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Re: Preprocessor expressions

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On 22/01/2017 15:52, jameskuyper@verizon.net wrote:
> On Sunday, January 22, 2017 at 8:24:30 AM UTC-5, Bart wrote:
>> These are the ones that occur after #if and #elif.
>>
>> The standard 6.10p1 says these are constant-expressions. But the latter
>> (A.2.1) can apparently include casts, ++, sizeof, ., ->, [] and  and all
>> the rest.
>
> Keep in mind that, in an #if constant-expression, all of the following occur
> before the condition is interpreted as a constant-expression:
> 1. macros that are not the argument of the "defined" unary operator are
> expanded.

> 2. occurrences of the "defined" unary operator are replaced by either 0 or 1,
> depending upon whether or not it's argument matches the name of a defined macro.
> 3. All remaining identifiers are replaced with the pp-number 0. This occurs
> BEFORE conversion of pre-processing tokens into tokens (6.10.1p4).
> Keep in mind that keywords qualify only as tokens, and not as pre-processing
> tokens (6.4p1). At this point in the translation process, things that will
> later be identified as keywords are parsed only as identifiers - and the
> conversion described above applies to those identifiers.
>
> Therefore, (struct tm*) would be converted to (0 0*), and (int (*)(double))
> would be converted to (0(*)(0), both of which are syntax errors. Offhand, I
> don't see any way for a #if condition to contain an actual cast.
>
> For both the prefix and postfix versions of the ++ and -- operators, there is a
> constraint that their argument be a modifiable lvalue. With all identifiers
> converted to 0, how could that constraint ever be satisfied?
>
> For both the "." and "->", the left operand is required to have a struct or
> union type. With all identifiers (including typedefs, struct and union tags,
> and all of the keywords used to specify types) replaced with 0, how could you
> form such an operand?

OK, thanks. That's starting to make more sense (but a dedicated grammar 
for pp expressions I think would have been useful).

> With all identifiers converted to 0, the only way to form an array is using a
> string literal. "abcdef"[0] is therefore permitted in a #if.

gcc (and most compilers I've tried) tells me that "abcdef" is not a 
valid pp token. And neither is "[", "->", ".", "++" and so on.

(I've been using trial and error to figure these things out, to try and 
build a reduced syntax for these expressions.)

-- 
Bartc

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Preprocessor expressions BartC <bc@freeuk.com> - 2017-01-22 13:24 +0000
  Re: Preprocessor expressions jameskuyper@verizon.net - 2017-01-22 07:52 -0800
    Re: Preprocessor expressions Ralf Damaschke <rwspam@gmx.de> - 2017-01-22 17:15 +0000
      Re: Preprocessor expressions "James R. Kuyper" <jameskuyper@verizon.net> - 2017-01-22 13:32 -0500
    Re: Preprocessor expressions BartC <bc@freeuk.com> - 2017-01-22 17:38 +0000
      Re: Preprocessor expressions Thiago Adams <thiago.adams@gmail.com> - 2017-02-01 03:12 -0800
        Re: Preprocessor expressions Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-02-01 11:44 +0000
        Re: Preprocessor expressions BartC <bc@freeuk.com> - 2017-02-01 11:56 +0000
        Re: Preprocessor expressions jameskuyper@verizon.net - 2017-02-01 08:35 -0800
          Re: Preprocessor expressions Thiago Adams <thiago.adams@gmail.com> - 2017-02-02 09:23 -0800
            Re: Preprocessor expressions BartC <bc@freeuk.com> - 2017-02-02 18:14 +0000
              Re: Preprocessor expressions Keith Thompson <kst-u@mib.org> - 2017-02-02 10:35 -0800
              Re: Preprocessor expressions Thiago Adams <thiago.adams@gmail.com> - 2017-02-02 17:09 -0800
                Re: Preprocessor expressions Thiago Adams <thiago.adams@gmail.com> - 2017-02-05 10:09 -0800
                Re: Preprocessor expressions Thiago Adams <thiago.adams@gmail.com> - 2017-02-05 10:38 -0800
                Re: Preprocessor expressions Thiago Adams <thiago.adams@gmail.com> - 2017-02-05 11:03 -0800
                Re: Preprocessor expressions BartC <bc@freeuk.com> - 2017-02-06 15:17 +0000
                Re: Preprocessor expressions Thiago Adams <thiago.adams@gmail.com> - 2017-02-07 05:36 -0800
                Re: Preprocessor expressions BartC <bc@freeuk.com> - 2017-02-07 21:01 +0000
                Re: Preprocessor expressions supercat@casperkitty.com - 2017-02-07 13:16 -0800
                Re: Preprocessor expressions BartC <bc@freeuk.com> - 2017-02-07 22:09 +0000
                Re: Preprocessor expressions supercat@casperkitty.com - 2017-02-07 14:33 -0800
                Re: Preprocessor expressions Keith Thompson <kst-u@mib.org> - 2017-02-07 14:52 -0800
                Re: Preprocessor expressions BartC <bc@freeuk.com> - 2017-02-08 00:13 +0000
                Re: Preprocessor expressions Keith Thompson <kst-u@mib.org> - 2017-02-07 16:39 -0800
                Re: Preprocessor expressions jameskuyper@verizon.net - 2017-02-07 16:47 -0800
                Re: Preprocessor expressions David Brown <david.brown@hesbynett.no> - 2017-02-08 10:03 +0100
                Re: Preprocessor expressions Keith Thompson <kst-u@mib.org> - 2017-02-08 09:19 -0800
                Re: Preprocessor expressions Thiago Adams <thiago.adams@gmail.com> - 2017-02-08 12:35 -0800
                Re: Preprocessor expressions Thiago Adams <thiago.adams@gmail.com> - 2017-02-09 05:25 -0800
                Re: Preprocessor expressions supercat@casperkitty.com - 2017-02-09 07:37 -0800
                Re: Preprocessor expressions BartC <bc@freeuk.com> - 2017-02-09 15:52 +0000
  Re: Preprocessor expressions BartC <bc@freeuk.com> - 2017-01-22 19:34 +0000
    Re: Preprocessor expressions jameskuyper@verizon.net - 2017-01-22 13:01 -0800
  Re: Preprocessor expressions Tim Rentsch <txr@alumni.caltech.edu> - 2017-01-30 05:30 -0800

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