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| From | Keith Thompson <kst-u@mib.org> |
|---|---|
| Newsgroups | comp.lang.c |
| Subject | Re: Issues with setting variables to true or false: how does the compiler understands it |
| Date | 2015-10-24 16:28 -0700 |
| Organization | None to speak of |
| Message-ID | <ln7fmbu9qd.fsf@kst-u.example.com> (permalink) |
| References | <ba14ded1-d9f7-477e-a00f-cc47104c31f6@googlegroups.com> <562BAA43.8090700@verizon.net> <ln1tckum2z.fsf@kst-u.example.com> <d84e0f4b-aeab-411c-bf96-49c96113647e@googlegroups.com> |
supercat@casperkitty.com writes:
> On Saturday, October 24, 2015 at 2:02:20 PM UTC-5, Keith Thompson wrote:
>> Not at all. I find `if (pointer != NULL)` clearer than `if (pointer)`
>> because the `!= NULL` makes it clear at a glance that it's a pointer
>> comparison. We're checking whether `pointer` is unequal to a null
>> pointer, not to 0, 0.0, or '\0'. (And in most real-world code, pointer
>> variables aren't named "pointer".)
>
> In expressions like "if (ptr != null)" or "if ((foo & 1)!= 0)" I consider
> the "null" and "0" to be "phony literals", since what the expressions are
You mean NULL, not null.
> really seeking to evaluate is the validity of ptr or the "zero-ness"
> of (foo & 1). While I appreciate that directly interpreting pointers or
> integers as true/false values is hokey, I regard phony literals as being
> somewhat hokey as well.
>
> When testing a pointer against null, one isn't really interested in
> comparing the pointer matches a null pointer, but whether it's valid.
No, if I write `if (ptr != NULL)`, I'm testing whether ptr is unequal to
the null pointer. If it's equal to NULL, it's because that's the most
recent value I stored in it. I might also be *interested* in knowing
whether it's valid, but I have no way of testing that, so I don't
bother.
> The fact that there is only one testable representation for a valid pointer
> is coincidental.
I disagree; there's nothing coincidental about it. `if (ptr)` and
`if (ptr != NULL)` are semantically identical. The latter expresses
my intent more clearly.
[...]
> If one were designing a language from scratch, it would probably make sense
> to have an operator to explicitly test whether something is null, or to
> test whether two values have any bits in common, but given that C implicitly
> regards null pointers and zeroes as "false" and other pointers and integer
> values as "true", there probably isn't a huge need for such things on modern
> systems (though on a sign-magnitude system I could see a use for the special
> bit-wise operator, since if one wanted to test whether a number was either
> negative or odd, the expression "if (num & -1)" wouldn't be problematic for
> negative even numbers, since even though -0 would have a bit set it would
> still be considered "false").
If I were designing a language from scratch, rather than having some
special operator to test whether a pointer is null, I'd just define
equality for pointers and a constant whose value is a null pointer --
and not permit pointer values to be used directly as conditions. I've
used several languages that work that way. If you happen to prefer the
way C does it, that's fine; I'm merely expressing my preference.
--
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"
Back to comp.lang.c | Previous | Next — Previous in thread | Next in thread | Find similar | Unroll thread
Issues with setting variables to true or false: how does the compiler understands it Alla _ <modelling.data@gmail.com> - 2015-10-24 02:17 -0700
Issues with setting variables to true or false: how does the compiler understands it asetofsymbols@gmail.com - 2015-10-24 03:29 -0700
Re: Issues with setting variables to true or false: how does the compiler understands it Alla _ <modelling.data@gmail.com> - 2015-10-24 03:38 -0700
Re: Issues with setting variables to true or false: how does the compiler understands it Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-10-24 04:30 -0700
Re: Issues with setting variables to true or false: how does the compiler understands it gazelle@shell.xmission.com (Kenny McCormack) - 2015-10-24 12:04 +0000
Issues with setting variables to true or false: how does the compiler understands it asetofsymbols@gmail.com - 2015-10-24 04:59 -0700
Re: Issues with setting variables to true or false: how does the compiler understands it Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-10-24 14:28 +0100
Re: Issues with setting variables to true or false: how does the compiler understands it asetofsymbols@gmail.com - 2015-10-24 07:26 -0700
Re: Issues with setting variables to true or false: how does the compiler understands it Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-10-24 17:06 +0100
Re: Issues with setting variables to true or false: how does the compiler understands it James Kuyper <jameskuyper@verizon.net> - 2015-10-24 11:56 -0400
Re: Issues with setting variables to true or false: how does the compiler understands it Keith Thompson <kst-u@mib.org> - 2015-10-24 12:02 -0700
Re: Issues with setting variables to true or false: how does the compiler understands it supercat@casperkitty.com - 2015-10-24 15:45 -0700
Re: Issues with setting variables to true or false: how does the compiler understands it Keith Thompson <kst-u@mib.org> - 2015-10-24 16:28 -0700
Re: Issues with setting variables to true or false: how does the compiler understands it Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-10-24 16:44 -0700
