Re: This will work or not?

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From	Bo Persson <bop@gmb.dk>
Newsgroups	comp.lang.c++
Subject	Re: This will work or not?
Date	Wed, 31 Aug 2016 18:53:00 +0200
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On 2016-08-31 15:20, David Brown wrote:
> On 31/08/16 14:59, Melzzzzz wrote:
>> Code in question is like this:
>>
>> struct Problem{
>> 	uint32_t a;
>> 	uint32_t b;
>> 	uint64_t c;
>> 	uint32_t d;
>> 	bool check_it() {
>> 		for (uint32_t* i = &a;i != &d; ++i)
>> 			if (*i>0) return true;
>> 		return false;
>> 	}
>>
>> };
>>
>> I am not sure if this will work, so if someone has clear explanation
>> why it will work (or not) , I would be grateful.
>>
>
> I don't think you are guaranteed that a, b, c, and d follow each other
> in the structure layout (though it is very likely to be the case).  With
> a pointer to a single item &a, incrementing the pointer is undefined
> behaviour - as is accessing *i.  And accessing the uint64_t member using
> an uint32_t* pointer contradicts aliasing restrictions.
>
> All in all, the compiler is free to assume that this code is full of
> undefined behaviour, therefore you don't really care what it does -
> therefore it can all be optimised to "return true", "return false", or
> even "launch nasal daemons".  (This last option is legal for the
> compiler, but thankfully unlikely in practice.)
>
> You can make it legal with:
>
> struct Problem {
> 	union {
> 		struct {
> 			uint32_t a;
> 			uint32_t b;
> 			uint64_t c;
> 			uint32_t d;
> 		}
> 		uint32_t as_array[5];
> 	}
> 	bool check_it() {
> 		for (uint32_t* i = &as_array[0]; i != &as_array[4]; ++i)
> 			if (*i > 0) return true;
> 		return false;
> 	}
> }
>
> (Any issues regarding anonymous unions/structs are left as an exercise
> for the reader :-)
>

This might work in C, but a problem is that the C++ standard says:

9.3 Unions
"In a union, a non-static data member is active if its name refers to an 
object whose lifetime has begun and has not ended (3.8). At most one of 
the non-static data members of an object of union type can be active at 
any time, that is, the value of at most one of the non-static data 
members can be stored in a union at any time. "

So, if the struct with a, b, c, and d is active, as_array is not 
"stored" and cannot be read. You can't read an object that isn't there!



