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| From | Keith Thompson <kst-u@mib.org> |
|---|---|
| Newsgroups | comp.lang.c |
| Subject | Re: Size of an arraylist in bytes |
| Date | 2012-02-01 13:53 -0800 |
| Organization | None to speak of |
| Message-ID | <lnzkd2te2w.fsf@nuthaus.mib.org> (permalink) |
| References | (10 earlier) <ae41b513-aef7-41ec-ae29-53f28d0aeb2f@v5g2000yqn.googlegroups.com> <lnhb1wva2q.fsf@nuthaus.mib.org> <kfnvco07wzg.fsf@x-alumni2.alumni.caltech.edu> <ln8vkv5yiz.fsf@nuthaus.mib.org> <kfnsjivz2sa.fsf@x-alumni2.alumni.caltech.edu> |
Tim Rentsch <txr@alumni.caltech.edu> writes:
> Keith Thompson <kst-u@mib.org> writes:
>> Tim Rentsch <txr@alumni.caltech.edu> writes:
>>> Keith Thompson <kst-u@mib.org> writes:
>>>> Nick Keighley <nick_keighley_nospam@hotmail.com> writes:
>>>>> On Nov 21, 9:25 pm, Joe Wright <joewwri...@comcast.net> wrote:
>>>>>> On 11/21/2011 15:06, Stefan Ram wrote:
>>>> [...]
>>>>>> A pointer is a variable object in storage which can hold the address of
>>>>>> another object in storage. The address is a value, the pointer is an object.
>>>>>>
>>>>>> This concept is too complicated (or too simple) to some of my old friends
>>>>>> but it works for me. Keith Thompson beat me into semi-submission several
>>>>>> years ago until I got too tired to play. But here I am again.
>>>>>>
>>>>>> A pointer is an object. A function cannot return an object.
>>>>>
>>>>> surely this happens all the time
>>>>>
>>>>> struct point
>>>>> {
>>>>> int x;
>>>>> int y;
>>>>> };
>>>>>
>>>>> struct point make_point (int a, int b)
>>>>> { struct point p; p.x = a; p.y = b; return p; }
>>>>>
>>>>> isn't that returning an object? In fact doesn't this return an object?
>>>>>
>>>>> int f (void) { return 1; }
>>>>
>>>> I'd say no. In both cases, the function returns a value, not an
>>>> object. (There might be a "region of data storage" (from C99 3.14,
>>>> the definition of "object") that's allocated to hold the value, but
>>>> logically what the function returns is the value, not the object.)
>>>>
>>>> The value returned by a call to
>>>>
>>>> int f (void) { return 1; }
>>>>
>>>> is no more an object than the evaluation of the RHS of
>>>>
>>>> n = 1;
>>>>
>>>> results in an object. In particular, objects generally have
>>>> addresses; function results don't.
>>>
>>> In C99 this point could be argued. Under the new Standard,
>>> however, it's clear that struct/union expressions do in
>>> fact return objects, not just values. See 6.2.4 p8 (N1548).
>>
>> (N1570 is newer.)
>
> It is. Are N1548 and N1570 different in any way other than the
> presence or absence of change bars?
I don't know (is there a "diff" for PDF files?), but they're dated
2010-12-02 and 2011-04-12, respectively. I presume the latter
reflects whatever changes the committee made in those 4+ months.
N1570 has change bars marking changes relative to N1539.
[...]
>> In C99, there's no more an "object" created by func() [which
>> returns a struct that has an array member] than there is one
>> created by "int foo(void) { return 42; }". [snip C11 comment]
>
> The Standard doesn't say so, but I argue that for all practical
> purposes there is, because of how array conversion works. In
> particular, the resulting pointer value is AFAIK valid, eg, can
> be compared against NULL or other pointers, etc.
>
>
>> If the struct returned by the function doesn't contain an array
>> member, then it's still covered by the old rules which don't imply
>> that there's an object. (Implementations might treat both cases
>> the same way, but they needn't do so.)
