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| From | Keith Thompson <kst-u@mib.org> |
|---|---|
| Newsgroups | comp.lang.c |
| Subject | Re: Is enum a suitable way to implement a "local define?" |
| Date | 2014-04-09 08:19 -0700 |
| Organization | None to speak of |
| Message-ID | <ln4n22bkec.fsf@nuthaus.mib.org> (permalink) |
| References | (12 earlier) <li093i$svh$1@dont-email.me> <TJ_0v.147431$Bj3.3238@fx09.am4> <li24jo$22j$1@dont-email.me> <lnha63avgt.fsf@nuthaus.mib.org> <li2tqd$aem$1@dont-email.me> |
David Brown <david.brown@hesbynett.no> writes:
> On 09/04/14 08:05, Keith Thompson wrote:
>> David Brown <david.brown@hesbynett.no> writes:
>>> On 09/04/14 00:27, BartC wrote:
>> [...]
>>>> Instead of a simple, logical extension to allow such a declaration,
>>>> where it will be completely obvious what it is and what it can do,
>>>> you instead propose using a *variable* for the purpose, which so many
>>>> issues associated with it that I hardly know where to start! But I
>>>> will have a go:
>>>
>>> A "const" is not a variable - it is a constant object. So instead of
>>> introducing some sort of new type of constant to the language, I would
>>> rather take the existing const concept and extend it slightly (as it is
>>> in C++) to cover the remaining needs for constants. That is far
>>> simpler, clearer and easier than having a new type of constant - and it
>>> already exists in C++.
>>
>> You're conflating "const" and "constant" in a context where the
>> distinction is extremely relevant.
>
> Perhaps - I certainly agree that we must be as clear as possible here.
[...]
>> It's not entirely clear what the word "variable" should mean in C. The
>> standard doesn't use the term; instead it uses "object". Is a
>> const-qualified object a "variable"? You could probably get three
>> different answers from three different people here; mine is that I avoid
>> using the word "variable" when there's any chance of confusion.
>
> I view "variable" to mean something that can legally be changed after
> its creation and initialisation.
That's not an unreasonable definition -- but it's still not entirely
unambiguous. Someone might refer to a const-qualified object as a
"variable". And a member of a non-const struct object is an object that
can be modified, but is it a variable or just part of one? What about
an array element? What about an object allocated by malloc()?
The words "const", "constant", and "object", have well-defined meanings
in C. "Variable" doesn't. I suggest just avoiding that word, at least
in this discussion.
> Thus any object defined as "const"
> will be a constant object, and not a variable.
Um, no, it will be a *read-only* object.
>> "const" in C simply means "read-only". "Constant" refers to something
>> that can (and must) be evaluated at compile time, as in "integer
>> constant" (what some languages call an "integer literal") or a "constant
>> expression" (an expression that can be used as if it were a literal).
>
> Yes, that's true (AFAIUI). But I think we must carefully distinguish
> between file scope const and block scope const, as the rules and effects
> are different.
>
> Because a file scope initialised "const" must be initialised by a
> "constant expression", and it is read only, the compiler knows its value
> at any time - and therefore any use of its value can be replaced by the
> compile-time constant expression. The exceptions here are for things
> like array dimensions, which C disallows even though the compiler is
> guaranteed the required knowledge to generate the code, and C++ can use
> the const objects in such circumstances.
There's not that much difference. The name of a const-qualified object
is not a constant expression, regardless of where it's defined.
Optimizing compilers can evaluate any expression at compile time if they
have enough information. A file-scope object might be defined in one
translation unit and declared in another; the compiler might see a
declaration "const int foo;" and have no way of knowing the value.
A compiler can replace:
int n = 42;
printf("n = %d\n", n);
by the equivalent of:
puts("n = 42");
even though there's no "const" on the declaration.
> At block level (local to a function), const objects become constant
> after their initialisation - you cannot legally change them. But
> (unless they are "static const") they do not necessarily have
> compile-time constant values, and are therefore named read-only copies
> of their initialisers.
