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Re: lvalues from the ternary op

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supercat@casperkitty.com writes:
> On Wednesday, January 18, 2017 at 2:45:36 PM UTC-6, Keith Thompson wrote:
>> > I don't think I'd seen that paper before.  It looks nice.  I don't think
>> > there word "potential" should be seen a problem in that context, though I
>> > think it might be better to say that an lvalue represents a "view" of a
>> > potential object.  A successful malloc(1000) creates a single object, and
>> > while code may create many views of different parts of it, that doesn't
>> > make each view "an object".
>> 
>> I'm not at all sure what point you're making here.
>> 
>> The word "potentially" was added to make it clear that *ptr is an lvalue
>> even if ptr's value happens (at run time) to be a null pointer, or some
>> other pointer value that cannot validly be dereferenced.
>
> I understood your use of the term "potential", but you seemed uneasy
> about it.  I see no reason for any unease.

I'm uneasy because I don't think it's entirely clear what it means.
If you're already aware of the particular issue (*ptr where ptr may
or may not be dereferenceable), it's probably fairly obvious that
the added word was meant to address it, but if you're not, it might
not be clear what "potentially designates" means.  I had hoped, when
I made the suggestion, that the committee would add some wording at
least informally explaining what "potentially" means in that context.

> A bigger issue is that an lvalue isn't really "an object", but rather a 
> view.

No, an lvalue is not an object, and nobody ever said it was.

An lvalue is a kind of expression.  As such, it is an entity that exists
in C source code, before any objects yet exist.

>        Given at file scope, what objects are created by:
>
>    struct { uint32_t x, y; } s1;
>    union { uint32_t x; double y; } u1;
>
> I would say those declarations would create two objects, s1 and s2.
>
> The lvalue s1 would be a view of all eight bytes of storage occupied by s1;
> s1.x would be a view of the first four bytes of that, and s1.y would be
> a view of the other four bytes.  Likewise u1 would be a view of the eight
> bytes occupied by u1, u1.x would be a view of the first four bytes, and
> u1.y would be a view of all eight bytes.

s1, s1.x, s1.y, u1, u1.x, and u1.y are all objects.  If you disagree,
see the standard's definition of "object", and explain how any of them
doesn't qualify.  I'm using "s1" as the name of the struct object
created by the declaration, not using "s1" as an expression; likewise
for the others.

> If an lvalue is defined as a view, then the effect of "." becomes straight-
> forward: given a view to a region of storage, yield a view to a subset of
> that storage.  Describing the effect in terms of "objects" would be less
> clear, since it would require a structure and union definitions each create
> multiple overlapping objects.

But structure and union definitions *do* create multiple overlapping
objects.  Introducing this idea of a "view" is neither necessary nor
helpful.

-- 
Keith Thompson (The_Other_Keith) kst-u@mib.org  <http://www.ghoti.net/~kst>
Working, but not speaking, for JetHead Development, Inc.
"We must do something.  This is something.  Therefore, we must do this."
    -- Antony Jay and Jonathan Lynn, "Yes Minister"

