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Re: Variable name that contains line number

Cross-posted to 2 groups.

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On 7/6/2011 17:27, Paul N wrote:
> On Jul 6, 12:06 pm, Michael DOUBEZ<michael.dou...@free.fr>  wrote:
>>
>> #define PCAT( x, y)  x##y
>> #define SET_VAR_NNN(n)  int PCAT(var,n)
>> #define SET_VAR SET_VAR_NNN(__LINE__)
>
> When I saw this question, I suspected the answer would be along those
> lines, but I'm afraid I still can't see why the second form works and
> the first form doesn't. Could someone explain to me?
>
> In my naive way, I would expect the preprocessor to either substitute
> the arguments and then the functions, or vice versa. So I would have
> expected either the first version to work as follows:
>
> SET_VAR; becomes
> SET_VAR_NNN(__LINE__); becomes
> SET_VAR_NNN(6); becomes
> var##6; becomes
> var6;
>
> (ie first version works)
>
> or for the second version to go:
>
> SET_VAR; becomes
> SET_VAR_NNN(__LINE__); becomes
> int PCAT(var,__LINE__); becomes
> int var##__LINE__; becomes
> int var__LINE__;
>
> (ie second version doesn't work).
>
> I can't see what magic it must do to allow the frst form not to work
> but the second to work. I've had a quick look at both the Standard and
> the FAQ, but the penny hasn't dropped.

The relevant portions of the Standard might be 6.10.3.3p2, 6.10.3.3p3, 
6.10.3.4, if I understand them correctly.

   SET_VAR
   /* Replacement list */
   SET_VAR_NNN(__LINE__)
   /* Rescan finds 'SET_VAR_NNN()' and substitutes replacement list */
   int PCAT(var,n)
     /* Argument substitution "all macros contained therein" */
     __LINE__
     6
   /* Argument substitution "after" */
   int PCAT(var,6)
   /* Rescan finds 'PCAT()' and substitutes replacement list*/
   x##y
   /* "...preceded or followed by a ##..." */
   var6
   /* Rescan finds nothing */

Contrast with the original:

   SET_VAR
   /* Replacement list */
   SET_VAR_NNN(__LINE__)
   /* Rescan finds 'SET_VAR_NNN()' and substitutes replacement list */
   int var##n
   /* "...preceded or followed by a ##..." */
   int var__LINE__
   /* Rescan finds nothing */

I hope this helps (and hope it doesn't contain an error)! :)

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Thread

Variable name that contains line number Alex Vinokur <alex.vinokur@gmail.com> - 2011-07-06 03:29 -0700
  Re: Variable name that contains line number Michael DOUBEZ <michael.doubez@free.fr> - 2011-07-06 04:06 -0700
    Re: Variable name that contains line number Alex Vinokur <alex.vinokur@gmail.com> - 2011-07-06 04:17 -0700
      Re: Variable name that contains line number Puppet_Sock <puppet_sock@hotmail.com> - 2011-07-06 06:40 -0700
        Re: Variable name that contains line number Juha Nieminen <nospam@thanks.invalid> - 2011-07-06 15:26 +0000
        Re: Variable name that contains line number "Balog Pal" <pasa@lib.hu> - 2011-07-07 01:17 +0200
    Re: Variable name that contains line number Paul N <gw7rib@aol.com> - 2011-07-06 14:27 -0700
      Re: Variable name that contains line number Alexander Bartolich <alexander.bartolich@gmx.at> - 2011-07-07 01:50 +0000
      Re: Variable name that contains line number Shao Miller <sha0.miller@gmail.com> - 2011-07-07 18:21 -0400

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