C's Limited Macro Capabilities

[T. William Wells]

In article <69517 at psuecl.bitnet> bpm at 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
bill at twwells.com