer to this?
>>>>
>>>> Is there some cryptic hidden thing in the standard that just might
>>>> "allow" for the following code to run without ub?
>>>
>>> Possibly.
>> [...]
>>> By my understanding it can easily be that it is not undefined behavior
>>> there at all. What is the motivation of writing code like that?
>>
>> Mainly to boil things down for the example. Also, one can use the base
>> object as a, say, sequence counter. The types in the union can be used
>> for, an event dispatch.
> 
> Hmm. I'm not sure I understand the example problem. What the counter
> counts and  why you do not keep it outside of union?
> 
>> Say the union object is being represented as an object_1.
>>
>> object_1* o1 = &o.o1;
>>
>> o1->b.p0 += 1;
>> o1->p0 += 1;
>> o1->p1 += 3;
> 
> Above looks ill-formed, diagnostic required.
> The p0 and p1 were of class types without operator+=(int) specified.
> 
>> o1->b.p0 += 1;
>>
>> Now, we can read the sequence count base now matter if we are working
>> with an object_0 or object_1.
>>
>> int seq = o.b.p0;
> 
> I'm still confused. This is all about some kind of aliases and references
> (that likely work fine). But existence of those does simplify nothing for
> me. There is apparent desire to have multiple members of union active
> at same time. That also does confuse me. For me it is not question if it
> is defined behavior but uncertainty about why I need it regardless if it
> is defined behavior or not.

I need this ability so I can have a swizzle class.

/Flibble

-- 
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Thread

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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