Re: Issues with setting variables to true or false: how does the compiler understands it supercat@casperkitty.com - 2015-10-24 19:34 -0700
Re: Issues with setting variables to true or false: how does the compiler understands it supercat@casperkitty.com - 2015-10-24 19:30 -0700
Re: Issues with setting variables to true or false: how does the compiler understands it Keith Thompson <kst-u@mib.org> - 2015-10-24 20:33 -0700
Re: Issues with setting variables to true or false: how does the compiler understands it Tim Rentsch <txr@alumni.caltech.edu> - 2015-10-28 11:16 -0700
Re: Issues with setting variables to true or false: how does the compiler understands it Eric Sosman <esosman@comcast-dot-net.invalid> - 2015-10-28 15:12 -0400
Re: Issues with setting variables to true or false: how does the compiler understands it Jorgen Grahn <grahn+nntp@snipabacken.se> - 2015-10-29 12:18 +0000
Re: Issues with setting variables to true or false: how does the compiler understands it raltbos@xs4all.nl (Richard Bos) - 2015-10-29 21:51 +0000
Re: Issues with setting variables to true or false: how does the compiler understands it Tim Rentsch <txr@alumni.caltech.edu> - 2015-10-30 08:38 -0700
Re: Issues with setting variables to true or false: how does the compiler understands it Tim Rentsch <txr@alumni.caltech.edu> - 2015-10-30 05:34 -0700
Re: Issues with setting variables to true or false: how does the compiler understands it Keith Thompson <kst-u@mib.org> - 2015-10-28 14:08 -0700
Re: Issues with setting variables to true or false: how does the compiler understands it Tim Rentsch <txr@alumni.caltech.edu> - 2015-10-30 06:14 -0700
Re: Issues with setting variables to true or false: how does the compiler understands it glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-10-28 21:50 +0000
Re: Issues with setting variables to true or false: how does the compiler understands it supercat@casperkitty.com - 2015-10-28 15:08 -0700
Re: Issues with setting variables to true or false: how does the compiler understands it glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-10-28 22:47 +0000
Re: Issues with setting variables to true or false: how does the compiler understands it Robert Wessel <robertwessel2@yahoo.com> - 2015-10-28 18:35 -0500
Re: Issues with setting variables to true or false: how does the compiler understands it supercat@casperkitty.com - 2015-10-28 18:12 -0700
Re: Issues with setting variables to true or false: how does the compiler understands it glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-10-29 01:35 +0000
Re: Issues with setting variables to true or false: how does the compiler understands it Robert Wessel <robertwessel2@yahoo.com> - 2015-10-28 21:12 -0500
Re: Issues with setting variables to true or false: how does the compiler understands it supercat@casperkitty.com - 2015-10-28 19:52 -0700
Re: Issues with setting variables to true or false: how does the compiler understands it supercat@casperkitty.com - 2015-10-28 18:01 -0700
Re: Issues with setting variables to true or false: how does the compiler understands it Tim Rentsch <txr@alumni.caltech.edu> - 2015-10-30 03:47 -0700
Re: Issues with setting variables to true or false: how does the compiler understands it Barry Schwarz <schwarzb@dqel.com> - 2015-10-24 09:54 -0700
Re: Issues with setting variables to true or false: how does the compiler understands it Keith Thompson <kst-u@mib.org> - 2015-10-24 12:13 -0700
Re: Issues with setting variables to true or false: how does the compiler understands it Richard Heathfield <rjh@cpax.org.uk> - 2015-10-24 21:50 +0100
Re: Issues with setting variables to true or false: how does the compiler understands it Alla _ <modelling.data@gmail.com> - 2015-10-25 04:04 -0700
Re: Issues with setting variables to true or false: how does the compiler understands it Barry Schwarz <schwarzb@dqel.com> - 2015-10-25 08:32 -0700
Re: Issues with setting variables to true or false: how does the compiler understands it Richard Heathfield <rjh@cpax.org.uk> - 2015-10-25 18:19 +0000
Re: Issues with setting variables to true or false: how does the compiler understands it Alla _ <modelling.data@gmail.com> - 2015-10-25 11:30 -0700
Re: Issues with setting variables to true or false: how does the compiler understands it Richard Heathfield <rjh@cpax.org.uk> - 2015-10-25 18:56 +0000
Re: Issues with setting variables to true or false: how does the compiler understands it Alla _ <modelling.data@gmail.com> - 2015-10-25 20:59 -0700
Re: Issues with setting variables to true or false: how does the compiler understands it Anand Hariharan <mailto.anand.hariharan@gmail.com> - 2015-10-25 21:49 -0700
Re: Issues with setting variables to true or false: how does the compiler understands it Richard Heathfield <rjh@cpax.org.uk> - 2015-10-26 08:10 +0000
Re: Issues with setting variables to true or false: how does the compiler understands it Anand Hariharan <mailto.anand.hariharan@gmail.com> - 2015-10-26 16:05 -0700
Re: Issues with setting variables to true or false: how does the compiler understands it Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-10-27 00:34 +0000
Re: Issues with setting variables to true or false: how does the compiler understands it Keith Thompson <kst-u@mib.org> - 2015-10-27 09:04 -0700
Re: Issues with setting variables to true or false: how does the compiler understands it Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-10-27 21:24 +0000
