C macro force evaluation of parameter

Question

I am trying to force the evaluation of a macro parameter.

I tried to use the pattern in : Macro evaluation in c preprocessor But for some reason here PP_NARG(VA_ARGS) does not expand correctly.

Do you know why the second line in main() is not compiling ? error: pasting "RUN_CODE_FOR_EACH" and "(" does not give a valid preprocessing token

#define PP_NARG(...)  (PP_NARG_(__VA_ARGS__,PP_RSEQ_N()) - \
    (sizeof(#__VA_ARGS__) == 1))
#define PP_NARG_(...)  PP_ARG_N(__VA_ARGS__)

#define PP_ARG_N( \
   _1, _2, _3, _4, _5, _6, _7, _8, _9,_10, \
  _11,_12,_13,_14,_15,_16,_17,_18,_19,_20, \
  _21,_22,_23,_24,_25,_26,_27,_28,_29,_30, \
  _31,_32,_33,_34,_35,_36,_37,_38,_39,_40, \
  _41,_42,_43,_44,_45,_46,_47,_48,_49,_50, \
  _51,_52,_53,_54,_55,_56,_57,_58,_59,_60, \
  _61,_62,_63, N, ...) N

#define PP_RSEQ_N() \
    63,62,61,60,          \
    59,58,57,56,55,54,53,52,51,50, \
    49,48,47,46,45,44,43,42,41,40, \
    39,38,37,36,35,34,33,32,31,30, \
    29,28,27,26,25,24,23,22,21,20, \
    19,18,17,16,15,14,13,12,11,10, \
    9,8,7,6,5,4,3,2,1,0

#define RUN_CODE(code, x) { \
    decltype(x) &&variadic_item = x; \
    code; \
}

#define RUN_CODE_FOR_EACH1(code, x) RUN_CODE(code, x);
#define RUN_CODE_FOR_EACH2(code, x, ...) RUN_CODE(code, x); RUN_CODE_FOR_EACH1(code, __VA_ARGS__)
#define RUN_CODE_FOR_EACH3(code, x, ...) RUN_CODE(code, x); RUN_CODE_FOR_EACH2(code, __VA_ARGS__)
#define RUN_CODE_FOR_EACH4(code, x, ...) RUN_CODE(code, x); RUN_CODE_FOR_EACH3(code, __VA_ARGS__)
#define RUN_CODE_FOR_EACH5(code, x, ...) RUN_CODE(code, x); RUN_CODE_FOR_EACH4(code, __VA_ARGS__)
#define RUN_CODE_FOR_EACH6(code, x, ...) RUN_CODE(code, x); RUN_CODE_FOR_EACH5(code, __VA_ARGS__)
#define RUN_CODE_FOR_EACH7(code, x, ...) RUN_CODE(code, x); RUN_CODE_FOR_EACH6(code, __VA_ARGS__)
#define RUN_CODE_FOR_EACH8(code, x, ...) RUN_CODE(code, x); RUN_CODE_FOR_EACH7(code, __VA_ARGS__)

#define RUN_CODE_FOR_EACH__(code, size, ...) RUN_CODE_FOR_EACH##size(code, __VA_ARGS__)
#define RUN_CODE_FOR_EACH_(code, size, ...) RUN_CODE_FOR_EACH__(code, size, __VA_ARGS__)
#define RUN_CODE_FOR_EACH(code, ...) RUN_CODE_FOR_EACH_(code, PP_NARG(__VA_ARGS__), __VA_ARGS__)


void print(int i){std::cout << "int: " << i << '\n';}
int print(double d){std::cout << "double: " << d << '\n';return 2;}

int main() {
    RUN_CODE_FOR_EACH_(print(variadic_item), 4, 1, 2., 3., 4); // Working
    RUN_CODE_FOR_EACH(print(variadic_item), 1, 2., 3., 4); // Compilation error
    std::cout << "size=" << PP_NARG(1, 2., 3., 4) << '\n'; // show size=4
    return 0;
}

Thank you

error: pasting "RUN_CODE_FOR_EACH" and "(" does not give a valid preprocessing token — infiniteLoop, Jan 25 '17 at 16:52
`PP_NARG` does not make sense, as you're using compile-time constructs at preprocessor-time. You will not get a valid preprocessor-time index. You need to count the arguments without resorting to things like `sizeof` - there are many solutions on SO. — Vittorio Romeo, Jan 25 '17 at 16:53
Question is tagged C, but this is C++ (in C you can not redefine a function like you do with `print`) — David Ranieri, Jan 25 '17 at 17:00
Oh thanks @VittorioRomeo. Could you post an answer so I can accept it ? — infiniteLoop, Jan 25 '17 at 17:00
Looks like the workaround in this page (http://smackerelofopinion.blogspot.com/2011/10/determining-number-of-arguments-in-c.html) is not completely a workaround — infiniteLoop, Jan 25 '17 at 17:02
@mikeDundee Hint: When talking to other user (not under his post), put @ and a name after it at the beginning of your comment. Otherwise there is a chance the user you're talking to won't get a notification. — HolyBlackCat, Jan 25 '17 at 17:03

score 0 · Accepted Answer · edited May 23 '17 at 12:09

PP_NARG does not make sense, as you're using compile-time constructs at preprocessor-time. You will not get a valid preprocessor-time argument count. You need to count the arguments without resorting to things like sizeof.

Here's an example on how that could be done from my vrm_pp library. First, I define the interface macros for argument counting:

#define VRM_PP_IMPL_N_ARG(...) VRM_PP_IMPL_NSEQ(__VA_ARGS__)

#define VRM_PP_IMPL_ARGCOUNT(...) \
    VRM_PP_IMPL_N_ARG(__VA_ARGS__, VRM_PP_IMPL_RSEQ())

#define VRM_PP_ARGCOUNT(...) VRM_PP_IMPL_ARGCOUNT(__VA_ARGS__)

I then use a generator script to generate the count sequences.

VRM_PP_IMPL_RSEQ is a reverse-counting sequence: 10, 9, 8, 7, 6 ...
VRM_PP_IMPL_NSEQ is a macro that takes N arguments (where N is the maximum supported number of arguments) and returns the N-th argument.

You can find a complete explanation of this solution on the "C++ preprocessor __VA_ARGS__ number of arguments" question.

C macro force evaluation of parameter

1 Answers1