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Groups > comp.lang.c > #162005
| From | John Forkosh <forkosh@panix.com> |
|---|---|
| Newsgroups | comp.lang.c |
| Subject | Re: How to disambiguate macro? |
| Date | 2021-07-21 01:48 +0000 |
| Organization | PANIX Public Access Internet and UNIX, NYC |
| Message-ID | <sd7ud2$hkh$1@reader1.panix.com> (permalink) |
| References | <sd6b8j$498$1@reader1.panix.com> <878s21gney.fsf@bsb.me.uk> <sd6h5j$hng$2@reader1.panix.com> <87fsw9m4qd.fsf@bsb.me.uk> |
Ben Bacarisse <ben.usenet@bsb.me.uk> wrote:
> John Forkosh <forkosh@panix.com> writes:
>> Ben Bacarisse <ben.usenet@bsb.me.uk> wrote:
>>> John Forkosh <forkosh@panix.com> writes:
>>>
>>>> Consider a macro of the form
>>>> /* --- set north=parent,south=child,east=next,west=previous link of node --- */
>>>> #define setlink(ptr,nsew,link) if ( checknode(ptr) ) \
>>>> ((node *)(ptr))->nsew = (node *)(link)
>>>> Not important exactly what it does, just the if(xxx)yyy construction.
>>>>
>>>> Now, suppose I want to use it like
>>>> if ( xxx ) setlink(a,b,c); else yyy;
>>>> Then the compiler warns that the binding of that else yyy; is ambiguous.
>>>>
>>>> I clearly mean
>>>> if ( xxx ) {setlink(a,b,c);} else yyy;
>>>> but it's cumbersome and inelegant to write it that way again and again.
>>>>
>>>> Likewise, if I try to define the macro with the {}'s (and the trailing ;)
>>>> #define setlink(ptr,nsew,link) { if ( checknode(ptr) ) \
>>>> ((node *)(ptr))->nsew = (node *)(link); }
>>>> Then writing setlink(a,b,c); in the code has that extraneous ; which
>>>> can then again cause the same kind of confusion. But omitting the ; just
>>>> looks wrong and ugly.
>>>>
>>>> So what's a way to define the macro that simultaneously removes
>>>> any potential semantic confusion without introducing the necessity of
>>>> any unpretty syntax?
>>>
>>> There is a conventional idiom for this:
>>>
>>> #define setlink(ptr,nsew,link) \
>>> do { \
>>> if ( checknode(ptr) ) \
>>> ((node *)(ptr))->nsew = (node *)(link); \
>>> } while (0)
>>>
>>> (The {}s are optional in this case.)
>>
>> Thanks. Okay, I'll try adopting that conventional style.
>> Looks a bit elaborate to me, but if that's the convention
>> then I guess I can get comfortable with it.
>
> Most C programmers won't bat an eyelid at it, but I agree it's clumsy.
>
> When the contingent action is actually just an expression, you can avoid
> the if (...) altogether like this:
>
> #define setlink(ptr,nsew,link) \
> ( checknode(ptr) && (((node *)(ptr))->nsew = (node *)(link)) )
>
> so that there are no 'else' issues at all.
Thanks, again, for the alternative solution. I actually like it
a little better since it avoids the klutzy do-while construction
that has no useful purpose and might only confuse the casual reader,
i.e., "What the heck is he doing that for?" But then again, I never
much liked relying that (a&&b) always evaluates a first, immediately
becoming 0 without evaluating b at all if a is itself 0. You never know
exactly what the next C standard might mess around with.
But your suggestion itself suggested the more explicit version
#define setlink(ptr,nsew,link) \
( checknode(ptr)? (((node *)(ptr))->nsew = (node *)(link)) : 0 )
which I think accomplishes the same thing with explicitly visible logic.