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Re: lvalues from the ternary op Manfred <noname@invalid.add> - 2017-01-04 19:46 +0100
  Re: lvalues from the ternary op supercat@casperkitty.com - 2017-01-04 11:08 -0800
  Re: lvalues from the ternary op Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-01-04 21:10 +0000
    Re: lvalues from the ternary op Keith Thompson <kst-u@mib.org> - 2017-01-04 13:48 -0800
      Re: lvalues from the ternary op Ian Collins <ian-news@hotmail.com> - 2017-01-05 11:07 +1300
        Re: lvalues from the ternary op Keith Thompson <kst-u@mib.org> - 2017-01-04 14:20 -0800
      Re: lvalues from the ternary op supercat@casperkitty.com - 2017-01-04 14:41 -0800
        Re: lvalues from the ternary op Keith Thompson <kst-u@mib.org> - 2017-01-04 15:21 -0800
          Re: lvalues from the ternary op supercat@casperkitty.com - 2017-01-04 15:52 -0800
            Re: lvalues from the ternary op Keith Thompson <kst-u@mib.org> - 2017-01-04 16:37 -0800
            Re: lvalues from the ternary op Keith Thompson <kst-u@mib.org> - 2017-01-04 18:31 -0800
              Re: lvalues from the ternary op Tim Rentsch <txr@alumni.caltech.edu> - 2017-01-13 17:26 -0800
                Re: lvalues from the ternary op Keith Thompson <kst-u@mib.org> - 2017-01-13 17:49 -0800
                Re: lvalues from the ternary op Tim Rentsch <txr@alumni.caltech.edu> - 2017-01-15 22:53 -0800
    Re: lvalues from the ternary op Tim Rentsch <txr@alumni.caltech.edu> - 2017-01-13 17:08 -0800
      Re: lvalues from the ternary op Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-01-14 02:15 +0000
        Re: lvalues from the ternary op Tim Rentsch <txr@alumni.caltech.edu> - 2017-01-15 22:51 -0800
      Re: lvalues from the ternary op luser droog <luser.droog@gmail.com> - 2017-01-16 23:09 -0800
        Re: lvalues from the ternary op Manfred <noname@invalid.add> - 2017-01-18 14:51 +0100
          Re: lvalues from the ternary op Keith Thompson <kst-u@mib.org> - 2017-01-18 09:59 -0800
            Re: lvalues from the ternary op Lew Pitcher <lew.pitcher@digitalfreehold.ca> - 2017-01-18 13:12 -0500
            Re: lvalues from the ternary op supercat@casperkitty.com - 2017-01-18 12:11 -0800
              Re: lvalues from the ternary op Keith Thompson <kst-u@mib.org> - 2017-01-18 12:45 -0800
                Re: lvalues from the ternary op supercat@casperkitty.com - 2017-01-18 14:07 -0800
                Re: lvalues from the ternary op Keith Thompson <kst-u@mib.org> - 2017-01-18 14:40 -0800
                Re: lvalues from the ternary op supercat@casperkitty.com - 2017-01-18 16:10 -0800
                Re: lvalues from the ternary op Keith Thompson <kst-u@mib.org> - 2017-01-18 18:08 -0800
                Re: lvalues from the ternary op supercat@casperkitty.com - 2017-01-19 06:44 -0800
                Re: lvalues from the ternary op Keith Thompson <kst-u@mib.org> - 2017-01-19 08:40 -0800
                Re: lvalues from the ternary op supercat@casperkitty.com - 2017-01-19 12:32 -0800
                Re: lvalues from the ternary op supercat@casperkitty.com - 2017-01-20 06:01 -0800
                Re: lvalues from the ternary op Keith Thompson <kst-u@mib.org> - 2017-01-20 08:15 -0800
                Re: lvalues from the ternary op supercat@casperkitty.com - 2017-01-20 10:51 -0800
                Re: lvalues from the ternary op Keith Thompson <kst-u@mib.org> - 2017-01-20 11:32 -0800
                Re: lvalues from the ternary op supercat@casperkitty.com - 2017-01-20 13:41 -0800
                Re: lvalues from the ternary op supercat@casperkitty.com - 2017-01-23 07:04 -0800
                Re: lvalues from the ternary op Ike Naar <ike@iceland.freeshell.org> - 2017-01-23 17:59 +0000
                Re: lvalues from the ternary op supercat@casperkitty.com - 2017-01-23 10:57 -0800
                Re: lvalues from the ternary op Tim Rentsch <txr@alumni.caltech.edu> - 2017-01-31 08:45 -0800
                Re: lvalues from the ternary op Keith Thompson <kst-u@mib.org> - 2017-01-31 09:13 -0800
