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| From | James Kuyper <jameskuyper@verizon.net> |
|---|---|
| Newsgroups | comp.lang.c |
| Subject | Re: Aliasing and subtyping structs |
| Date | 2016-11-01 11:41 -0400 |
| Organization | A noiseless patient Spider |
| Message-ID | <nvad38$piq$1@dont-email.me> (permalink) |
| References | <87oa1zgzw0.fsf@starless.ames.family> |
On 11/01/2016 08:15 AM, Andreas wrote:
> Hi all,
>
> while searching the web I've found a lot of assertions that a common
> idiom of subtyping structs in C is well-defined by virtue of the first
> struct member always having the same address as the surrounding struct.
> What I'm concerned about is aliasing in this context.
>
> struct sub_type
> {
> uint8_t super_tag;
> ...
> };
>
> struct super_type
> {
> struct sub_type super;
> ...
> };
>
> struct hyper_type
> {
> struct super_type super;
> ...
> };
>
> void foo (struct sub_type* base)
> {
> struct super_type* up = 0;
> struct hyper_type* upper = 0;
> if (base->super_tag == UP_ID)
> {
> up = (struct super_type*) base; // *NOT aliasing!?*
> ...
> }
> else if (base->super_tag == UP_ID)
> {
> upper = (struct hyper_type*) base; // *NOT aliasing!?*
> ...
> }
> ...
> }
>
> void bar (struct super_type* up, struct hyper_type* upper)
> {
> struct sub_type* base1 = (struct sub_type*) up; // *ALIASING?*
> struct sub_type* base2 = (struct sub_type*) upper; // *ALIASING?*
Yes, it is guaranteed that "A pointer to a structure object, suitably
converted, points to its initial member (or if that member is a
bit-field, then to the unit in which it resides), and vice versa.".
(6.7.2.1p15). Technically, that means that you'd have to use (struct
sub_type*)(struct super_type*) co convert upper to base2. The standard
does NOT guarantee that this pair of conversions has the same effect as
the direct conversion to struct sub_type*. However, in practice, that's
unlikely to be problematic.
*base1 is now an alias for up.super, and *base2 is now an alias for
upper.super.super. This is not a problem in itself, but it is something
to be aware of.
> struct sub_type* base1 = up.super;
> struct sub_type* base2 = upper.super.super;
That code contains at least two errors on each line, possibly more, so
I'm not sure what was intended. It looks like you are defining two new
variables, base1 and base2, in the same scope where you've already
defined variables with those names. I see two possibilities: you
intended this as an alternative to the first pair of lines - in that
case, you should have used #if #else #endif to separate the two
alternatives. The other possibility is that you forgot to give these
variables new names, such as base3 and base4.
The next problem is that the initializers for those definitions have the
type "struct sub_type", while those variables have the type "struct
sub_type*". I suspect that there's a missing "&" after the "=" in each
of those definitions.
