A group of variadic macros

Question

I would like to have a group of variable number of arguments passed into a macro. I have following macros which is incorrect:

#define M_NARGS(...) M_NARGS_(__VA_ARGS__, 10, 9, 8, 7, 6, 5, 4, 3, 2, 1, 0)
#define M_NARGS_(_10, _9, _8, _7, _6, _5, _4, _3, _2, _1, N, ...) N

#define M_CONC(A, B) M_CONC_(A, B)
#define M_CONC_(A, B) A##B
#define M_ID(...) __VA_ARGS__

#define M_LEFT(L, R) L
#define M_RIGHT(L, R) R

#define M_FOR_EACH(ACTN, ...) M_CONC(M_FOR_EACH_, M_NARGS(__VA_ARGS__)) (ACTN, __VA_ARGS__)

#define M_FOR_EACH_0(ACTN, E) E
#define M_FOR_EACH_1(ACTN, E) ACTN(E)
#define M_FOR_EACH_2(ACTN, E, ...) ACTN(E) M_FOR_EACH_1(ACTN, __VA_ARGS__)
#define M_FOR_EACH_3(ACTN, E, ...) ACTN(E) M_FOR_EACH_2(ACTN, __VA_ARGS__)
#define M_FOR_EACH_4(ACTN, E, ...) ACTN(E) M_FOR_EACH_3(ACTN, __VA_ARGS__)
#define M_FOR_EACH_5(ACTN, E, ...) ACTN(E) M_FOR_EACH_4(ACTN, __VA_ARGS__)

#define FRUITS (apple, banana, cherry)
#define ANIMALS (dog, monkey)

#define ZOO_BLOCK(NAME, FRTS, ANMLS) struct NAME##Block { \
  M_FOR_EACH(DEFINE_FRUITS, FRTS) \  // Wrong, see my question below
  M_FOR_EACH(DEFINE_ANIMAL, ANMLS) \  // Wrong
}

#define DEFINE_FRUITS(F) Fruit F;
#define DEFINE_ANIMALS(F) Animal F;

ZOO_BLOCK(MyZoo, FRUITS, ANIMALS);

By M_FOR_EACH(DEFINE_FRUITS, FRTS), I would like to do M_FOR_EACH(DEFINE_FRUITS, __VA_ARGS__) actually and __VA_ARGS__ are all from FRUITS (i.e. apple, banana, cherry). How can I change my macros to do this?

Answer 1

I'm not sure whether this is what you are looking for, but the parenthesised fruit and animal groups are not resolved. You can "flatten" them with your M_IDmacro, e.g.:

#define M_ID(...) __VA_ARGS__

#define FRUITS M_ID(apple, banana, cherry)
#define ANIMALS M_ID(dog, monkey)

#define ZOO_BLOCK(NAME, FRTS, ANMLS) struct NAME##Block {   \
  M_FOR_EACH(DEFINE_FRUITS, FRTS)                           \
  M_FOR_EACH(DEFINE_ANIMALS, ANMLS)                          \
}

#define DEFINE_FRUITS(F) Fruit F;
#define DEFINE_ANIMALS(F) Animal F;

ZOO_BLOCK(MyZoo, FRUITS, ANIMALS);

This, together with correcting a minor typo in DEFINE_ANIMAL/S yields:

struct MyZooBlock { 
    Fruit apple;
    Fruit banana;
    Fruit cherry;
    Animal dog;
    Animal monkey;
};

Answer 2

If you want to generate structs based on lists I would use higher order macros. This does not require you to have another macro that actually does the loop resolution.

#define FRUITS(V) \
    V(apple) \
    V(banana) \
    V(cherry)

#define ANIMALS(V) \
    V(dog) \
    V(monkey)

#define VISIT_ANI_STRUCT(A) \
    Animal A;

#define VISIT_FRU_STRUCT(F) \
    Fruit F;

#define ZOO_BLOCK(NAME, GEN_ANI,GEN_FRU) \
    struct NAME ## Block { \
        ANIMALS(GEN_ANI) \
        FRUITS(GEN_FRU) \
    }

ZOO_BLOCK(Zoo, VISIT_ANI_STRUCT, VISIT_FRU_STRUCT);

Will result in:

struct ZooBlock { Animal dog; Animal monkey; Fruit apple; Fruit banana; Fruit cherry; };

Or if you need the other way round

#define ZOO_BLOCK(NAME, A, F) \
    struct NAME ## Block { \
        A(VISIT_ANI_STRUCT) \
        F(VISIT_FRU_STRUCT) \
    }

ZOO_BLOCK(Zoo, VISIT_ANI_STRUCT, VISIT_FRU_STRUCT);