c++cc-preprocessorvariadic-macros

C++ preprocessor __VA_ARGS__ number of arguments


Simple question for which I could not find answer on the net. In variadic argument macros, how to find the number of arguments? I am okay with boost preprocessor, if it has the solution.

If it makes a difference, I am trying to convert variable number of macro arguments to boost preprocessor sequence, list, or array for further reprocessing.


Solution

  • This is actually compiler dependent, and not supported by any standard.

    Here however you have a macro implementation that does the count:

    #define PP_NARG(...) \
             PP_NARG_(__VA_ARGS__,PP_RSEQ_N())
    #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
    
    /* Some test cases */
    
    
    PP_NARG(A) -> 1
    PP_NARG(A,B) -> 2
    PP_NARG(A,B,C) -> 3
    PP_NARG(A,B,C,D) -> 4
    PP_NARG(A,B,C,D,E) -> 5
    PP_NARG(1,2,3,4,5,6,7,8,9,0,
             1,2,3,4,5,6,7,8,9,0,
             1,2,3,4,5,6,7,8,9,0,
             1,2,3,4,5,6,7,8,9,0,
             1,2,3,4,5,6,7,8,9,0,
             1,2,3,4,5,6,7,8,9,0,
             1,2,3) -> 63