The same is true of file-scope const objects. Such an object has an
address, and the value of the initializer is stored at that address.
The ability to refer to that value without necessarily loading it from
memory is merely an optimization.
>> Remember that
>>
>> const int r = rand();
>>
>> is perfectly valid (at block scope); r is const (read-only), but clearly
>> its value cannot be determined until run time.
>
> Correct - but after its initialisation, and throughout its lifetime, "r"
> is constant.
No, it's read-only, not constant. In C, the word "constant"
refers to something that can be evaluated at compile time. We can
reasonably discuss this only if we use words in ways that are
consistent with with the way they're defined by the C standard.
[...]
--
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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Is enum a suitable way to implement a "local define?" partremmaps@gmail.com - 2014-04-05 13:07 -0700
Re: Is enum a suitable way to implement a "local define?" Keith Thompson <kst-u@mib.org> - 2014-04-05 13:55 -0700
Re: Is enum a suitable way to implement a "local define?" Keith Thompson <kst-u@mib.org> - 2014-04-05 14:14 -0700
Re: Is enum a suitable way to implement a "local define?" David Brown <david.brown@hesbynett.no> - 2014-04-06 22:18 +0200
Re: Is enum a suitable way to implement a "local define?" Keith Thompson <kst-u@mib.org> - 2014-04-06 13:28 -0700
Re: Is enum a suitable way to implement a "local define?" James Kuyper <jameskuyper@verizon.net> - 2014-04-06 16:35 -0400
Re: Is enum a suitable way to implement a "local define?" Ian Collins <ian-news@hotmail.com> - 2014-04-07 08:55 +1200
Re: Is enum a suitable way to implement a "local define?" David Brown <david.brown@hesbynett.no> - 2014-04-06 23:30 +0200
Re: Is enum a suitable way to implement a "local define?" Ian Collins <ian-news@hotmail.com> - 2014-04-07 10:12 +1200
Re: Is enum a suitable way to implement a "local define?" James Kuyper <jameskuyper@verizon.net> - 2014-04-06 18:22 -0400
Re: Is enum a suitable way to implement a "local define?" Malcolm McLean <malcolm.mclean5@btinternet.com> - 2014-04-05 14:30 -0700
Re: Is enum a suitable way to implement a "local define?" "BartC" <bc@freeuk.com> - 2014-04-05 22:39 +0100
Re: Is enum a suitable way to implement a "local define?" David Brown <david.brown@hesbynett.no> - 2014-04-06 22:27 +0200
Re: Is enum a suitable way to implement a "local define?" Ian Collins <ian-news@hotmail.com> - 2014-04-07 08:52 +1200
Re: Is enum a suitable way to implement a "local define?" "BartC" <bc@freeuk.com> - 2014-04-06 23:20 +0100
Re: Is enum a suitable way to implement a "local define?" David Brown <david.brown@hesbynett.no> - 2014-04-07 02:17 +0200
Re: Is enum a suitable way to implement a "local define?" Keith Thompson <kst-u@mib.org> - 2014-04-06 19:00 -0700
Re: Is enum a suitable way to implement a "local define?" glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2014-04-07 04:44 +0000
Re: Is enum a suitable way to implement a "local define?" "BartC" <bc@freeuk.com> - 2014-04-07 10:15 +0100
Re: Is enum a suitable way to implement a "local define?" glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2014-04-07 18:32 +0000
Re: Is enum a suitable way to implement a "local define?" Kaz Kylheku <kaz@kylheku.com> - 2014-04-07 18:47 +0000
Re: Is enum a suitable way to implement a "local define?" Alain Ketterlin <alain@dpt-info.u-strasbg.fr> - 2014-04-07 11:39 +0200
