I want a macro that expands to a different one depending on the presence or absence of arguments. Example:

_return(); /* расширяется в return(0xDEAFBEEF); */ _return(a+b); /* расширяется в return(a+b); */

How?

In principle, I came across such a branch , but then what? Well, we learned how many arguments, and ...

  • By the way (hooliganism, of course), but you can also directly return() and return(expr) (without underscores) so expand. - avp
  • Can. But I would like to clearly see with my eyes where the honest version is used and where the substitutions are. - PinkTux
  • And this is the right approach. - avp
  • #define TRUE (time (0) & 1) and let them suffer with debugging :) - sercxjo

1 answer 1

Slightly corrected the original set of macros by reference. Here's what happened:

 #define __VA_NARG__(...) \ __VA_NARG_(_0, ## __VA_ARGS__, __RSEQ_N()) #define __VA_NARG_(...) \ __VA_ARG_N(__VA_ARGS__) #define __VA_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 __RSEQ_N() \ 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 JOIN(x,y) _JOIN(x,y) #define _JOIN(x,y) x##y #define r(...) JOIN(r,__VA_NARG__(__VA_ARGS__))(__VA_ARGS__) #define r0() nullargs() #define r1(x) oneargs(x) r(x); r();

I'm testing on cpp from gcc. Received as a result of all substitutions:

 oneargs(x); nullargs();

Double pass JOIN -> _ JOIN -> ## - otherwise macroarguments are not deployed.

The preprocessor operation ## sticks together without spaces and a new identifier is obtained. In macros with a variable number of arguments, ## after a comma and before __VA_ARGS__ removes a comma, if there are no arguments, this is an extension to gcc, so it may not work in other compilers.

  • one
    Thanks It works! And the principle is clear :) I also have gcc, interesting, will it work somewhere else? - PinkTux
  • It turned out great. Practically function overload in C. Just describe what you need ## in __VA_NARG_(_0, ## __VA_ARGS__, __RSEQ_N()) (just in case, it ’s written here ). - avp
  • "Practically function overload in C" - I was recently shown lambda for C shown :) - PinkTux
  • @PinkTux, gcc has nested functions, i.e. with practical tz. the same as lambda. - avp