                Re: lvalues from the ternary op supercat@casperkitty.com - 2017-01-31 10:17 -0800
                Re: lvalues from the ternary op Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-02-01 01:51 +0000
                Re: lvalues from the ternary op supercat@casperkitty.com - 2017-02-01 14:24 -0800
                Re: lvalues from the ternary op Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-02-02 03:07 +0000
                Re: lvalues from the ternary op Tim Rentsch <txr@alumni.caltech.edu> - 2017-02-04 04:43 -0800
                Re: lvalues from the ternary op supercat@casperkitty.com - 2017-02-04 08:01 -0800
                Re: lvalues from the ternary op Tim Rentsch <txr@alumni.caltech.edu> - 2017-02-09 09:06 -0800
                Re: lvalues from the ternary op supercat@casperkitty.com - 2017-02-09 11:56 -0800
                Re: lvalues from the ternary op Tim Rentsch <txr@alumni.caltech.edu> - 2017-02-12 10:29 -0800
                Re: lvalues from the ternary op supercat@casperkitty.com - 2017-02-12 13:43 -0800
                Re: lvalues from the ternary op Tim Rentsch <txr@alumni.caltech.edu> - 2017-02-19 21:44 -0800
                Re: lvalues from the ternary op supercat@casperkitty.com - 2017-02-20 08:15 -0800
                Re: lvalues from the ternary op Tim Rentsch <txr@alumni.caltech.edu> - 2017-02-24 07:58 -0800
                Re: lvalues from the ternary op supercat@casperkitty.com - 2017-02-24 10:00 -0800
                Re: lvalues from the ternary op scott@slp53.sl.home (Scott Lurndal) - 2017-02-24 18:58 +0000
                Re: lvalues from the ternary op supercat@casperkitty.com - 2017-02-24 13:03 -0800
                Re: lvalues from the ternary op Keith Thompson <kst-u@mib.org> - 2017-02-24 14:11 -0800
                Re: lvalues from the ternary op Keith Thompson <kst-u@mib.org> - 2017-02-24 12:42 -0800
                Re: lvalues from the ternary op Keith Thompson <kst-u@mib.org> - 2017-02-24 13:49 -0800
                Re: lvalues from the ternary op supercat@casperkitty.com - 2017-02-24 13:58 -0800
                Re: lvalues from the ternary op Keith Thompson <kst-u@mib.org> - 2017-02-24 14:56 -0800
                Re: lvalues from the ternary op supercat <flatfinger@casperkitty.com> - 2017-02-25 14:00 -0800
                Re: lvalues from the ternary op Tim Rentsch <txr@alumni.caltech.edu> - 2017-02-25 09:23 -0800
                Re: lvalues from the ternary op Tim Rentsch <txr@alumni.caltech.edu> - 2017-02-25 09:43 -0800
                Re: lvalues from the ternary op Kelsey Bjarnason <kbjarnason@gmail.com> - 2017-02-01 22:47 -0800
                Re: lvalues from the ternary op supercat@casperkitty.com - 2017-02-02 10:12 -0800
                Re: lvalues from the ternary op Tim Rentsch <txr@alumni.caltech.edu> - 2017-02-04 07:51 -0800
                Re: lvalues from the ternary op supercat@casperkitty.com - 2017-02-04 08:18 -0800
                Re: lvalues from the ternary op Tim Rentsch <txr@alumni.caltech.edu> - 2017-02-09 08:58 -0800
                Re: lvalues from the ternary op Tim Rentsch <txr@alumni.caltech.edu> - 2017-02-04 04:00 -0800
                Re: lvalues from the ternary op Kelsey Bjarnason <kbjarnason@gmail.com> - 2017-02-04 09:35 -0800
                Re: lvalues from the ternary op Tim Rentsch <txr@alumni.caltech.edu> - 2017-02-04 09:35 -0800
                Re: lvalues from the ternary op Robert Wessel <robertwessel2@yahoo.com> - 2017-01-31 13:36 -0600
                Re: lvalues from the ternary op supercat@casperkitty.com - 2017-01-31 12:23 -0800
                Re: lvalues from the ternary op Tim Rentsch <txr@alumni.caltech.edu> - 2017-02-01 13:48 -0800
                Re: lvalues from the ternary op Tim Rentsch <txr@alumni.caltech.edu> - 2017-01-31 08:53 -0800
            Re: lvalues from the ternary op Manfred <noname@invalid.add> - 2017-01-19 14:10 +0100
        Re: lvalues from the ternary op Tim Rentsch <txr@alumni.caltech.edu> - 2017-01-30 12:05 -0800

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