Re: Is enum a suitable way to implement a "local define?" David Brown <david.brown@hesbynett.no> - 2014-04-07 08:58 +0200
Re: Is enum a suitable way to implement a "local define?" Ian Collins <ian-news@hotmail.com> - 2014-04-07 19:34 +1200
Re: Is enum a suitable way to implement a "local define?" "BartC" <bc@freeuk.com> - 2014-04-07 09:41 +0100
Re: Is enum a suitable way to implement a "local define?" Ian Collins <ian-news@hotmail.com> - 2014-04-07 21:45 +1200
Re: Is enum a suitable way to implement a "local define?" "BartC" <bc@freeuk.com> - 2014-04-07 11:03 +0100
Re: Is enum a suitable way to implement a "local define?" David Brown <david.brown@hesbynett.no> - 2014-04-07 12:54 +0200
Re: Is enum a suitable way to implement a "local define?" "BartC" <bc@freeuk.com> - 2014-04-07 13:48 +0100
Re: Is enum a suitable way to implement a "local define?" David Brown <david.brown@hesbynett.no> - 2014-04-07 15:27 +0200
Re: Is enum a suitable way to implement a "local define?" Ian Collins <ian-news@hotmail.com> - 2014-04-07 23:00 +1200
Re: Is enum a suitable way to implement a "local define?" David Brown <david.brown@hesbynett.no> - 2014-04-07 11:31 +0200
Re: Is enum a suitable way to implement a "local define?" Les Cargill <lcargill99@comcast.com> - 2014-04-07 07:24 -0500
Re: Is enum a suitable way to implement a "local define?" David Brown <david.brown@hesbynett.no> - 2014-04-07 15:02 +0200
Re: Is enum a suitable way to implement a "local define?" "BartC" <bc@freeuk.com> - 2014-04-07 10:19 +0100
Re: Is enum a suitable way to implement a "local define?" David Brown <david.brown@hesbynett.no> - 2014-04-07 12:43 +0200
Re: Is enum a suitable way to implement a "local define?" "BartC" <bc@freeuk.com> - 2014-04-07 13:27 +0100
Re: Is enum a suitable way to implement a "local define?" David Brown <david.brown@hesbynett.no> - 2014-04-07 15:16 +0200
Re: Is enum a suitable way to implement a "local define?" "BartC" <bc@freeuk.com> - 2014-04-07 14:43 +0100
Re: Is enum a suitable way to implement a "local define?" David Brown <david.brown@hesbynett.no> - 2014-04-07 16:44 +0200
Re: Is enum a suitable way to implement a "local define?" "BartC" <bc@freeuk.com> - 2014-04-07 16:52 +0100
Re: Is enum a suitable way to implement a "local define?" James Kuyper <jameskuyper@verizon.net> - 2014-04-07 12:53 -0400
Re: Is enum a suitable way to implement a "local define?" James Kuyper <jameskuyper@verizon.net> - 2014-04-07 12:59 -0400
Re: Is enum a suitable way to implement a "local define?" "BartC" <bc@freeuk.com> - 2014-04-07 18:12 +0100
Re: Is enum a suitable way to implement a "local define?" Richard <rgrdev_@gmail.com> - 2014-04-07 18:27 +0100
Re: Is enum a suitable way to implement a "local define?" "BartC" <bc@freeuk.com> - 2014-04-08 23:22 +0100
Re: Is enum a suitable way to implement a "local define?" James Kuyper <jameskuyper@verizon.net> - 2014-04-08 18:44 -0400
Re: Is enum a suitable way to implement a "local define?" "BartC" <bc@freeuk.com> - 2014-04-09 00:22 +0100
Re: Is enum a suitable way to implement a "local define?" David Brown <david.brown@hesbynett.no> - 2014-04-08 09:39 +0200
Re: Is enum a suitable way to implement a "local define?" "BartC" <bc@freeuk.com> - 2014-04-08 23:27 +0100
Re: Is enum a suitable way to implement a "local define?" Ian Collins <ian-news@hotmail.com> - 2014-04-09 10:52 +1200
Re: Is enum a suitable way to implement a "local define?" "BartC" <bc@freeuk.com> - 2014-04-09 00:24 +0100
