Foreach macro in C

There’s times when we want to do weird things with macros like this:

#define PRINT(ARG) printf(“%s\n”, #ARG);

int main() {
FOREACH(PRINT, foo, bar)
}

The output will be:

foo
bar

Basic steps

First we need to think how t…


This content originally appeared on DEV Community and was authored by Horu

There's times when we want to do weird things with macros like this:

#define PRINT(ARG) printf("%s\n", #ARG);

int main() {
    FOREACH(PRINT, foo, bar)
}

The output will be:

foo
bar

Basic steps

First we need to think how to start this challenge, in my last post I explained how to create a repeat macro which repeats N times whatever we want, I recommend you read it to understand this one. view post.

First we need to calculate the number of arguments, so then we will can repeat N times over it like we did with the repeat macro.

How to calculate the number of arguments?

We can start defining our N_VA_ARGS macro like this

#define N_VA_ARGS_(_9,_8,_7,_6,_5,_4,_3,_2,_1, N, ...) N
#define N_VA_ARGS(...) N_VA_ARGS_(__VA_ARGS__ __VA_OPT__(,) 9,8,7,6,5,4,3,2,1,0)

__VA_ARGS__ __VA_OPT__(,) will paste our arguments before the sequence, we will use __VA_OPT__(,) to set a comma if it is needed.
According how many arguments we pass, it will move the sequence to the right, in this case we have the sequence (9,8,7,6,5,4,3,2,1,0) so N_VA_ARGS_ will take the 0 when there's no arguments, then if we pass an argument the sequence will move to the right and N will be 1 in this case and 0 will be discarded by the .... So with this way we can calculate how many arguments we will pass to our foreach macro (only until 9 arguments).

The foreach macro

Once we have our N_VA_ARGS macro, it is time to build our foreach macro.

#define N_VA_ARGS_(_9,_8,_7,_6,_5,_4,_3,_2,_1, N, ...) N
#define N_VA_ARGS(...) N_VA_ARGS_(__VA_ARGS__ __VA_OPT__(,) 9,8,7,6,5,4,3,2,1,0)

#define FOREACH_0(FN, ...) 
#define FOREACH_1(FN, E, ...)  FN(E) 
#define FOREACH_2(FN, E, ...)  FN(E) FOREACH_1(FN, __VA_ARGS__)
#define FOREACH_3(FN, E, ...)  FN(E) FOREACH_2(FN, __VA_ARGS__)
#define FOREACH_4(FN, E, ...)  FN(E) FOREACH_3(FN, __VA_ARGS__)
#define FOREACH_5(FN, E, ...)  FN(E) FOREACH_4(FN, __VA_ARGS__)
#define FOREACH_6(FN, E, ...)  FN(E) FOREACH_5(FN, __VA_ARGS__)
#define FOREACH_7(FN, E, ...)  FN(E) FOREACH_6(FN, __VA_ARGS__)
#define FOREACH_8(FN, E, ...)  FN(E) FOREACH_7(FN, __VA_ARGS__)
#define FOREACH_9(FN, E, ...)  FN(E) FOREACH_8(FN, __VA_ARGS__)

#define FOREACH__(FN, NARGS, ...) FOREACH_##NARGS(FN, __VA_ARGS__) 
#define FOREACH_(FN, NARGS, ...) FOREACH__(FN, NARGS, __VA_ARGS__)
#define FOREACH(FN, ...) FOREACH_(FN, N_VA_ARGS(__VA_ARGS__), __VA_ARGS__)

FOREACH will calculate the number of arguments using N_VA_ARGS, and then will call FOREACH_X depending how many arguments we pass.

FOREACH_X will call recursively until reach FOREACH_1, every call will recursively grab the first argument and forward the rest to the next function always callling to FN macro.

Completed foreach macro

This is the completed version of foreach macro, it supports until 100 arguments.