>
> Since member selection can be applied to the results of such
> functions, the semantic descriptions of '.' and '->' imply that
> there is.
Here's what both N1256 (C99) and N1570 (C11) 6.5.2.3p3 say about the
semantics of "." (C90 is very similar.)
A postfix expression followed by the . operator and an identifier
designates a member of a structure or union object. The value
is that of the named member, and is an lvalue if the first
expression is an lvalue. If the first expression has qualified
type, the result has the so-qualified version of the type of
the designated member.
The first sentence clearly implies that the prefix must refer to
an object. The second sentence, though talks about what happens
if the prefix is not an lvalue. An lvalue is an expression that
(potentially(!)) designates an object.
So if the prefix isn't an lvalue, and therefore doesn't designate an
object, what object's member does the expression designate?
Personally, I think it would have made more sense to say that the
prefix is an expression (lvalue or not) of struct or union type, and
that x.y, if x is not an lvalue, yields the value of the member of
the struct or union value. In other words, values can have member
values even if there's no object in sight. (Probably not the best
possible wording.)
(An aside: I've complained before that the C90 and C99 standards'
definitions of "lvalue" were flawed, and suggested "expression that
potentially designates an object" to fix the problem. The word
"potentially" is needed to ensure that *ptr is an lvalue even if
ptr==NULL. I just now noticed that N1570 has this updated wording.
Looks like somebody was listening. I am inordinately pleased.)
--
Keith Thompson (The_Other_Keith) kst-u@mib.org <http://www.ghoti.net/~kst>
Will write code for food.
"We must do something. This is something. Therefore, we must do this."
-- Antony Jay and Jonathan Lynn, "Yes Minister"
Back to comp.lang.c | Previous | Next — Previous in thread | Next in thread | Find similar | Unroll thread
Re: Size of an arraylist in bytes Joe Wright <joewwright@comcast.net> - 2011-11-21 16:25 -0500
Re: Size of an arraylist in bytes Keith Thompson <kst-u@mib.org> - 2011-11-21 14:03 -0800
Re: Size of an arraylist in bytes pete <pfiland@mindspring.com> - 2011-11-21 17:45 -0500
Re: Size of an arraylist in bytes Ark <akhasin@macroexpressions.com> - 2011-11-21 21:19 -0500
Re: Size of an arraylist in bytes Eric Sosman <esosman@ieee-dot-org.invalid> - 2011-11-21 22:06 -0500
Re: Size of an arraylist in bytes Ark <akhasin@macroexpressions.com> - 2011-11-22 03:07 -0500
Re: Size of an arraylist in bytes James Kuyper <jameskuyper@verizon.net> - 2011-11-22 06:51 -0500
Re: an instance of an object (was: Size of an arraylist in bytes) Keith Thompson <kst-u@mib.org> - 2011-11-21 21:54 -0800
Re: Size of an arraylist in bytes pete <pfiland@mindspring.com> - 2011-11-22 01:49 -0500
Re: Size of an arraylist in bytes Ark <akhasin@macroexpressions.com> - 2011-11-22 03:17 -0500
Re: Size of an arraylist in bytes Keith Thompson <kst-u@mib.org> - 2011-11-22 00:48 -0800
Re: Size of an arraylist in bytes pete <pfiland@mindspring.com> - 2011-11-23 05:16 -0500