Re: Is enum a suitable way to implement a "local define?" Ian Collins <ian-news@hotmail.com> - 2014-04-09 11:32 +1200
Re: Is enum a suitable way to implement a "local define?" David Brown <david.brown@hesbynett.no> - 2014-04-09 08:58 +0200
Re: Is enum a suitable way to implement a "local define?" James Kuyper <jameskuyper@verizon.net> - 2014-04-08 19:54 -0400
Re: Is enum a suitable way to implement a "local define?" Kaz Kylheku <kaz@kylheku.com> - 2014-04-09 00:41 +0000
Re: Is enum a suitable way to implement a "local define?" glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2014-04-08 23:05 +0000
Re: Is enum a suitable way to implement a "local define?" Stephen Sprunk <stephen@sprunk.org> - 2014-04-08 18:24 -0500
Re: Is enum a suitable way to implement a "local define?" James Kuyper <jameskuyper@verizon.net> - 2014-04-08 19:39 -0400
Re: Is enum a suitable way to implement a "local define?" Keith Thompson <kst-u@mib.org> - 2014-04-08 22:41 -0700
Re: Is enum a suitable way to implement a "local define?" David Brown <david.brown@hesbynett.no> - 2014-04-09 09:16 +0200
Re: Is enum a suitable way to implement a "local define?" James Kuyper <jameskuyper@verizon.net> - 2014-04-09 07:27 -0400
Re: Is enum a suitable way to implement a "local define?" David Brown <david.brown@hesbynett.no> - 2014-04-09 15:32 +0200
Re: Is enum a suitable way to implement a "local define?" Keith Thompson <kst-u@mib.org> - 2014-04-09 08:32 -0700
Re: Is enum a suitable way to implement a "local define?" David Brown <david.brown@hesbynett.no> - 2014-04-10 11:03 +0200
Re: Is enum a suitable way to implement a "local define?" James Kuyper <jameskuyper@verizon.net> - 2014-04-10 07:27 -0400
Re: Is enum a suitable way to implement a "local define?" David Brown <david.brown@hesbynett.no> - 2014-04-10 14:02 +0200
Re: Is enum a suitable way to implement a "local define?" Keith Thompson <kst-u@mib.org> - 2014-04-10 08:34 -0700
Re: Is enum a suitable way to implement a "local define?" glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2014-04-10 16:36 +0000
Re: Is enum a suitable way to implement a "local define?" James Kuyper <jameskuyper@verizon.net> - 2014-04-10 12:58 -0400
Re: Is enum a suitable way to implement a "local define?" James Kuyper <jameskuyper@verizon.net> - 2014-04-09 11:36 -0400
Re: Is enum a suitable way to implement a "local define?" glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2014-04-09 18:36 +0000
Re: Is enum a suitable way to implement a "local define?" Stephen Sprunk <stephen@sprunk.org> - 2014-04-09 14:11 -0500
Re: Is enum a suitable way to implement a "local define?" David Brown <david.brown@hesbynett.no> - 2014-04-10 13:55 +0200
Re: Is enum a suitable way to implement a "local define?" David Brown <david.brown@hesbynett.no> - 2014-04-09 02:35 +0200
Re: Is enum a suitable way to implement a "local define?" Keith Thompson <kst-u@mib.org> - 2014-04-08 23:05 -0700
Re: Is enum a suitable way to implement a "local define?" David Brown <david.brown@hesbynett.no> - 2014-04-09 09:45 +0200
Re: Is enum a suitable way to implement a "local define?" Keith Thompson <kst-u@mib.org> - 2014-04-09 08:19 -0700
Re: Is enum a suitable way to implement a "local define?" Richard <rgrdev_@gmail.com> - 2014-04-10 09:08 +0100
Re: Is enum a suitable way to implement a "local define?" Malcolm McLean <malcolm.mclean5@btinternet.com> - 2014-04-10 02:57 -0700
