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Groups > comp.lang.c++ > #71328
| From | James Kuyper <jameskuyper@alumni.caltech.edu> |
|---|---|
| Newsgroups | comp.lang.c++ |
| Subject | Re: Union type punning in C++ |
| Date | 2020-01-13 21:47 -0500 |
| Organization | A noiseless patient Spider |
| Message-ID | <qvja44$ad$1@dont-email.me> (permalink) |
| References | (4 earlier) <140b2e3a-cc88-45f9-a292-efd471e3c789@googlegroups.com> <86r203guiq.fsf@linuxsc.com> <2e087cbf-8a15-4109-b199-f200487745f6@googlegroups.com> <074aa134-b041-43f8-b44e-f6643eee5645@googlegroups.com> <a7279af6-380a-4939-9a4b-c3b8a4648753@googlegroups.com> |
On 1/13/20 5:15 PM, Öö Tiib wrote:
> On Monday, 13 January 2020 22:48:59 UTC+2, James Kuyper wrote:
>> Could you please identify the clauses in the standard that define layout
>> compatibility in terms of addresses? The only ones I can find that seem
>> relevant specify that layout compatibility depends upon the order and
>> declared types of the members of structs.
>
> If a standard-layout class object has any non-static data members, its
> address is the same as the address of its first non-static data member.
> [class.mem]24
That's not what I'm looking for. That's a rule that allows you to reach
the conclusion that two objects are at the same address. I'm looking for
the rule who's existence is implied by your argument, which allows you
to take the fact that two struct members must be at the same address,
and from that fact conclude that they are corresponding members of the
common initial sequence of two struct types.
> So address of standard layout class object of type "base" and
> address of standard layout class object whose first non-static
> data member is of type "base" are both addresses of object of
> type "base". Two standard layout objects of type "base" are
> layout compatible, aren't those?
True - but irrelevant. You've dropped the relevant code, but I'm going
to reinstate it in order to make it clear what I'm referring to.
On 1/2/20 6:39 PM, Chris M. Thomasson wrote:
> struct base
> {
> int p0;
> };
>
> struct object_0
> {
> base b;
> int p0;
> };
>
> struct object_1
> {
> base b;
> int p0;
> int p1;
> };
>
> union object
> {
> base b;
> object_0 o0;
> object_1 o1;
> };
>
>
> int main() {
> object o;
I'm in perfect agreement that o.b, o.o0.b and o.o1.b all have the same
type, and therefore have layout-compatible types. I'm also in agreement
that all three have the same address. The fact that o.o0.b and o.01.b
have layout compatible types means that it's perfectly safe to write to
o.o0.b.p0, and read from o.o1.b.p0 - the value you read is guaranteed to
be the same as the the value that was written, because of 9.2p19.
However, no similar conclusion can be derived from the fact that o.b is
layout compatible with the other two objects, nor from the fact that it
has the same address as they do. If you feel otherwise, please cite the
rule that allows it.
The first point to note is that in order to apply 9.2p19, the two
structs being compared must be members of the union. So if you're
looking at o.b and o.o0.b, the struct types that you need to compare are
struct base and struct object_0. o.o0.b is not a member of the union -
it's a member of a member of the union, so it's type is not directly
relevant to the question of whether or not 9.2p19 applies.
The second point to note is that the common initial sequence of two
struct types A and B is an ordered list of pairs of members, each pair
containing one non-static data member of struct A, and one non-static
data member of struct B. The list must be in the same order as the
declarations of the members in each struct types, and the members of
each pair must have layout compatible types.
The only member of o.b is named "p0" and has the type "int". There are
two members of o.o0, named "b" and "p0", which have the types "struct
base" and "int" respectively. If you claim that struct base and struct
object_0 have a common initial sequence, then the first and only member
of that common initial sequence would have to be the first member of o.b
(which is o.b.p0) paired up with the first member of o.o0 (which is
o.o0.b). But o.b.p0 has the type "int", while o.o0.b has the type
"struct base". There's no rule that allows you to conclude that any
struct type is layout compatible with any scalar type.
Let me address that last point in more detail. 3.9p11 identifies three
ways that you can reach the conclusion that two types are layout-compatible:
1. The types cv1 T1 and cv2 T2 (where cv1 and cv2 are optionally empty
lists of type qualifiers) are compatible only if T1 and T2 are the same
type. But "int" and "struct base" are not the same type.
2. Two enumerations can be layout compatible if they have the same
underlying type (7.2p9). But neither "int" nor "struct base" is an
enumeration.
3. Two standard-layout class types can be layout compatible (9.2p17) if
they have the same number of non-static data members and each such
member of one struct type is layout-compatible with the corresponding
member of the other struct type. But "int" is not a class type, and
therefore, in particular, cannot qualify as a standard-layout class type.
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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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