--
John Forkosh ( mailto: j@f.com where j=john and f=forkosh )
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How to disambiguate macro? John Forkosh <forkosh@panix.com> - 2021-07-20 11:15 +0000
Re: How to disambiguate macro? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2021-07-20 13:49 +0100
Re: How to disambiguate macro? John Forkosh <forkosh@panix.com> - 2021-07-20 12:56 +0000
Re: How to disambiguate macro? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2021-07-20 15:36 +0100
Re: How to disambiguate macro? John Forkosh <forkosh@panix.com> - 2021-07-21 01:48 +0000
Re: How to disambiguate macro? Andrey Tarasevich <andreytarasevich@hotmail.com> - 2021-07-20 19:18 -0700
Re: How to disambiguate macro? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2021-07-20 20:27 -0700
Re: How to disambiguate macro? John Forkosh <forkosh@panix.com> - 2021-07-21 04:15 +0000
Re: How to disambiguate macro? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2021-07-21 02:00 -0700
Re: How to disambiguate macro? James Kuyper <jameskuyper@alumni.caltech.edu> - 2021-07-21 06:02 -0400
Re: How to disambiguate macro? James Kuyper <jameskuyper@alumni.caltech.edu> - 2021-07-21 06:00 -0400
Re: How to disambiguate macro? John Forkosh <forkosh@panix.com> - 2021-07-21 10:10 +0000
Re: How to disambiguate macro? Bart <bc@freeuk.com> - 2021-07-21 11:30 +0100
Re: How to disambiguate macro? David Brown <david.brown@hesbynett.no> - 2021-07-21 13:31 +0200
Re: How to disambiguate macro? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2021-07-21 13:02 +0100
Re: How to disambiguate macro? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2021-07-21 14:20 +0100
Re: How to disambiguate macro? Bart <bc@freeuk.com> - 2021-07-21 15:15 +0100
Re: How to disambiguate macro? David Brown <david.brown@hesbynett.no> - 2021-07-21 16:28 +0200
Re: How to disambiguate macro? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2021-07-21 16:20 +0100
Re: How to disambiguate macro? Bart <bc@freeuk.com> - 2021-07-21 17:34 +0100
Re: How to disambiguate macro? David Brown <david.brown@hesbynett.no> - 2021-07-21 19:46 +0200
Re: How to disambiguate macro? Manfred <noname@add.invalid> - 2021-07-21 20:42 +0200
Re: How to disambiguate macro? David Brown <david.brown@hesbynett.no> - 2021-07-22 08:37 +0200
Re: How to disambiguate macro? Bart <bc@freeuk.com> - 2021-07-21 20:47 +0100
Re: How to disambiguate macro? David Brown <david.brown@hesbynett.no> - 2021-07-22 08:47 +0200
Re: How to disambiguate macro? Bart <bc@freeuk.com> - 2021-07-22 11:31 +0100
Re: How to disambiguate macro? antispam@math.uni.wroc.pl - 2021-07-22 14:37 +0000
Re: How to disambiguate macro? David Brown <david.brown@hesbynett.no> - 2021-07-22 18:15 +0200
Re: How to disambiguate macro? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2021-07-21 21:19 +0100
Re: How to disambiguate macro? Manfred <noname@add.invalid> - 2021-07-21 15:42 +0200
Re: How to disambiguate macro? scott@slp53.sl.home (Scott Lurndal) - 2021-07-21 15:39 +0000
Re: How to disambiguate macro? Andrey Tarasevich <andreytarasevich@hotmail.com> - 2021-07-21 09:22 -0700
Re: How to disambiguate macro? James Kuyper <jameskuyper@alumni.caltech.edu> - 2021-07-21 17:24 -0400
Re: How to disambiguate macro? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2021-07-21 16:42 -0700
Re: How to disambiguate macro? John Forkosh <forkosh@panix.com> - 2021-07-22 05:49 +0000
Re: How to disambiguate macro? James Kuyper <jameskuyper@alumni.caltech.edu> - 2021-07-22 19:11 -0400
Re: How to disambiguate macro? John Forkosh <forkosh@panix.com> - 2021-07-22 05:30 +0000
Re: How to disambiguate macro? scott@slp53.sl.home (Scott Lurndal) - 2021-07-22 14:14 +0000
Re: How to disambiguate macro? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2021-07-22 17:06 +0100
OT: possessive adjectives (Was: How to disambiguate macro?) Manfred <noname@add.invalid> - 2021-07-22 15:04 +0200
Re: OT: possessive adjectives Tim Rentsch <tr.17687@z991.linuxsc.com> - 2021-08-14 13:37 -0700
Re: How to disambiguate macro? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2021-08-14 13:58 -0700
Re: How to disambiguate macro? David Brown <david.brown@hesbynett.no> - 2021-07-20 14:50 +0200
Re: How to disambiguate macro? John Forkosh <forkosh@panix.com> - 2021-07-20 13:19 +0000
Re: How to disambiguate macro? Bart <bc@freeuk.com> - 2021-07-20 14:40 +0100
Re: How to disambiguate macro? David Brown <david.brown@hesbynett.no> - 2021-07-20 21:02 +0200
Re: How to disambiguate macro? David Brown <david.brown@hesbynett.no> - 2021-07-20 21:00 +0200
Re: How to disambiguate macro? David Brown <david.brown@hesbynett.no> - 2021-07-21 08:15 +0200
Re: How to disambiguate macro? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2021-08-14 23:17 -0700
Re: How to disambiguate macro? John Forkosh <forkosh@panix.com> - 2021-07-20 12:51 +0000
Re: How to disambiguate macro? Andrey Tarasevich <andreytarasevich@hotmail.com> - 2021-07-20 08:47 -0700
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