Re: Is enum a suitable way to implement a "local define?" Richard <rgrdev_@gmail.com> - 2014-04-10 11:50 +0100
Re: Is enum a suitable way to implement a "local define?" Seungbeom Kim <musiphil@bawi.org> - 2014-04-10 11:37 -0700
Re: Is enum a suitable way to implement a "local define?" James Kuyper <jameskuyper@verizon.net> - 2014-04-10 14:52 -0400
Re: Is enum a suitable way to implement a "local define?" Kaz Kylheku <kaz@kylheku.com> - 2014-04-10 18:53 +0000
Re: Is enum a suitable way to implement a "local define?" glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2014-04-10 19:36 +0000
Re: Is enum a suitable way to implement a "local define?" James Kuyper <jameskuyper@verizon.net> - 2014-04-10 15:59 -0400
Re: Is enum a suitable way to implement a "local define?" glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2014-04-10 20:12 +0000
Re: Is enum a suitable way to implement a "local define?" James Kuyper <jameskuyper@verizon.net> - 2014-04-10 16:18 -0400
Re: Is enum a suitable way to implement a "local define?" Martin Shobe <martin.shobe@yahoo.com> - 2014-04-11 10:24 -0500
Re: Is enum a suitable way to implement a "local define?" Martin Shobe <martin.shobe@yahoo.com> - 2014-04-11 14:36 -0500
Re: Is enum a suitable way to implement a "local define?" David Thompson <dave.thompson2@verizon.net> - 2014-05-25 16:44 -0400
Re: Is enum a suitable way to implement a "local define?" Keith Thompson <kst-u@mib.org> - 2014-04-10 12:47 -0700
Re: Is enum a suitable way to implement a "local define?" James Kuyper <jameskuyper@verizon.net> - 2014-04-10 16:05 -0400
Re: Is enum a suitable way to implement a "local define?" Keith Thompson <kst-u@mib.org> - 2014-04-10 13:34 -0700
Re: Is enum a suitable way to implement a "local define?" James Kuyper <jameskuyper@verizon.net> - 2014-04-10 16:59 -0400
Re: Is enum a suitable way to implement a "local define?" Keith Thompson <kst-u@mib.org> - 2014-04-10 16:01 -0700
Re: Is enum a suitable way to implement a "local define?" Tim Rentsch <txr@alumni.caltech.edu> - 2014-04-14 21:24 -0700
Re: Is enum a suitable way to implement a "local define?" Tim Rentsch <txr@alumni.caltech.edu> - 2014-04-14 21:36 -0700
Re: Is enum a suitable way to implement a "local define?" glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2014-04-15 05:26 +0000
Re: Is enum a suitable way to implement a "local define?" James Kuyper <jameskuyper@verizon.net> - 2014-04-15 07:18 -0400
Re: Is enum a suitable way to implement a "local define?" Tim Rentsch <txr@alumni.caltech.edu> - 2014-04-18 09:01 -0700
Please ignore trolls Noob <root@127.0.0.1> - 2014-04-09 15:26 +0200
Re: Please ignore trolls David Brown <david.brown@hesbynett.no> - 2014-04-09 15:38 +0200
Re: Please ignore trolls James Kuyper <jameskuyper@verizon.net> - 2014-04-09 12:02 -0400
Re: Please ignore trolls Kaz Kylheku <kaz@kylheku.com> - 2014-04-09 19:18 +0000
Re: Please ignore trolls "BartC" <bc@freeuk.com> - 2014-04-10 20:18 +0100
Re: Please ignore trolls James Kuyper <jameskuyper@verizon.net> - 2014-04-10 15:49 -0400
Re: Please ignore trolls gazelle@shell.xmission.com (Kenny McCormack) - 2014-04-10 20:14 +0000
Re: Please ignore trolls "BartC" <bc@freeuk.com> - 2014-04-11 14:35 +0100
Re: Please ignore trolls Malcolm McLean <malcolm.mclean5@btinternet.com> - 2014-04-11 07:35 -0700
Re: Please ignore trolls "BartC" <bc@freeuk.com> - 2014-04-11 15:51 +0100
Re: Please ignore trolls Jorgen Grahn <grahn+nntp@snipabacken.se> - 2014-04-12 15:48 +0000