#define N_VA_ARGS_(_100,_99,_98,_97,_96,_95,_94,_93,_92,_91,_90,_89,_88,_87,_86,_85,_84,_83,_82,_81,_80,_79,_78,_77,_76,_75,_74,_73,_72,_71,_70,_69,_68,_67,_66,_65,_64,_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, N, ...) N
#define N_VA_ARGS(...) N_VA_ARGS_(__VA_ARGS__ __VA_OPT__(,) 100,99,98,97,96,95,94,93,92,91,90,89,88,87,86,85,84,83,82,81,80,79,78,77,76,75,74,73,72,71,70,69,68,67,66,65,64,63,62,61,60,59,58,57,56,55,54,53,52,51,50,49,48,47,46,45,44,43,42,41,40,39,38,37,36,35,34,33,32,31,30,29,28,27,26,25,24,23,22,21,20,19,18,17,16,15,14,13,12,11,10,9,8,7,6,5,4,3,2,1,0)

#define FOREACH_0(FN, ...) 
#define FOREACH_1(FN, E, ...) FN(E) 
#define FOREACH_2(FN, E, ...) FN(E) FOREACH_1(FN, __VA_ARGS__)
#define FOREACH_3(FN, E, ...) FN(E) FOREACH_2(FN, __VA_ARGS__)
#define FOREACH_4(FN, E, ...) FN(E) FOREACH_3(FN, __VA_ARGS__)
#define FOREACH_5(FN, E, ...) FN(E) FOREACH_4(FN, __VA_ARGS__)
#define FOREACH_6(FN, E, ...) FN(E) FOREACH_5(FN, __VA_ARGS__)
#define FOREACH_7(FN, E, ...) FN(E) FOREACH_6(FN, __VA_ARGS__)
#define FOREACH_8(FN, E, ...) FN(E) FOREACH_7(FN, __VA_ARGS__)
#define FOREACH_9(FN, E, ...) FN(E) FOREACH_8(FN, __VA_ARGS__)
#define FOREACH_10(FN, E, ...) FN(E) FOREACH_9(FN, __VA_ARGS__)
#define FOREACH_11(FN, E, ...) FN(E) FOREACH_10(FN, __VA_ARGS__)
#define FOREACH_12(FN, E, ...) FN(E) FOREACH_11(FN, __VA_ARGS__)
#define FOREACH_13(FN, E, ...) FN(E) FOREACH_12(FN, __VA_ARGS__)
#define FOREACH_14(FN, E, ...) FN(E) FOREACH_13(FN, __VA_ARGS__)
#define FOREACH_15(FN, E, ...) FN(E) FOREACH_14(FN, __VA_ARGS__)
#define FOREACH_16(FN, E, ...) FN(E) FOREACH_15(FN, __VA_ARGS__)
#define FOREACH_17(FN, E, ...) FN(E) FOREACH_16(FN, __VA_ARGS__)
#define FOREACH_18(FN, E, ...) FN(E) FOREACH_17(FN, __VA_ARGS__)
#define FOREACH_19(FN, E, ...) FN(E) FOREACH_18(FN, __VA_ARGS__)
#define FOREACH_20(FN, E, ...) FN(E) FOREACH_19(FN, __VA_ARGS__)
#define FOREACH_21(FN, E, ...) FN(E) FOREACH_20(FN, __VA_ARGS__)
#define FOREACH_22(FN, E, ...) FN(E) FOREACH_21(FN, __VA_ARGS__)
#define FOREACH_23(FN, E, ...) FN(E) FOREACH_22(FN, __VA_ARGS__)
#define FOREACH_24(FN, E, ...) FN(E) FOREACH_23(FN, __VA_ARGS__)
#define FOREACH_25(FN, E, ...) FN(E) FOREACH_24(FN, __VA_ARGS__)
#define FOREACH_26(FN, E, ...) FN(E) FOREACH_25(FN, __VA_ARGS__)
#define FOREACH_27(FN, E, ...) FN(E) FOREACH_26(FN, __VA_ARGS__)
#define FOREACH_28(FN, E, ...) FN(E) FOREACH_27(FN, __VA_ARGS__)
#define FOREACH_29(FN, E, ...) FN(E) FOREACH_28(FN, __VA_ARGS__)
