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Groups > comp.lang.c > #161995
| From | Ben Bacarisse <ben.usenet@bsb.me.uk> |
|---|---|
| Newsgroups | comp.lang.c |
| Subject | Re: How to disambiguate macro? |
| Date | 2021-07-20 15:36 +0100 |
| Organization | A noiseless patient Spider |
| Message-ID | <87fsw9m4qd.fsf@bsb.me.uk> (permalink) |
| References | <sd6b8j$498$1@reader1.panix.com> <878s21gney.fsf@bsb.me.uk> <sd6h5j$hng$2@reader1.panix.com> |
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 'else' issues at all.
--
Ben.
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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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