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| From | Keith Thompson <kst-u@mib.org> |
|---|---|
| Newsgroups | comp.lang.c |
| Subject | Re: Casts on lvalues |
| Date | 2012-12-05 16:09 -0800 |
| Organization | None to speak of |
| Message-ID | <ln1uf4qbws.fsf@nuthaus.mib.org> (permalink) |
| References | (4 earlier) <tVKvs.786834$A%.8284@fx26.am4> <k9o2b3$bcb$1@dont-email.me> <KBMvs.279623$kt5.191450@fx08.am4> <U4Ovs.586$ww5.519@newsfe01.iad> <qFPvs.640121$9D6.90678@fx27.am4> |
"BartC" <bc@freeuk.com> writes:
> "Greg Martin" <greg@softsprocket.com> wrote in message
> news:U4Ovs.586$ww5.519@newsfe01.iad...
>> On 12-12-05 10:39 AM, BartC wrote:
>>> Plenty of terms can appear interchangeably on both left and right sides.
>>>
>>> But while A can appear on either side, (T)A can't. And there doesn't
>>> appear to be a convincing reason why not.
>>
>> Since you aren't actually changing A by (T)A but rather saying treat it's
>> value like it's a T rather then A's declared type, it doesn't really make
>> sense.
>
> Isn't that the point of having casts?
No. The point of a cast is to explicitly convert a *value* of some
type to a value of some other specified type.
There are several ways in C to treat an object of some type as if it
were an object of some other type. A simple cast is not one of them.
Depending on what you're trying to do, you can take the object's
address, cast it to some other pointer type, and then dereference the
result. You do so at your own risk; it can fail badly if the object
doesn't have the proper alignment for the target type, or if the target
type is larger than the object, or if you assign an invalid value (such
as a trap representation) to the object.
Or you can use a union; this avoids alignment issues, but the other
problems still apply.
Or you can use memcpy() to create a *copy* of an object's representation
in an object of a different type.
>> I can't see the value of momentarily treating A like it's a T just to fool
>> the compiler.
>
> On a platform that you know inside-out, and where the alternative is to do
> exactly the same but using assembly code with all of it's disadvantages,
> there there is plenty of value in doing it.
There are *plenty* of ways to do what you want to do without
resorting to assembly code. It's true that they're not as
syntactically "clean" as a hypothetical lvalue cast. Personally,
I think that's a good thing; type-punning is dangerous enough that
it shouldn't be done casually. If you think it should be easier
to do, that's fine. I don't think *anyone* is 100% happy with the
way C is defined.
I suppose you could define a macros to make it easier:
#include <stdio.h>
#define PUN(object, type) (*(type*)&(object))
int main(void) {
unsigned u;
PUN(u, float) = 1.0/3.0;
printf("u (as float) = %g\n", PUN(u, float));
printf("u = 0x%x\n", u);
return 0;
}
[...]
--
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
Casts on lvalues "BartC" <bc@freeuk.com> - 2012-12-02 13:23 +0000
Re: Casts on lvalues SG <sgesemann@gmail.invalid> - 2012-12-02 14:40 +0100
Re: Casts on lvalues Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-12-02 13:53 +0000
Re: Casts on lvalues "BartC" <bc@freeuk.com> - 2012-12-06 12:26 +0000
Re: Casts on lvalues Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-12-06 15:15 +0000
Re: Casts on lvalues Keith Thompson <kst-u@mib.org> - 2012-12-06 12:05 -0800
Re: Casts on lvalues Keith Thompson <kst-u@mib.org> - 2012-12-02 12:34 -0800
Re: Casts on lvalues "BartC" <bc@freeuk.com> - 2012-12-02 21:18 +0000
Re: Casts on lvalues Willem <willem@turtle.stack.nl> - 2012-12-05 22:36 +0000
Re: Casts on lvalues "BartC" <bc@freeuk.com> - 2012-12-05 23:46 +0000
Re: Casts on lvalues Willem <willem@turtle.stack.nl> - 2012-12-06 18:35 +0000
Re: Casts on lvalues pete <pfiland@mindspring.com> - 2012-12-02 16:21 -0500
Re: Casts on lvalues Eric Sosman <esosman@comcast-dot-net.invalid> - 2012-12-02 17:10 -0500
Re: Casts on lvalues pete <pfiland@mindspring.com> - 2012-12-02 18:50 -0500
Re: Casts on lvalues James Kuyper <jameskuyper@verizon.net> - 2012-12-02 19:14 -0500
Re: Casts on lvalues falk@rahul.net (Edward A. Falk) - 2012-12-04 23:57 +0000
Re: Casts on lvalues "BartC" <bc@freeuk.com> - 2012-12-05 01:35 +0000
Re: Casts on lvalues Piotr Kalinowski <pitkali@gmail.com> - 2012-12-05 16:07 +0100
Re: Casts on lvalues "BartC" <bc@freeuk.com> - 2012-12-05 16:47 +0000
Re: Casts on lvalues Eric Sosman <esosman@comcast-dot-net.invalid> - 2012-12-05 13:04 -0500
Re: Casts on lvalues "BartC" <bc@freeuk.com> - 2012-12-05 18:39 +0000