#define FOREACH_30(FN, E, ...) FN(E) FOREACH_29(FN, __VA_ARGS__)
#define FOREACH_31(FN, E, ...) FN(E) FOREACH_30(FN, __VA_ARGS__)
#define FOREACH_32(FN, E, ...) FN(E) FOREACH_31(FN, __VA_ARGS__)
#define FOREACH_33(FN, E, ...) FN(E) FOREACH_32(FN, __VA_ARGS__)
#define FOREACH_34(FN, E, ...) FN(E) FOREACH_33(FN, __VA_ARGS__)
#define FOREACH_35(FN, E, ...) FN(E) FOREACH_34(FN, __VA_ARGS__)
#define FOREACH_36(FN, E, ...) FN(E) FOREACH_35(FN, __VA_ARGS__)
#define FOREACH_37(FN, E, ...) FN(E) FOREACH_36(FN, __VA_ARGS__)
#define FOREACH_38(FN, E, ...) FN(E) FOREACH_37(FN, __VA_ARGS__)
#define FOREACH_39(FN, E, ...) FN(E) FOREACH_38(FN, __VA_ARGS__)
#define FOREACH_40(FN, E, ...) FN(E) FOREACH_39(FN, __VA_ARGS__)
#define FOREACH_41(FN, E, ...) FN(E) FOREACH_40(FN, __VA_ARGS__)
#define FOREACH_42(FN, E, ...) FN(E) FOREACH_41(FN, __VA_ARGS__)
#define FOREACH_43(FN, E, ...) FN(E) FOREACH_42(FN, __VA_ARGS__)
#define FOREACH_44(FN, E, ...) FN(E) FOREACH_43(FN, __VA_ARGS__)
#define FOREACH_45(FN, E, ...) FN(E) FOREACH_44(FN, __VA_ARGS__)
#define FOREACH_46(FN, E, ...) FN(E) FOREACH_45(FN, __VA_ARGS__)
#define FOREACH_47(FN, E, ...) FN(E) FOREACH_46(FN, __VA_ARGS__)
#define FOREACH_48(FN, E, ...) FN(E) FOREACH_47(FN, __VA_ARGS__)
#define FOREACH_49(FN, E, ...) FN(E) FOREACH_48(FN, __VA_ARGS__)
#define FOREACH_50(FN, E, ...) FN(E) FOREACH_49(FN, __VA_ARGS__)
#define FOREACH_51(FN, E, ...) FN(E) FOREACH_50(FN, __VA_ARGS__)
#define FOREACH_52(FN, E, ...) FN(E) FOREACH_51(FN, __VA_ARGS__)
#define FOREACH_53(FN, E, ...) FN(E) FOREACH_52(FN, __VA_ARGS__)
#define FOREACH_54(FN, E, ...) FN(E) FOREACH_53(FN, __VA_ARGS__)
#define FOREACH_55(FN, E, ...) FN(E) FOREACH_54(FN, __VA_ARGS__)
#define FOREACH_56(FN, E, ...) FN(E) FOREACH_55(FN, __VA_ARGS__)
#define FOREACH_57(FN, E, ...) FN(E) FOREACH_56(FN, __VA_ARGS__)
#define FOREACH_58(FN, E, ...) FN(E) FOREACH_57(FN, __VA_ARGS__)
#define FOREACH_59(FN, E, ...) FN(E) FOREACH_58(FN, __VA_ARGS__)
#define FOREACH_60(FN, E, ...) FN(E) FOREACH_59(FN, __VA_ARGS__)
#define FOREACH_61(FN, E, ...) FN(E) FOREACH_60(FN, __VA_ARGS__)
#define FOREACH_62(FN, E, ...) FN(E) FOREACH_61(FN, __VA_ARGS__)
#define FOREACH_63(FN, E, ...) FN(E) FOREACH_62(FN, __VA_ARGS__)
#define FOREACH_64(FN, E, ...) FN(E) FOREACH_63(FN, __VA_ARGS__)
#define FOREACH_65(FN, E, ...) FN(E) FOREACH_64(FN, __VA_ARGS__)
#define FOREACH_66(FN, E, ...) FN(E) FOREACH_65(FN, __VA_ARGS__)
#define FOREACH_67(FN, E, ...) FN(E) FOREACH_66(FN, __VA_ARGS__)
