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| From | Kaz Kylheku <kaz@kylheku.com> |
|---|---|
| Newsgroups | comp.lang.c |
| Subject | Re: Size of an arraylist in bytes |
| Date | 2012-02-03 22:35 +0000 |
| Organization | A noiseless patient Spider |
| Message-ID | <20120203135041.829@kylheku.com> (permalink) |
| References | (12 earlier) <kfnvco07wzg.fsf@x-alumni2.alumni.caltech.edu> <ln8vkv5yiz.fsf@nuthaus.mib.org> <kfnsjivz2sa.fsf@x-alumni2.alumni.caltech.edu> <lnzkd2te2w.fsf@nuthaus.mib.org> <eketv8-fdf.ln1@jones.homeip.net> |
On 2012-02-03, lawrence.jones@siemens.com <lawrence.jones@siemens.com> wrote:
> Keith Thompson <kst-u@mib.org> wrote:
>>
>> (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.)
>
> You're welcome. :-)
A very simple, define an lvalue is as an expression which satisfies these three
conditions:
- [lvalue syntactic form] An lvalue is either a certain kind of primary
expression, (identifier or string literal) or its operator falls into a
certain set:
- struct/union member selection (. operator) applied to an expression which
itself has lvalue syntactic form).
- pointer member selection (-> operator)
- pointer dereference or array indexing
If E has the syntactic form of an lvalue, then so does (E) and
denotes the same lvalue as E.
- [lvalue context] An lvalue is the argument of certain operators in certain
positions (those which require lvalues):
- destination operand of assignment
- operand of & address-of
- operand of increment
- ... etc
Expressions not in these contexts are not lvalues even if they have
lvalue syntactic form.
- [lvalue type] An lvalue has object type.
Further, lvalues are then classified as modifiable if they are not array types,
and not const-qualified, and do not have any member which is const-qualified,
(if they are struct or unions).
That's it. These rules should cover everything. There is no need to mention
anything about lvalues anywhere else, except to state that an lvalue (or
perhaps a modifiable one) is required.
And note how it is not necessary to inherit the lvalue attribute semantically.
According to standard C, f().d cannot be an lvalue because the struct returned
by f() is not an lvalue, and x.y is only an lvalue if x is. According to the
above rules, however, f().d is not an lvalue simply because, although it is
a member selection, the base expression f() lacks lvalue syntactic form.
(If you can determine an attribute purely from syntax, there is no need
to compute it inheritance of abstract attributes!)
Also note another subtlety in my rules. Even when x.y is an lvalue, x isn't!
x is only an expression which has the syntactic form of an lvalue. But x
is not in lvalue context (the member selection postifix operator does not
establish one). In the standard rules, x has to be an lvalue for y to be,
because y inherits that attribute from x. But x is not being assigned and its
address isn't being taken: why must it be an lvalue?
Incidentally, wow about f().a where a is an array? Here, C99 introduced a
relaxation from C90, allowing the usage. In C90, this required a diagnostic
because only an lvalue array could produce a pointer to the first element. The
C90 rule could be expressed like this.
Except when it is the operand of the sizeof operator or the unary &
operator, or is a string literal used to initialize an array, an
expression E that has type ‘‘array of type’’, AND THAT HAS LVALUE
SYNTACTIC FORM, is converted to an expression with type ‘‘pointer to
type’’ that points to the initial element of the array object to a
pointer.
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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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