Re: Casts on lvalues James Kuyper <jameskuyper@verizon.net> - 2012-12-05 14:23 -0500
Re: Casts on lvalues "BartC" <bc@freeuk.com> - 2012-12-05 22:11 +0000
Re: Casts on lvalues Ian Collins <ian-news@hotmail.com> - 2012-12-06 11:51 +1300
Re: Casts on lvalues "BartC" <bc@freeuk.com> - 2012-12-05 23:58 +0000
Re: Casts on lvalues Piotr Kalinowski <pitkali@gmail.com> - 2012-12-06 22:00 +0100
Re: Casts on lvalues glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2012-12-06 03:05 +0000
Re: Casts on lvalues Eric Sosman <esosman@comcast-dot-net.invalid> - 2012-12-05 14:43 -0500
Re: Casts on lvalues Greg Martin <greg@softsprocket.com> - 2012-12-05 12:25 -0800
Re: Casts on lvalues "BartC" <bc@freeuk.com> - 2012-12-05 21:57 +0000
Re: Casts on lvalues Keith Thompson <kst-u@mib.org> - 2012-12-05 16:09 -0800
Re: Casts on lvalues Philipp Thomas <kthomas@linux01.gwdg.de> - 2012-12-07 01:24 +0100
Re: Casts on lvalues Keith Thompson <kst-u@mib.org> - 2012-12-06 17:22 -0800
Re: Casts on lvalues Öö Tiib <ootiib@hot.ee> - 2012-12-07 00:31 -0800
Re: Casts on lvalues James Kuyper <jameskuyper@verizon.net> - 2012-12-07 08:18 -0500
Re: Casts on lvalues Öö Tiib <ootiib@hot.ee> - 2012-12-07 09:07 -0800
Re: Casts on lvalues Keith Thompson <kst-u@mib.org> - 2012-12-07 11:01 -0800
Re: Casts on lvalues glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2012-12-07 21:57 +0000
Re: Casts on lvalues Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-12-07 22:29 +0000
Re: Casts on lvalues glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2012-12-08 00:04 +0000
Re: Casts on lvalues lawrence.jones@siemens.com - 2012-12-15 16:51 -0500
Re: Casts on lvalues Keith Thompson <kst-u@mib.org> - 2012-12-15 15:48 -0800
Re: Casts on lvalues lawrence.jones@siemens.com - 2012-12-17 16:37 -0500
Re: Casts on lvalues Tim Rentsch <txr@alumni.caltech.edu> - 2012-12-17 02:58 -0800
Re: Casts on lvalues Tim Rentsch <txr@alumni.caltech.edu> - 2012-12-17 02:53 -0800
Re: Casts on lvalues Tim Rentsch <txr@alumni.caltech.edu> - 2012-12-17 02:43 -0800
Re: Casts on lvalues Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-12-05 22:07 +0000
Re: Casts on lvalues "BartC" <bc@freeuk.com> - 2012-12-05 22:20 +0000
Re: Casts on lvalues Keith Thompson <kst-u@mib.org> - 2012-12-05 16:10 -0800
Re: Casts on lvalues Philip Lantz <prl@canterey.us> - 2012-12-06 22:49 -0800
Re: Casts on lvalues "BartC" <bc@freeuk.com> - 2012-12-07 12:54 +0000
Re: Casts on lvalues James Kuyper <jameskuyper@verizon.net> - 2012-12-07 08:22 -0500
Re: Casts on lvalues "BartC" <bc@freeuk.com> - 2012-12-07 13:48 +0000
Re: Casts on lvalues James Kuyper <jameskuyper@verizon.net> - 2012-12-07 11:17 -0500
Re: Casts on lvalues glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2012-12-07 17:33 +0000
Re: Casts on lvalues "BartC" <bc@freeuk.com> - 2012-12-07 18:28 +0000
Re: Casts on lvalues glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2012-12-07 22:18 +0000
Re: Casts on lvalues "BartC" <bc@freeuk.com> - 2012-12-07 18:27 +0000
Re: Casts on lvalues James Kuyper <jameskuyper@verizon.net> - 2012-12-07 14:14 -0500
Re: Casts on lvalues "BartC" <bc@freeuk.com> - 2012-12-07 20:16 +0000
Re: Casts on lvalues James Kuyper <jameskuyper@verizon.net> - 2012-12-07 15:41 -0500
Re: Casts on lvalues Tim Rentsch <txr@alumni.caltech.edu> - 2012-12-17 03:03 -0800
Re: Casts on lvalues Phil Carmody <thefatphil_demunged@yahoo.co.uk> - 2012-12-09 17:54 +0200
Re: Casts on lvalues James Kuyper <jameskuyper@verizon.net> - 2012-12-09 12:38 -0500
Re: Casts on lvalues "BartC" <bc@freeuk.com> - 2012-12-09 18:50 +0000
Re: Casts on lvalues James Kuyper <jameskuyper@verizon.net> - 2012-12-09 14:31 -0500
Re: Casts on lvalues Keith Thompson <kst-u@mib.org> - 2012-12-09 20:12 -0800
Re: Casts on lvalues Heikki Kallasjoki <fis+usenet@zem.fi> - 2012-12-10 07:38 +0000
Re: Casts on lvalues James Kuyper <jameskuyper@verizon.net> - 2012-12-10 08:12 -0500
Re: Casts on lvalues Heikki Kallasjoki <fis+usenet@zem.fi> - 2012-12-10 17:46 +0000
Re: Casts on lvalues James Kuyper <jameskuyper@verizon.net> - 2012-12-10 12:59 -0500
Re: Casts on lvalues Keith Thompson <kst-u@mib.org> - 2012-12-10 12:07 -0800
Re: Casts on lvalues Tim Rentsch <txr@alumni.caltech.edu> - 2012-12-17 03:11 -0800
Re: Casts on lvalues Ian Collins <ian-news@hotmail.com> - 2012-12-10 20:50 +1300
Re: Casts on lvalues James Kuyper <jameskuyper@verizon.net> - 2012-12-10 07:49 -0500
Re: Casts on lvalues Rui Maciel <rui.maciel@gmail.com> - 2012-12-07 15:19 +0000
Re: Casts on lvalues "BartC" <bc@freeuk.com> - 2012-12-07 16:54 +0000
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