#define FOREACH_68(FN, E, ...) FN(E) FOREACH_67(FN, __VA_ARGS__)
#define FOREACH_69(FN, E, ...) FN(E) FOREACH_68(FN, __VA_ARGS__)
#define FOREACH_70(FN, E, ...) FN(E) FOREACH_69(FN, __VA_ARGS__)
#define FOREACH_71(FN, E, ...) FN(E) FOREACH_70(FN, __VA_ARGS__)
#define FOREACH_72(FN, E, ...) FN(E) FOREACH_71(FN, __VA_ARGS__)
#define FOREACH_73(FN, E, ...) FN(E) FOREACH_72(FN, __VA_ARGS__)
#define FOREACH_74(FN, E, ...) FN(E) FOREACH_73(FN, __VA_ARGS__)
#define FOREACH_75(FN, E, ...) FN(E) FOREACH_74(FN, __VA_ARGS__)
#define FOREACH_76(FN, E, ...) FN(E) FOREACH_75(FN, __VA_ARGS__)
#define FOREACH_77(FN, E, ...) FN(E) FOREACH_76(FN, __VA_ARGS__)
#define FOREACH_78(FN, E, ...) FN(E) FOREACH_77(FN, __VA_ARGS__)
#define FOREACH_79(FN, E, ...) FN(E) FOREACH_78(FN, __VA_ARGS__)
#define FOREACH_80(FN, E, ...) FN(E) FOREACH_79(FN, __VA_ARGS__)
#define FOREACH_81(FN, E, ...) FN(E) FOREACH_80(FN, __VA_ARGS__)
#define FOREACH_82(FN, E, ...) FN(E) FOREACH_81(FN, __VA_ARGS__)
#define FOREACH_83(FN, E, ...) FN(E) FOREACH_82(FN, __VA_ARGS__)
#define FOREACH_84(FN, E, ...) FN(E) FOREACH_83(FN, __VA_ARGS__)
#define FOREACH_85(FN, E, ...) FN(E) FOREACH_84(FN, __VA_ARGS__)
#define FOREACH_86(FN, E, ...) FN(E) FOREACH_85(FN, __VA_ARGS__)
#define FOREACH_87(FN, E, ...) FN(E) FOREACH_86(FN, __VA_ARGS__)
#define FOREACH_88(FN, E, ...) FN(E) FOREACH_87(FN, __VA_ARGS__)
#define FOREACH_89(FN, E, ...) FN(E) FOREACH_88(FN, __VA_ARGS__)
#define FOREACH_90(FN, E, ...) FN(E) FOREACH_89(FN, __VA_ARGS__)
#define FOREACH_91(FN, E, ...) FN(E) FOREACH_90(FN, __VA_ARGS__)
#define FOREACH_92(FN, E, ...) FN(E) FOREACH_91(FN, __VA_ARGS__)
#define FOREACH_93(FN, E, ...) FN(E) FOREACH_92(FN, __VA_ARGS__)
#define FOREACH_94(FN, E, ...) FN(E) FOREACH_93(FN, __VA_ARGS__)
#define FOREACH_95(FN, E, ...) FN(E) FOREACH_94(FN, __VA_ARGS__)
#define FOREACH_96(FN, E, ...) FN(E) FOREACH_95(FN, __VA_ARGS__)
#define FOREACH_97(FN, E, ...) FN(E) FOREACH_96(FN, __VA_ARGS__)
#define FOREACH_98(FN, E, ...) FN(E) FOREACH_97(FN, __VA_ARGS__)
#define FOREACH_99(FN, E, ...) FN(E) FOREACH_98(FN, __VA_ARGS__)
#define FOREACH_100(FN, E, ...) FN(E) FOREACH_99(FN, __VA_ARGS__)

#define FOREACH__(FN, NARGS, ...) FOREACH_##NARGS(FN, __VA_ARGS__) 
#define FOREACH_(FN, NARGS, ...) FOREACH__(FN, NARGS, __VA_ARGS__)
#define FOREACH(FN, ...) FOREACH_(FN, N_VA_ARGS(__VA_ARGS__), __VA_ARGS__)


This content originally appeared on DEV Community and was authored by Horu


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