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From: bill@twwells.com (T. William Wells)
Newsgroups: comp.lang.c
Subject: Re: C's Limited Macro Capabilities
Message-ID: <1989Dec4.033809.16802@twwells.com>
Date: 4 Dec 89 03:38:09 GMT
References: <69517@psuecl.bitnet>
Organization: None, Ft. Lauderdale, FL
Lines: 26

In article <69517@psuecl.bitnet> bpm@psuecl.bitnet (Brian Moquin) writes:
: A student in my C class asked me an interesting question:  can you have
: conditional compilation directives embedded within '#define' macros?
: The answer is no.  The student pointed out that this then
: severely limits the macro capabilities of C.  He said that in assembler
: (MASM, I'm sure), he can write macros which contain arguments that
: determine how the macros get expanded.  I had trouble coming up with
: a good analogous example in C, but here's one to think about:
:         #define cast(flag,x)    #if flag=='I' \
:                                         ((int)(x)) \
:                                 #elif flag=='F' \
:                                         ((float)(x)) \
:                                 #endif
: This is not legal C, but if it were I think it would enhance the power
: of macro expansions significantly.  Thoughts?

This particular one can be dealt with in ANSI C, I think.
Something like:

#define cast_F(x)       ((float)(x))
#define cast_I(x)       ((int)(x))
#define cast(flag,x)    cast_##flag((x))

---
Bill                    { uunet | novavax | ankh | sunvice } !twwells!bill
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