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| From | Keith Thompson <kst-u@mib.org> |
|---|---|
| Newsgroups | comp.lang.c |
| Subject | Re: Array sizes and const |
| Date | 2012-06-02 14:59 -0700 |
| Organization | None to speak of |
| Message-ID | <lnmx4lfk8a.fsf@nuthaus.mib.org> (permalink) |
| References | <jqdtou$koe$1@dont-email.me> |
Les Cargill <lcargill99@comcast.com> writes:
> I am using Mingw 3.4.5 on a 'Doze box, but it pretty much
> follows any Gnu compiler I've run into.
>
> Why is this:
>
> ---------------------------
> #define SIZE 20
> const int size = SIZE;
> char array[SIZE];
> ---------------------------
>
> legal, yet
>
> ---------------------------
> #define SIZE 20
> const int size = SIZE;
> char array[size];
> ---------------------------
>
> not?
Both are legal if they occur inside a function definition, where the
second is a variable-length array (VLA) declaration.
At file scope, VLAs are not allowed.
Given:
const int size = 20;
"size" is still considered an ordinary object; a reference to it is
*not* a constant expression. Any compiler worth its salt will convert a
reference to that object to a literal 20 (assuming that results in
faster code), but that's an optimization; in the abstract machine it
fetches the value of the object (which will always be 20).
The key thing to remember is that the "const" keyword doesn't mean
"constant". A "constant" expression is one that can be (and in many
cases must be) evaluated at compile time. The "const" keyword really
means read-only. For example, the following is a perfectly legal
declaration (inside a function):
const int r = rand();
(Note that C++ has slightly different rules; in C++, a reference to
"size" *is* a constant expression.)
--
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"
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Array sizes and const Les Cargill <lcargill99@comcast.com> - 2012-06-02 15:43 -0500
Re: Array sizes and const Ian Collins <ian-news@hotmail.com> - 2012-06-03 08:56 +1200
Re: Array sizes and const Eric Sosman <esosman@ieee-dot-org.invalid> - 2012-06-02 16:59 -0400
Re: Array sizes and const Les Cargill <lcargill99@comcast.com> - 2012-06-02 16:35 -0500
Re: Array sizes and const jgk@panix.com (Joe keane) - 2012-06-08 20:05 +0000
Re: Array sizes and const jgk@panix.com (Joe keane) - 2012-06-09 21:36 +0000
Re: Array sizes and const Les Cargill <lcargill99@comcast.com> - 2012-06-02 16:43 -0500
Re: Array sizes and const Keith Thompson <kst-u@mib.org> - 2012-06-02 14:59 -0700
Re: Array sizes and const Les Cargill <lcargill99@comcast.com> - 2012-06-02 17:12 -0500
Re: Array sizes and const Keith Thompson <kst-u@mib.org> - 2012-06-02 18:27 -0700
Re: Array sizes and const Les Cargill <lcargill99@comcast.com> - 2012-06-02 22:27 -0500
Re: Array sizes and const "BartC" <bc@freeuk.com> - 2012-06-03 11:24 +0100
Re: Array sizes and const Keith Thompson <kst-u@mib.org> - 2012-06-03 13:51 -0700
Re: Array sizes and const "BartC" <bc@freeuk.com> - 2012-06-04 12:20 +0100
Re: Array sizes and const Jens Gustedt <jens.gustedt@loria.fr> - 2012-06-04 13:54 +0200
Re: Array sizes and const Keith Thompson <kst-u@mib.org> - 2012-06-04 14:32 -0700
Re: Array sizes and const Nobody <nobody@nowhere.com> - 2012-06-02 23:50 +0100
Re: Array sizes and const Ian Collins <ian-news@hotmail.com> - 2012-06-03 10:53 +1200
Re: Array sizes and const Nobody <nobody@nowhere.com> - 2012-06-03 14:35 +0100
Re: Array sizes and const James Kuyper <jameskuyper@verizon.net> - 2012-06-02 21:11 -0400
Re: Array sizes and const Malcolm McLean <malcolm.mclean5@btinternet.com> - 2012-06-03 07:39 -0700
Re: Array sizes and const James Kuyper <jameskuyper@verizon.net> - 2012-06-03 16:03 -0400
Re: Array sizes and const Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-06-03 21:49 +0100
Re: Array sizes and const Keith Thompson <kst-u@mib.org> - 2012-06-03 14:49 -0700
Re: Array sizes and const Malcolm McLean <malcolm.mclean5@btinternet.com> - 2012-06-03 15:02 -0700
Re: Array sizes and const Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-06-04 01:21 +0100
Re: Array sizes and const Ian Collins <ian-news@hotmail.com> - 2012-06-04 09:08 +1200
Re: Array sizes and const Malcolm McLean <malcolm.mclean5@btinternet.com> - 2012-06-03 15:08 -0700
Re: Array sizes and const Ian Collins <ian-news@hotmail.com> - 2012-06-04 10:37 +1200
Re: Array sizes and const "BartC" <bc@freeuk.com> - 2012-06-03 23:47 +0100
Re: Array sizes and const Ian Collins <ian-news@hotmail.com> - 2012-06-04 10:55 +1200
Re: Array sizes and const Malcolm McLean <malcolm.mclean5@btinternet.com> - 2012-06-03 16:04 -0700
Re: Array sizes and const Ian Collins <ian-news@hotmail.com> - 2012-06-04 11:14 +1200
Re: Array sizes and const "BartC" <bc@freeuk.com> - 2012-06-04 00:23 +0100
Re: Array sizes and const Ian Collins <ian-news@hotmail.com> - 2012-06-04 11:30 +1200
Re: Array sizes and const "BartC" <bc@freeuk.com> - 2012-06-04 00:52 +0100
Re: Array sizes and const Ian Collins <ian-news@hotmail.com> - 2012-06-04 12:04 +1200
Re: Array sizes and const Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-06-04 01:39 +0100
Re: Array sizes and const Malcolm McLean <malcolm.mclean5@btinternet.com> - 2012-06-04 02:35 -0700
Re: Array sizes and const Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-06-04 01:33 +0100
Re: Array sizes and const Ian Collins <ian-news@hotmail.com> - 2012-06-04 12:35 +1200
Re: Array sizes and const pacman@kosh.dhis.org (Alan Curry) - 2012-06-04 21:28 +0000
Re: Array sizes and const James Kuyper <jameskuyper@verizon.net> - 2012-06-04 12:08 -0400
Re: Array sizes and const jgk@panix.com (Joe keane) - 2012-06-04 22:49 +0000
Re: Array sizes and const David Brown <david.brown@hesbynett.no> - 2012-06-03 15:46 -0500
Re: Array sizes and const Tim Rentsch <txr@alumni.caltech.edu> - 2012-06-08 02:08 -0700
Re: Array sizes and const Les Cargill <lcargill99@comcast.com> - 2012-06-08 06:45 -0500
Re: Array sizes and const Tim Rentsch <txr@alumni.caltech.edu> - 2012-06-08 10:49 -0700
Re: Array sizes and const Les Cargill <lcargill99@comcast.com> - 2012-06-08 18:05 -0500
Re: Array sizes and const Keith Thompson <kst-u@mib.org> - 2012-06-08 13:31 -0700
Re: Array sizes and const Tim Rentsch <txr@alumni.caltech.edu> - 2012-06-08 20:30 -0700
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