Re: Size of an arraylist in bytes Kaz Kylheku <kaz@kylheku.com> - 2011-11-22 16:49 +0000
Re: Size of an arraylist in bytes pete <pfiland@mindspring.com> - 2011-11-23 09:30 -0500
Re: Size of an arraylist in bytes Nick Keighley <nick_keighley_nospam@hotmail.com> - 2011-11-22 00:07 -0800
Re: Size of an arraylist in bytes Keith Thompson <kst-u@mib.org> - 2011-11-22 00:24 -0800
Re: Size of an arraylist in bytes Tim Rentsch <txr@alumni.caltech.edu> - 2012-01-25 05:06 -0800
Re: Size of an arraylist in bytes Keith Thompson <kst-u@mib.org> - 2012-01-25 12:15 -0800
Re: Size of an arraylist in bytes Tim Rentsch <txr@alumni.caltech.edu> - 2012-01-31 18:49 -0800
Re: Size of an arraylist in bytes Keith Thompson <kst-u@mib.org> - 2012-02-01 13:53 -0800
Re: Size of an arraylist in bytes lawrence.jones@siemens.com - 2012-02-01 18:53 -0500
Re: Size of an arraylist in bytes Keith Thompson <kst-u@mib.org> - 2012-02-01 20:06 -0800
Re: Size of an arraylist in bytes Shao Miller <sha0.miller@gmail.com> - 2012-02-01 21:40 -0500
Re: Size of an arraylist in bytes jgk@panix.com (Joe keane) - 2012-02-02 03:45 +0000
Re: Size of an arraylist in bytes Keith Thompson <kst-u@mib.org> - 2012-02-01 20:14 -0800
Re: Size of an arraylist in bytes Eric Sosman <esosman@ieee-dot-org.invalid> - 2012-02-01 23:14 -0500
Re: Size of an arraylist in bytes Shao Miller <sha0.miller@gmail.com> - 2012-02-02 18:38 -0500
Re: Size of an arraylist in bytes jgk@panix.com (Joe keane) - 2012-02-03 16:32 +0000
Re: Size of an arraylist in bytes James Kuyper <jameskuyper@verizon.net> - 2012-02-03 11:57 -0500
Re: Size of an arraylist in bytes Shao Miller <sha0.miller@gmail.com> - 2012-02-03 20:07 -0500
Re: Size of an arraylist in bytes jgk@panix.com (Joe keane) - 2012-02-04 22:35 +0000
Re: Size of an arraylist in bytes James Kuyper <jameskuyper@verizon.net> - 2012-02-04 18:43 -0500
Re: Size of an arraylist in bytes Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-02-05 01:50 +0000
Re: Size of an arraylist in bytes James Kuyper <jameskuyper@verizon.net> - 2012-02-04 21:37 -0500
Re: Size of an arraylist in bytes Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-02-05 14:48 +0000
Re: Size of an arraylist in bytes James Kuyper <jameskuyper@verizon.net> - 2012-02-05 10:20 -0500
Re: Size of an arraylist in bytes Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-02-05 16:25 +0000
Re: Size of an arraylist in bytes James Kuyper <jameskuyper@verizon.net> - 2012-02-05 12:06 -0500
Re: Size of an arraylist in bytes Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-02-05 19:21 +0000
Re: Size of an arraylist in bytes Shao Miller <sha0.miller@gmail.com> - 2012-02-05 15:29 -0500
Re: Size of an arraylist in bytes Tim Rentsch <txr@alumni.caltech.edu> - 2012-03-08 10:46 -0800
Re: Size of an arraylist in bytes Kaz Kylheku <kaz@kylheku.com> - 2012-02-05 02:39 +0000
Re: Size of an arraylist in bytes Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-02-05 15:17 +0000
Re: Size of an arraylist in bytes James Kuyper <jameskuyper@verizon.net> - 2012-02-05 12:06 -0500
Re: Size of an arraylist in bytes Shao Miller <sha0.miller@gmail.com> - 2012-02-05 15:06 -0500
Re: Size of an arraylist in bytes jgk@panix.com (Joe keane) - 2012-02-07 17:35 +0000