     Bo Persson

Thread

This will work or not? Melzzzzz <mel@zzzzz.com> - 2016-08-31 14:59 +0200
  Re: This will work or not? David Brown <david.brown@hesbynett.no> - 2016-08-31 15:20 +0200
    Re: This will work or not? mark <mark@invalid.invalid> - 2016-08-31 16:05 +0200
      Re: This will work or not? David Brown <david.brown@hesbynett.no> - 2016-08-31 16:19 +0200
    Re: This will work or not? scott@slp53.sl.home (Scott Lurndal) - 2016-08-31 14:20 +0000
    Re: This will work or not? Bo Persson <bop@gmb.dk> - 2016-08-31 18:53 +0200
      Re: This will work or not? Tim Rentsch <txr@alumni.caltech.edu> - 2016-09-01 09:03 -0700
        Re: This will work or not? David Brown <david.brown@hesbynett.no> - 2016-09-01 22:45 +0200
          Re: This will work or not? Tim Rentsch <txr@alumni.caltech.edu> - 2016-09-01 18:08 -0700
            Re: This will work or not? David Brown <david.brown@hesbynett.no> - 2016-09-02 09:24 +0200
              Re: This will work or not? Tim Rentsch <txr@alumni.caltech.edu> - 2016-09-04 06:31 -0700
                Re: This will work or not? David Brown <david.brown@hesbynett.no> - 2016-09-04 20:19 +0200
  Re: This will work or not? Manfred <noname@invalid.add> - 2016-08-31 15:45 +0200
    Re: This will work or not? Melzzzzz <mel@zzzzz.com> - 2016-08-31 15:59 +0200
    Re: This will work or not? David Brown <david.brown@hesbynett.no> - 2016-08-31 16:22 +0200
      Re: This will work or not? Manfred <noname@invalid.add> - 2016-08-31 17:06 +0200
        Re: This will work or not? David Brown <david.brown@hesbynett.no> - 2016-08-31 19:29 +0200
          Re: This will work or not? Chris Vine <chris@cvine--nospam--.freeserve.co.uk> - 2016-08-31 21:52 +0100
            Re: This will work or not? David Brown <david.brown@hesbynett.no> - 2016-09-01 09:07 +0200
              Re: This will work or not? Chris Vine <chris@cvine--nospam--.freeserve.co.uk> - 2016-09-01 12:30 +0100
            Re: This will work or not? Tim Rentsch <txr@alumni.caltech.edu> - 2016-09-01 09:01 -0700
              Re: This will work or not? Chris Vine <chris@cvine--nospam--.freeserve.co.uk> - 2016-09-01 18:16 +0100
                Re: This will work or not? Tim Rentsch <txr@alumni.caltech.edu> - 2016-09-01 18:37 -0700
                Re: This will work or not? Chris Vine <chris@cvine--nospam--.freeserve.co.uk> - 2016-09-02 10:42 +0100
                Re: This will work or not? Tim Rentsch <txr@alumni.caltech.edu> - 2016-09-04 06:59 -0700
          Re: This will work or not? Jerry Stuckle <jstucklex@attglobal.net> - 2016-08-31 23:17 -0400
            Re: This will work or not? David Brown <david.brown@hesbynett.no> - 2016-09-01 09:35 +0200
              Re: This will work or not? Jerry Stuckle <jstucklex@attglobal.net> - 2016-09-01 15:34 -0400
                Re: This will work or not? me <crisdunbar@gmail.com> - 2016-09-01 13:50 -0700
                Re: This will work or not? Jerry Stuckle <jstucklex@attglobal.net> - 2016-09-01 21:01 -0400
                Re: This will work or not? me <crisdunbar@gmail.com> - 2016-09-01 19:02 -0700
                Re: This will work or not? Jerry Stuckle <jstucklex@attglobal.net> - 2016-09-01 22:17 -0400
                Re: This will work or not? me <crisdunbar@gmail.com> - 2016-09-02 08:19 -0700
                Re: This will work or not? Jerry Stuckle <jstucklex@attglobal.net> - 2016-09-02 12:39 -0400
                Re: This will work or not? me <crisdunbar@gmail.com> - 2016-09-02 12:26 -0700
                Re: This will work or not? Jerry Stuckle <jstucklex@attglobal.net> - 2016-09-02 16:38 -0400
                Re: This will work or not? me <crisdunbar@gmail.com> - 2016-09-02 13:51 -0700
                Re: This will work or not? me <crisdunbar@gmail.com> - 2016-09-02 14:16 -0700
                Re: This will work or not? Jerry Stuckle <jstucklex@attglobal.net> - 2016-09-02 19:58 -0400
                Re: This will work or not? Paavo Helde <myfirstname@osa.pri.ee> - 2016-09-02 23:56 +0300
                Re: This will work or not? David Brown <david.brown@hesbynett.no> - 2016-09-03 17:58 +0200
                Re: This will work or not? Manfred <noname@invalid.add> - 2016-09-02 11:41 +0200
                Re: This will work or not? David Brown <david.brown@hesbynett.no> - 2016-09-02 12:27 +0200
