Trying to find information on variadic macros.

Can you get specific parameters?

#define Test(...) printf("%s", ##__VAR_ARGS__)

I can't do this to get the first parameter

#define Test(...) printf("%s", var_arg(__VAR_ARGS__, char*))

or

#define Test(...) printf("%s", var_arg(*__VAR_ARGS__, char*))

or

#define Test(...) printf("%s", var_arg(*##__VAR_ARGS__, char*))




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Michael D. Borghardt wrote:
 > Trying to find information on variadic macros.

Variadic macros are not currently part of C++, so you should probably ask in a C
newsgroup.

 > Can you get specific parameters?

Yes, however sometimes a little preprocessor metaprogramming is required to
extract them.  Here's an example:

#define CAT(a, ...) PRIMITIVE_CAT(a, __VA_ARGS__)
#define PRIMITIVE_CAT(a, ...) a ## __VA_ARGS__

#define EXPAND(...) __VA_ARGS__

#define VARIADIC_ELEM(n, ...) \
     EXPAND( \
         PRIMITIVE_CAT(VARIADIC_ELEM_, n) \
         VARIADIC_ELEM_I(__VA_ARGS__,,,,,,,,,,) \
     ) \
     /**/
#define VARIADIC_ELEM_I(a, b, c, d, e, f, g, h, i, j, ...) \
     (a, b, c, d, e, f, g, h, i, j) \
     /**/
#define VARIADIC_ELEM_0(a, b, c, d, e, f, g, h, i, j) a
#define VARIADIC_ELEM_1(a, b, c, d, e, f, g, h, i, j)
#define VARIADIC_ELEM_1(a, b, c, d, e, f, g, h, i, j)
#define VARIADIC_ELEM_1(a, b, c, d, e, f, g, h, i, j)
#define VARIADIC_ELEM_1(a, b, c, d, e, f, g, h, i, j)
#define VARIADIC_ELEM_1(a, b, c, d, e, f, g, h, i, j)
#define VARIADIC_ELEM_1(a, b, c, d, e, f, g, h, i, j)
#define VARIADIC_ELEM_1(a, b, c, d, e, f, g, h, i, j)
#define VARIADIC_ELEM_1(a, b, c, d, e, f, g, h, i, j)



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Michael D. Borghardt wrote:
 > Trying to find information on variadic macros.

Variadic macros are not currently part of C++, so you should probably ask in a C
newsgroup.

 > Can you get specific parameters?

Yes, however sometimes a little preprocessor metaprogramming is required to
extract them.  Here's an example:

#define CAT(a, ...) PRIMITIVE_CAT(a, __VA_ARGS__)
#define PRIMITIVE_CAT(a, ...) a ## __VA_ARGS__

#define EXPAND(...) __VA_ARGS__

#define VARIADIC_ELEM(n, ...) \
     EXPAND( \
         PRIMITIVE_CAT(VARIADIC_ELEM_, n) \
         VARIADIC_ELEM_I(__VA_ARGS__,,,,,,,,,,) \
     ) \
     /**/
#define VARIADIC_ELEM_I(a, b, c, d, e, f, g, h, i, j, ...) \
     (a, b, c, d, e, f, g, h, i, j) \
     /**/
#define VARIADIC_ELEM_0(a, b, c, d, e, f, g, h, i, j) a
#define VARIADIC_ELEM_1(a, b, c, d, e, f, g, h, i, j) b
#define VARIADIC_ELEM_2(a, b, c, d, e, f, g, h, i, j) c
#define VARIADIC_ELEM_3(a, b, c, d, e, f, g, h, i, j) d
#define VARIADIC_ELEM_4(a, b, c, d, e, f, g, h, i, j) e
#define VARIADIC_ELEM_5(a, b, c, d, e, f, g, h, i, j) f
#define VARIADIC_ELEM_6(a, b, c, d, e, f, g, h, i, j) g
#define VARIADIC_ELEM_7(a, b, c, d, e, f, g, h, i, j) h
#define VARIADIC_ELEM_8(a, b, c, d, e, f, g, h, i, j) i
#define VARIADIC_ELEM_9(a, b, c, d, e, f, g, h, i, j) j

VARIADIC_ELEM(0, +, -, *, /, %) // +
VARIADIC_ELEM(1, +, -, *, /, %) // -
VARIADIC_ELEM(2, +, -, *, /, %) // *
VARIADIC_ELEM(3, +, -, *, /, %) // /
VARIADIC_ELEM(4, +, -, *, /, %) // %

Regards,
Paul Mensonides



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