Re: Size of an arraylist in bytes Shao Miller <sha0.miller@gmail.com> - 2012-02-07 19:51 -0500
Re: Size of an arraylist in bytes Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-02-08 01:14 +0000
Re: Size of an arraylist in bytes Kaz Kylheku <kaz@kylheku.com> - 2012-02-05 02:31 +0000
Re: Size of an arraylist in bytes Eric Sosman <esosman@ieee-dot-org.invalid> - 2012-02-03 21:26 -0500
Re: Size of an arraylist in bytes Eric Sosman <esosman@ieee-dot-org.invalid> - 2012-02-03 21:31 -0500
Re: Size of an arraylist in bytes James Kuyper <jameskuyper@verizon.net> - 2012-02-04 06:57 -0500
Re: Size of an arraylist in bytes Kaz Kylheku <kaz@kylheku.com> - 2012-02-04 18:32 +0000
Re: Size of an arraylist in bytes Shao Miller <sha0.miller@gmail.com> - 2012-02-05 22:51 -0500
Re: Size of an arraylist in bytes jgk@panix.com (Joe keane) - 2012-02-04 21:25 +0000
Re: Size of an arraylist in bytes Eric Sosman <esosman@ieee-dot-org.invalid> - 2012-02-04 17:08 -0500
Re: Size of an arraylist in bytes Tim Rentsch <txr@alumni.caltech.edu> - 2012-03-08 10:52 -0800
Re: Size of an arraylist in bytes James Kuyper <jameskuyper@verizon.net> - 2012-02-04 17:32 -0500
Re: Size of an arraylist in bytes Kaz Kylheku <kaz@kylheku.com> - 2012-02-05 02:19 +0000
Re: Size of an arraylist in bytes Eric Sosman <esosman@ieee-dot-org.invalid> - 2012-02-05 09:20 -0500
Re: Size of an arraylist in bytes James Kuyper <jameskuyper@verizon.net> - 2012-02-05 09:42 -0500
Re: Size of an arraylist in bytes Shao Miller <sha0.miller@gmail.com> - 2012-02-05 16:17 -0500
Re: Size of an arraylist in bytes Zoltan Kocsi <zoltan@bendor.com.au> - 2012-02-05 17:49 +1100
Re: Size of an arraylist in bytes Shao Miller <sha0.miller@gmail.com> - 2012-02-05 16:48 -0500
Re: Size of an arraylist in bytes Shao Miller <sha0.miller@gmail.com> - 2012-02-05 16:52 -0500
Re: Size of an arraylist in bytes Shao Miller <sha0.miller@gmail.com> - 2012-02-02 01:07 -0500
Re: Size of an arraylist in bytes lawrence.jones@siemens.com - 2012-02-03 11:29 -0500
Re: Size of an arraylist in bytes Keith Thompson <kst-u@mib.org> - 2012-02-03 12:42 -0800
lvalue "potentially" designates an object (was Re: Size of an arraylist in bytes) Kenneth Brody <kenbrody@spamcop.net> - 2012-02-04 12:47 -0500
Re: lvalue "potentially" designates an object (was Re: Size of an arraylist in bytes) Kaz Kylheku <kaz@kylheku.com> - 2012-02-04 18:39 +0000
Re: Size of an arraylist in bytes lawrence.jones@siemens.com - 2012-02-04 14:46 -0500
Re: Size of an arraylist in bytes Kaz Kylheku <kaz@kylheku.com> - 2012-02-03 22:35 +0000
Re: Size of an arraylist in bytes Shao Miller <sha0.miller@gmail.com> - 2012-02-06 00:01 -0500
Re: Size of an arraylist in bytes Kaz Kylheku <kaz@kylheku.com> - 2012-02-06 08:46 +0000
Re: Size of an arraylist in bytes Shao Miller <sha0.miller@gmail.com> - 2012-02-06 23:17 -0500
Re: Size of an arraylist in bytes lawrence.jones@siemens.com - 2012-02-01 18:34 -0500
Re: Size of an arraylist in bytes Tim Rentsch <txr@alumni.caltech.edu> - 2012-03-08 10:54 -0800
Re: Size of an arraylist in bytes Tim Rentsch <txr@alumni.caltech.edu> - 2012-01-25 05:00 -0800
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