                Re: This will work or not? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-09-02 12:37 +0100
                Re: This will work or not? David Brown <david.brown@hesbynett.no> - 2016-09-02 17:39 +0200
                Re: This will work or not? Melzzzzz <mel@zzzzz.com> - 2016-09-02 17:40 +0200
                Re: This will work or not? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-09-02 16:54 +0100
                Re: This will work or not? David Brown <david.brown@hesbynett.no> - 2016-09-02 20:22 +0200
                Re: This will work or not? Richard Damon <Richard@Damon-Family.org> - 2016-09-02 08:04 -0400
                Re: This will work or not? BartC <bc@freeuk.com> - 2016-09-02 13:51 +0100
                Re: This will work or not? Richard Damon <Richard@Damon-Family.org> - 2016-09-02 21:48 -0400
                Re: This will work or not? Manfred <noname@invalid.add> - 2016-09-02 15:17 +0200
                Re: This will work or not? David Brown <david.brown@hesbynett.no> - 2016-09-02 17:46 +0200
                Re: This will work or not? David Brown <david.brown@hesbynett.no> - 2016-09-01 23:14 +0200
                Re: This will work or not? Jerry Stuckle <jstucklex@attglobal.net> - 2016-09-01 21:02 -0400
                Re: This will work or not? David Brown <david.brown@hesbynett.no> - 2016-09-02 09:35 +0200
                Re: This will work or not? Jerry Stuckle <jstucklex@attglobal.net> - 2016-09-02 08:50 -0400
                Re: This will work or not? David Brown <david.brown@hesbynett.no> - 2016-09-02 17:59 +0200
                Re: This will work or not? Jerry Stuckle <jstucklex@attglobal.net> - 2016-09-02 12:49 -0400
                Re: This will work or not? David Brown <david.brown@hesbynett.no> - 2016-09-02 20:36 +0200
                Re: This will work or not? Jerry Stuckle <jstucklex@attglobal.net> - 2016-09-02 16:32 -0400
                Re: This will work or not? David Brown <david.brown@hesbynett.no> - 2016-09-03 18:08 +0200
                Re: This will work or not? Jerry Stuckle <jstucklex@attglobal.net> - 2016-09-03 18:25 -0400
                Re: This will work or not? Mr Flibble <flibbleREMOVETHISBIT@i42.co.uk> - 2016-09-03 23:51 +0100
                Re: This will work or not? Jerry Stuckle <jstucklex@attglobal.net> - 2016-09-03 21:48 -0400
                Re: This will work or not? Mr Flibble <flibbleREMOVETHISBIT@i42.co.uk> - 2016-09-04 03:48 +0100
                Re: This will work or not? Jerry Stuckle <jstucklex@attglobal.net> - 2016-09-03 23:05 -0400
                Re: This will work or not? Mr Flibble <flibbleREMOVETHISBIT@i42.co.uk> - 2016-09-04 05:02 +0100
                Re: This will work or not? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-09-04 10:57 +0100
                Re: This will work or not? Jerry Stuckle <jstucklex@attglobal.net> - 2016-09-04 09:40 -0400
                Re: This will work or not? Mr Flibble <flibbleREMOVETHISBIT@i42.co.uk> - 2016-09-04 16:39 +0100
                Re: This will work or not? Jerry Stuckle <jstucklex@attglobal.net> - 2016-09-04 12:46 -0400
                Re: This will work or not? Mr Flibble <flibbleREMOVETHISBIT@i42.co.uk> - 2016-09-04 17:51 +0100
                Re: This will work or not? Jerry Stuckle <jstucklex@attglobal.net> - 2016-09-04 13:07 -0400
                Re: This will work or not? Mr Flibble <flibbleREMOVETHISBIT@i42.co.uk> - 2016-09-04 18:37 +0100
                Re: This will work or not? Jerry Stuckle <jstucklex@attglobal.net> - 2016-09-04 17:16 -0400
                Re: This will work or not? Mr Flibble <flibbleREMOVETHISBIT@i42.co.uk> - 2016-09-04 22:23 +0100
                Re: This will work or not? Jerry Stuckle <jstucklex@attglobal.net> - 2016-09-04 18:54 -0400
                Re: This will work or not? "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2016-09-04 09:58 -0700
                Re: This will work or not? Jerry Stuckle <jstucklex@attglobal.net> - 2016-09-04 13:08 -0400
                Re: This will work or not? "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2016-09-04 10:21 -0700
                Re: This will work or not? Jerry Stuckle <jstucklex@attglobal.net> - 2016-09-04 17:18 -0400