Re: Please ignore trolls Kaz Kylheku <kaz@kylheku.com> - 2014-04-10 21:16 +0000
Re: Please ignore trolls "BartC" <bc@freeuk.com> - 2014-04-12 15:35 +0100
Re: Please ignore trolls Keith Thompson <kst-u@mib.org> - 2014-04-09 08:03 -0700
Re: Please ignore trolls gazelle@shell.xmission.com (Kenny McCormack) - 2014-04-09 15:25 +0000
Re: Is enum a suitable way to implement a "local define?" Ben Bacarisse <ben.usenet@bsb.me.uk> - 2014-04-09 21:28 +0100
Re: Is enum a suitable way to implement a "local define?" Ian Collins <ian-news@hotmail.com> - 2014-04-10 08:39 +1200
Re: Is enum a suitable way to implement a "local define?" Kaz Kylheku <kaz@kylheku.com> - 2014-04-09 23:38 +0000
Re: Is enum a suitable way to implement a "local define?" Ben Bacarisse <ben.usenet@bsb.me.uk> - 2014-04-10 02:07 +0100
Re: Is enum a suitable way to implement a "local define?" Ike Naar <ike@iceland.freeshell.org> - 2014-04-09 05:18 +0000
Re: Is enum a suitable way to implement a "local define?" Keith Thompson <kst-u@mib.org> - 2014-04-09 08:05 -0700
Re: Is enum a suitable way to implement a "local define?" Keith Thompson <kst-u@mib.org> - 2014-04-07 08:33 -0700
Re: Is enum a suitable way to implement a "local define?" "BartC" <bc@freeuk.com> - 2014-04-07 17:16 +0100
Re: Is enum a suitable way to implement a "local define?" Keith Thompson <kst-u@mib.org> - 2014-04-07 11:07 -0700
Re: Is enum a suitable way to implement a "local define?" David Brown <david.brown@hesbynett.no> - 2014-04-08 09:49 +0200
Re: Is enum a suitable way to implement a "local define?" Ian Collins <ian-news@hotmail.com> - 2014-04-08 20:33 +1200
Re: Is enum a suitable way to implement a "local define?" David Brown <david.brown@hesbynett.no> - 2014-04-08 12:31 +0200
Re: Is enum a suitable way to implement a "local define?" James Kuyper <jameskuyper@verizon.net> - 2014-04-08 21:59 -0400
Re: Is enum a suitable way to implement a "local define?" Keith Thompson <kst-u@mib.org> - 2014-04-08 07:37 -0700
Re: Is enum a suitable way to implement a "local define?" Stephen Sprunk <stephen@sprunk.org> - 2014-04-08 08:35 -0500
Re: Is enum a suitable way to implement a "local define?" Seungbeom Kim <musiphil@bawi.org> - 2014-04-08 11:30 -0700
Re: Is enum a suitable way to implement a "local define?" "BartC" <bc@freeuk.com> - 2014-04-08 23:46 +0100
Re: Is enum a suitable way to implement a "local define?" Stephen Sprunk <stephen@sprunk.org> - 2014-04-08 18:28 -0500
Re: Is enum a suitable way to implement a "local define?" Seungbeom Kim <musiphil@bawi.org> - 2014-04-08 17:21 -0700
Re: Is enum a suitable way to implement a "local define?" "BartC" <bc@freeuk.com> - 2014-04-09 09:07 +0100
Re: Is enum a suitable way to implement a "local define?" James Kuyper <jameskuyper@verizon.net> - 2014-04-09 07:22 -0400
Re: Is enum a suitable way to implement a "local define?" Keith Thompson <kst-u@mib.org> - 2014-04-06 16:25 -0700
Re: Is enum a suitable way to implement a "local define?" David Brown <david.brown@hesbynett.no> - 2014-04-07 02:22 +0200
Re: Is enum a suitable way to implement a "local define?" Ike Naar <ike@iceland.freeshell.org> - 2014-04-06 06:08 +0000
Re: Is enum a suitable way to implement a "local define?" jacob navia <jacob@spamsink.net> - 2014-04-06 08:54 +0200
Re: Is enum a suitable way to implement a "local define?" partremmaps@gmail.com - 2014-04-07 23:48 -0700
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