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Aliasing and subtyping structs Andreas <nospam@invalid.invalid> - 2016-11-01 13:15 +0100
Re: Aliasing and subtyping structs Andreas <nospam@invalid.invalid> - 2016-11-01 13:34 +0100
Re: Aliasing and subtyping structs Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-11-01 15:27 +0000
Re: Aliasing and subtyping structs Andreas <nospam@invalid.invalid> - 2016-11-02 09:47 +0100
Re: Aliasing and subtyping structs Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-11-02 15:47 +0000
Re: Aliasing and subtyping structs Andreas <nospam@invalid.invalid> - 2016-11-08 00:24 +0100
Re: Aliasing and subtyping structs supercat@casperkitty.com - 2016-11-07 21:42 -0800
Re: Aliasing and subtyping structs <william@wilbur.25thandClement.com> - 2016-11-03 18:53 -0700
Re: Aliasing and subtyping structs supercat@casperkitty.com - 2016-11-04 09:56 -0700
Re: Aliasing and subtyping structs Andreas <nospam@invalid.invalid> - 2016-11-08 21:53 +0100
Re: Aliasing and subtyping structs James Kuyper <jameskuyper@verizon.net> - 2016-11-01 11:41 -0400
Re: Aliasing and subtyping structs Andreas <nospam@invalid.invalid> - 2016-11-02 09:55 +0100
"Object-oriented programming in C" [was: Re: Aliasing and subtyping structs] Anton Shepelev <anton.txt@g{oogle}mail.com> - 2016-11-21 16:59 +0300
Re: "Object-oriented programming in C" [was: Re: Aliasing and subtyping structs] supercat@casperkitty.com - 2016-11-21 07:32 -0800
Re: "Object-oriented programming in C" [was: Re: Aliasing and subtyping structs] "James R. Kuyper" <jameskuyper@verizon.net> - 2016-11-21 11:15 -0500
Re: "Object-oriented programming in C" [was: Re: Aliasing and subtyping structs] supercat@casperkitty.com - 2016-11-21 08:35 -0800
Re: "Object-oriented programming in C" [was: Re: Aliasing and subtyping structs] Anton Shepelev <anton.txt@g{oogle}mail.com> - 2016-11-22 11:42 +0300
Re: "Object-oriented programming in C" [was: Re: Aliasing and subtyping structs] supercat@casperkitty.com - 2016-11-22 06:50 -0800
Re: "Object-oriented programming in C" [was: Re: Aliasing and subtyping structs] Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-11-22 16:07 +0000
Re: "Object-oriented programming in C" [was: Re: Aliasing and subtyping structs] supercat@casperkitty.com - 2016-11-22 12:02 -0800
Re: "Object-oriented programming in C" [was: Re: Aliasing and subtyping structs] Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-11-22 23:36 +0000
Re: "Object-oriented programming in C" [was: Re: Aliasing and subtyping structs] supercat@casperkitty.com - 2016-11-22 16:24 -0800
Re: "Object-oriented programming in C" [was: Re: Aliasing and subtyping structs] Richard Damon <Richard@Damon-Family.org> - 2016-11-22 22:21 -0500
Re: "Object-oriented programming in C" [was: Re: Aliasing and subtyping structs] supercat@casperkitty.com - 2016-11-23 08:31 -0800
Re: "Object-oriented programming in C" [was: Re: Aliasing and subtyping structs] me <crisdunbar@gmail.com> - 2016-11-23 09:01 -0800
Re: "Object-oriented programming in C" [was: Re: Aliasing and subtyping structs] supercat@casperkitty.com - 2016-11-23 12:05 -0800
Re: "Object-oriented programming in C" [was: Re: Aliasing and subtyping structs] Anton Shepelev <anton.txt@gmail.com> - 2016-11-22 23:23 +0300
Re: "Object-oriented programming in C" [was: Re: Aliasing and subtyping structs] "James R. Kuyper" <jameskuyper@verizon.net> - 2016-11-22 16:02 -0500
Re: "Object-oriented programming in C" [was: Re: Aliasing and subtyping structs] supercat@casperkitty.com - 2016-11-22 14:13 -0800
Re: container_of() macro Noob <root@127.0.0.1> - 2016-11-23 01:10 +0100
Re: container_of() macro <william@wilbur.25thandClement.com> - 2016-11-22 20:10 -0800
Re: container_of() macro "Chris M. Thomasson" <invalid@invalid.invalid> - 2016-11-23 15:10 -0800
Re: container_of() macro David Brown <david.brown@hesbynett.no> - 2016-11-23 11:31 +0100