                Re: This will work or not? Mr Flibble <flibbleREMOVETHISBIT@i42.co.uk> - 2016-09-04 22:24 +0100
                Re: This will work or not? Jerry Stuckle <jstucklex@attglobal.net> - 2016-09-04 18:56 -0400
                Re: This will work or not? Mr Flibble <flibbleREMOVETHISBIT@i42.co.uk> - 2016-09-05 00:27 +0100
                Re: This will work or not? Jerry Stuckle <jstucklex@attglobal.net> - 2016-09-04 22:53 -0400
                Re: This will work or not? Mr Flibble <flibbleREMOVETHISBIT@i42.co.uk> - 2016-09-05 18:18 +0100
                Re: This will work or not? Jerry Stuckle <jstucklex@attglobal.net> - 2016-09-05 15:27 -0400
                Re: This will work or not? Mr Flibble <flibbleREMOVETHISBIT@i42.co.uk> - 2016-09-05 20:32 +0100
                Re: This will work or not? woodbrian77@gmail.com - 2016-09-05 14:07 -0700
                Re: This will work or not? Jerry Stuckle <jstucklex@attglobal.net> - 2016-09-05 18:15 -0400
                Re: This will work or not? Ike Naar <ike@iceland.freeshell.org> - 2016-09-05 20:12 +0000
                Re: This will work or not? Mr Flibble <flibbleREMOVETHISBIT@i42.co.uk> - 2016-09-05 22:05 +0100
                Re: This will work or not? Jerry Stuckle <jstucklex@attglobal.net> - 2016-09-05 18:16 -0400
                Re: This will work or not? David Brown <david.brown@hesbynett.no> - 2016-09-06 08:47 +0200
                Re: This will work or not? Jerry Stuckle <jstucklex@attglobal.net> - 2016-09-06 09:12 -0400
                Re: This will work or not? David Brown <david.brown@hesbynett.no> - 2016-09-06 16:31 +0200
                Re: This will work or not? Jerry Stuckle <jstucklex@attglobal.net> - 2016-09-06 11:20 -0400
                Re: This will work or not? bighe.trole@gmail.com - 2016-09-04 18:55 -0700
                Re: This will work or not? Jerry Stuckle <jstucklex@attglobal.net> - 2016-09-04 22:55 -0400
                Re: This will work or not? "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2016-09-04 17:17 -0700
                Re: This will work or not? Jerry Stuckle <jstucklex@attglobal.net> - 2016-09-04 22:57 -0400
                Re: This will work or not? "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2016-09-05 11:20 -0700
                Re: This will work or not? Luuk <luuk@invalid.lan> - 2016-09-04 18:54 +0200
                Re: This will work or not? Mr Flibble <flibbleREMOVETHISBIT@i42.co.uk> - 2016-09-04 17:59 +0100
                Re: This will work or not? Jerry Stuckle <jstucklex@attglobal.net> - 2016-09-04 13:08 -0400
                Re: This will work or not? David Brown <david.brown@hesbynett.no> - 2016-09-04 20:25 +0200
                Re: This will work or not? Jerry Stuckle <jstucklex@attglobal.net> - 2016-09-04 17:20 -0400
                Re: This will work or not? Öö Tiib <ootiib@hot.ee> - 2016-09-04 02:36 -0700
                Re: This will work or not? Jerry Stuckle <jstucklex@attglobal.net> - 2016-09-04 09:41 -0400
                Re: This will work or not? Jerry Stuckle <jstucklex@attglobal.net> - 2016-09-03 21:49 -0400
                Re: This will work or not? Mr Flibble <flibbleREMOVETHISBIT@i42.co.uk> - 2016-09-04 03:49 +0100
                Re: This will work or not? Jerry Stuckle <jstucklex@attglobal.net> - 2016-09-03 23:06 -0400
                Re: This will work or not? woodbrian77@gmail.com - 2016-09-03 21:16 -0700
  Re: This will work or not? Vir Campestris <vir.campestris@invalid.invalid> - 2016-08-31 21:08 +0100
  Re: This will work or not? "Alf P. Steinbach" <alf.p.steinbach+usenet@gmail.com> - 2016-09-01 02:28 +0200
    Re: This will work or not? Öö Tiib <ootiib@hot.ee> - 2016-09-01 01:28 -0700
    Re: This will work or not? "Chris M. Thomasson" <invalid@invalid.invalid> - 2016-09-02 12:06 -0700
  Re: This will work or not? Öö Tiib <ootiib@hot.ee> - 2016-09-05 02:21 -0700
    Re: This will work or not? Melzzzzz <mel@zzzzz.com> - 2016-09-05 13:38 +0200
  Re: This will work or not? Juha Nieminen <nospam@thanks.invalid> - 2016-09-05 11:34 +0000
    Re: This will work or not? Melzzzzz <mel@zzzzz.com> - 2016-09-05 13:36 +0200
      Re: This will work or not? Vir Campestris <vir.campestris@invalid.invalid> - 2016-09-05 21:25 +0100

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