C Macro to get only even variadic arguments passed-CodePudding

Would it be possible to make a ONLY_EVEN() macro to perform the following translations:

ONLY_EVEN0()
ONLY_EVEN2(a,b) b
ONLY_EVEN4(a,b,c,d) b,d

With any number of arguments, using variadic macro parameters?

CodePudding user response：

Probably not for an unbounded number of arguments.

However it is possible for any variable number of arguments up to an arbitrarily chosen bound. For example, below is a self-contained macro solution for any number of arguments in [0..6], which can be extended in the obvious way to support up to any limit you wish.

#define ONLY_EVEN0(a) 
#define ONLY_EVEN1(a) 
#define ONLY_EVEN2(a,b) b
#define ONLY_EVEN3(a,b,c) b
#define ONLY_EVEN4(a,b,c,d) b,d
#define ONLY_EVEN5(a,b,c,d,e) b,d
#define ONLY_EVEN6(a,b,c,d,e,f) b,d,f
#define ONLY_EVEN_N(a,b,c,d,e,f,n,...) ONLY_EVEN##n
#define ONLY_EVEN(...) ONLY_EVEN_N(__VA_ARGS__,6,5,4,3,2,1,0)(__VA_ARGS__)

// example:
0: ONLY_EVEN()
1: ONLY_EVEN(A)
2: ONLY_EVEN(A,B)
3: ONLY_EVEN(A,B,C)
4: ONLY_EVEN(A,B,C,D)
5: ONLY_EVEN(A,B,C,D,E)
6: ONLY_EVEN(A,B,C,D,E,F)

$ g   -std=c  11 -pedantic -E onlyeven.cpp | grep -v '#'
0:
1:
2: B
3: B
4: B,D
5: B,D
6: B,D,F

Note that by C/C spec the preprocessor is only guaranteed to accept up to 256 arguments.