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From: aglew@ccvaxa.UUCP
Newsgroups: comp.lang.c
Subject: Re: #if inside #define -- easy to do, b
Message-ID: <28700020@ccvaxa>
Date: Fri, 2-Oct-87 20:44:00 EDT
Article-I.D.: ccvaxa.28700020
Posted: Fri Oct  2 20:44:00 1987
Date-Received: Wed, 7-Oct-87 03:14:03 EDT
References: <157@decvax.UUCP>
Lines: 41
Nf-ID: #R:decvax.UUCP:157:ccvaxa:28700020:000:1229
Nf-From: ccvaxa.UUCP!aglew    Oct  2 19:44:00 1987


>rbutterworth@orchid.waterloo.edu
>In article <29339@sun.uucp>, david@sun.uucp (David DiGiacomo) writes:
>> In article <157@decvax.UUCP> minow@decvax.UUCP (Martin Minow) writes:
>> >From time immemorial, C has forbidden # control lines inside a
>> >macro definition.
>> It would be a nice feature to have, but it's not really necessary for your
>> example
>
>OK, here's a real example where it would be useful
>(in fact I can't see any nice way of doing this without the ability):
>
>    #if !defined(compiler_A)
>    #if !defined(compiler_B)
>    #if !defined(compiler_C)
>    ^%$%^#^%$&^% syntax error since we don't know this compiler.
>
>    #else  /* compiler_C */
>    #   define ARGSUSED /*ARGSUSED*/
>    #endif /* compiler_C */
>
>    ...


Not the subject of this discussion (although I would like to have a real
macro language standard for cpp), but the construct above shows the importance
of (1) the #elif construct, and (2) a need for a standard abort-compilation
construct:

#if defined(compiler_A)
..
#elif defined(compiler_B)
..
#elif defined(compiler_C)
..
#else
#abort Don't know this compiler - can't define AGSUSED
#endif

I thibk it was Whitesmith's C that had the #abort construct, and
it was very useful.