Re: container_of() macro Anton Shepelev <anton.txt@g{oogle}mail.com> - 2016-11-23 14:07 +0300
Re: container_of() macro supercat@casperkitty.com - 2016-11-23 08:41 -0800
Re: "Object-oriented programming in C" [was: Re: Aliasing and subtyping structs] Anton Shepelev <anton.txt@gmail.com> - 2016-11-30 00:14 +0300
Re: "Object-oriented programming in C" [was: Re: Aliasing and subtyping structs] "James R. Kuyper" <jameskuyper@verizon.net> - 2016-11-29 17:23 -0500
Re: "Object-oriented programming in C" [was: Re: Aliasing and subtyping structs] Anton Shepelev <anton.txt@gmail.com> - 2016-12-04 18:48 +0300
Re: "Object-oriented programming in C" [was: Re: Aliasing and subtyping structs] "James R. Kuyper" <jameskuyper@verizon.net> - 2016-12-04 12:17 -0500
Re: "Object-oriented programming in C" [was: Re: Aliasing and subtyping structs] Anton Shepelev <anton.txt@gmail.com> - 2016-12-04 22:15 +0300
Re: "Object-oriented programming in C" [was: Re: Aliasing and subtyping structs] "James R. Kuyper" <jameskuyper@verizon.net> - 2016-12-04 15:06 -0500
Re: "Object-oriented programming in C" [was: Re: Aliasing and subtyping structs] Anton Shepelev <anton.txt@gmail.com> - 2016-12-12 22:31 +0300
Re: "Object-oriented programming in C" [was: Re: Aliasing and subtyping structs] "Chris M. Thomasson" <invalid@invalid.invalid> - 2016-11-23 13:39 -0800
Re: "Object-oriented programming in C" [was: Re: Aliasing and subtyping structs] luser droog <luser.droog@gmail.com> - 2016-11-23 15:10 -0800
Re: "Object-oriented programming in C" [was: Re: Aliasing and subtyping structs] "Chris M. Thomasson" <invalid@invalid.invalid> - 2016-11-23 15:13 -0800
Re: "Object-oriented programming in C" [was: Re: Aliasing and subtyping structs] luser droog <luser.droog@gmail.com> - 2016-11-23 18:29 -0800
Re: "Object-oriented programming in C" [was: Re: Aliasing and subtyping structs] Anton Shepelev <anton.txt@g{oogle}mail.com> - 2016-11-24 11:41 +0300
Re: "Object-oriented programming in C" [was: Re: Aliasing and subtyping structs] Thiago Adams <thiago.adams@gmail.com> - 2016-11-24 03:29 -0800
Re: "Object-oriented programming in C" [was: Re: Aliasing and subtyping structs] "Chris M. Thomasson" <invalid@invalid.invalid> - 2016-11-24 17:37 -0800
Re: "Object-oriented programming in C" [was: Re: Aliasing and subtyping structs] supercat@casperkitty.com - 2016-11-25 09:55 -0800
Re: "Object-oriented programming in C" [was: Re: Aliasing and subtyping structs] Thiago Adams <thiago.adams@gmail.com> - 2016-11-25 15:22 -0800
Re: "Object-oriented programming in C" [was: Re: Aliasing and subtyping structs] "Chris M. Thomasson" <invalid@invalid.invalid> - 2016-11-28 18:18 -0800
Re: "Object-oriented programming in C" [was: Re: Aliasing and subtyping structs] Thiago Adams <thiago.adams@gmail.com> - 2016-11-29 04:48 -0800
Re: "Object-oriented programming in C" [was: Re: Aliasing and subtyping structs] supercat@casperkitty.com - 2016-11-29 07:28 -0800
Re: "Object-oriented programming in C" [was: Re: Aliasing and subtyping structs] Thiago Adams <thiago.adams@gmail.com> - 2016-11-30 08:26 -0800
Re: "Object-oriented programming in C" [was: Re: Aliasing and subtyping structs] supercat@casperkitty.com - 2016-11-30 09:13 -0800
Re: "Object-oriented programming in C" [was: Re: Aliasing and subtyping structs] "Chris M. Thomasson" <invalid@invalid.invalid> - 2016-12-12 16:44 -0800
Re: "Object-oriented programming in C" [was: Re: Aliasing and subtyping structs] Melzzzzz <mel@zzzzz.com> - 2016-12-13 02:18 +0100
Re: "Object-oriented programming in C" [was: Re: Aliasing and subtyping structs] Thiago Adams <thiago.adams@gmail.com> - 2016-12-14 09:47 -0800
Re: "Object-oriented programming in C" [was: Re: Aliasing and subtyping structs] Thiago Adams <thiago.adams@gmail.com> - 2016-12-14 10:14 -0800
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