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From: Keith Thompson <kst-u@mib.org>
Newsgroups: comp.lang.c
Subject: Re: Question about #error
Date: Sat, 30 Mar 2019 15:19:49 -0700
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Thiago Adams <thiago.adams@gmail.com> writes:
> On Friday, March 29, 2019 at 12:05:53 PM UTC-3, James Kuyper wrote:
> ...
>> #if expressions are evaluated during translation phase 4. Per the rules
>> currently in effect, sizeof(int) gets converted into 0(0) before
>> evaluation (6.10.1p4), resulting in a syntax error.
>
>
> Does anyone uses or find uses of identifiers not defined in 
> #if expressions?
>
> Like 
> #if IDENTIFIER
>
> and IDENTIFIER was never #defined.
> The normal usage is 
> #if defined(X) or 
> #if X  
> where X is defined as 1 or 0
>
>
> I did a search in windows include directory for #if identifier
> and I didn't find any usage where identifier was not defined.
>
> I think this behavior could be deprecated and generate warning
> or error.

The meaning of

#if UNDEFINED_IDENTIFIER

is currently well defined.  Treating it as a fatal error could easily
break existing code.  (A Windows include directory is not likely to be a
representative sample.)

What would be the advantage of deprecating it?

-- 
Keith Thompson (The_Other_Keith) kst-u@mib.org  <http://www.ghoti.net/~kst>
Will write code for food.
void Void(void) { Void(); } /* The recursive call of the void */