Re: Issues with setting variables to true or false: how does the compiler understands it Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-10-26 18:16 -0700
Re: Issues with setting variables to true or false: how does the compiler understands it Richard Heathfield <rjh@cpax.org.uk> - 2015-10-27 02:33 +0000
Re: Issues with setting variables to true or false: how does the compiler understands it Keith Thompson <kst-u@mib.org> - 2015-10-27 09:08 -0700
Re: Issues with setting variables to true or false: how does the compiler understands it Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-10-26 11:22 +0000
Re: Issues with setting variables to true or false: how does the compiler understands it Richard Heathfield <rjh@cpax.org.uk> - 2015-10-26 11:35 +0000
Re: Issues with setting variables to true or false: how does the compiler understands it Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-10-26 17:00 +0000
Re: Issues with setting variables to true or false: how does the compiler understands it Richard Heathfield <rjh@cpax.org.uk> - 2015-10-26 17:59 +0000
Re: Issues with setting variables to true or false: how does the compiler understands it Keith Thompson <kst-u@mib.org> - 2015-10-26 07:55 -0700
Re: Issues with setting variables to true or false: how does the compiler understands it Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-10-26 17:06 +0000
Re: Issues with setting variables to true or false: how does the compiler understands it supercat@casperkitty.com - 2015-10-26 10:56 -0700
Re: Issues with setting variables to true or false: how does the compiler understands it glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-10-26 20:14 +0000
Re: Issues with setting variables to true or false: how does the compiler understands it Keith Thompson <kst-u@mib.org> - 2015-10-26 17:09 -0700
Re: Issues with setting variables to true or false: how does the compiler understands it Bartc <bc@freeuk.com> - 2015-10-26 23:39 +0000
Re: Issues with setting variables to true or false: how does the compiler understands it Jorgen Grahn <grahn+nntp@snipabacken.se> - 2015-10-27 12:31 +0000
Re: Issues with setting variables to true or false: how does the compiler understands it Richard Heathfield <rjh@cpax.org.uk> - 2015-10-27 12:45 +0000
Re: Issues with setting variables to true or false: how does the compiler understands it Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-10-27 13:24 +0000
Re: Issues with setting variables to true or false: how does the compiler understands it Richard Heathfield <rjh@cpax.org.uk> - 2015-10-27 13:41 +0000
Re: Issues with setting variables to true or false: how does the compiler understands it Jorgen Grahn <grahn+nntp@snipabacken.se> - 2015-10-27 19:43 +0000
Re: Issues with setting variables to true or false: how does the compiler understands it Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-10-27 19:47 +0000
Re: Issues with setting variables to true or false: how does the compiler understands it Bartc <bc@freeuk.com> - 2015-10-27 13:20 +0000
Re: Issues with setting variables to true or false: how does the compiler understands it Richard Heathfield <rjh@cpax.org.uk> - 2015-10-27 13:28 +0000
Re: Issues with setting variables to true or false: how does the compiler understands it Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-10-27 13:54 +0000
Re: Issues with setting variables to true or false: how does the compiler understands it supercat@casperkitty.com - 2015-10-27 07:51 -0700
Re: Issues with setting variables to true or false: how does the compiler understands it Bartc <bc@freeuk.com> - 2015-10-27 16:46 +0000
Re: Issues with setting variables to true or false: how does the compiler understands it supercat@casperkitty.com - 2015-10-27 10:52 -0700
Re: Issues with setting variables to true or false: how does the compiler understands it Bartc <bc@freeuk.com> - 2015-10-27 18:51 +0000
Re: Issues with setting variables to true or false: how does the compiler understands it supercat@casperkitty.com - 2015-10-27 12:17 -0700
Re: Issues with setting variables to true or false: how does the compiler understands it Bartc <bc@freeuk.com> - 2015-10-27 19:34 +0000
Re: Issues with setting variables to true or false: how does the compiler understands it supercat@casperkitty.com - 2015-10-27 13:10 -0700
Re: Issues with setting variables to true or false: how does the compiler understands it Keith Thompson <kst-u@mib.org> - 2015-10-27 13:41 -0700
Re: Issues with setting variables to true or false: how does the compiler understands it Bartc <bc@freeuk.com> - 2015-10-27 23:11 +0000
Re: Issues with setting variables to true or false: how does the compiler understands it Keith Thompson <kst-u@mib.org> - 2015-10-27 19:02 -0700
Re: Issues with setting variables to true or false: how does the compiler understands it gazelle@shell.xmission.com (Kenny McCormack) - 2015-10-28 07:09 +0000
Re: Issues with setting variables to true or false: how does the compiler understands it Jorgen Grahn <grahn+nntp@snipabacken.se> - 2015-10-28 11:27 +0000
Re: Issues with setting variables to true or false: how does the compiler understands it Keith Thompson <kst-u@mib.org> - 2015-10-27 12:27 -0700
Re: Issues with setting variables to true or false: how does the compiler understands it supercat@casperkitty.com - 2015-10-27 13:18 -0700
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