basic generic iterators in C

iterators.c

#include <stdio.h>
#include <stdlib.h>

#define ITERATOR_IMPL(T, state_t)                                                                                      \
    struct                                                                                                             \
    {                                                                                                                  \
        T (*__next__)(state_t *);                                                                                      \
    }
#define ITERATOR_STATE(T)                                                                                              \
    struct iterator_state_##T                                                                                          \
    {                                                                                                                  \
        bool finished;                                                                                                 \
        T data;                                                                                                        \
    }
#define ITERATOR(T)                                                                                                    \
    struct iterator_##T                                                                                                \
    {                                                                                                                  \
        ITERATOR_STATE(T) state;                                                                                       \
        ITERATOR_IMPL(T, ITERATOR_STATE(T)) impl;                                                                      \
    }

#define ITERATOR_NEW(T, init, next)                                                                                    \
    {                                                                                                                  \
        (ITERATOR_STATE(T)){                                                                                           \
            false,                                                                                                     \
            (init),                                                                                                    \
        },                                                                                                             \
        {                                                                                                              \
            (next)                                                                                                     \
        }                                                                                                              \
    }
#define NEXT(iterator)                                                                                                 \
    ({                                                                                                                 \
        if ((iterator).state.finished)                                                                                 \
        {                                                                                                              \
            fprintf(stderr, "Error: iterator is finished\n");                                                          \
            exit(1);                                                                                                   \
        }                                                                                                              \
        (iterator).impl.__next__(&(iterator).state);                                                                   \
    })

#define STOP_ITERATION(iterator) (iterator).state.finished
#define YIELD(value) return (value)
#define ITERATOR_RETURN(state_p) (state_p)->finished = true

int foo(ITERATOR_STATE(int) * state)
{
    if (state->finished)
    {
        return -1;
    }

    int value = state->data;
    state->data++;

    if (state->data > 9)
    {
        ITERATOR_RETURN(state);
    }

    YIELD(value);
}

int main()
{
    ITERATOR(int) x = ITERATOR_NEW(int, 0, foo);

    while (!STOP_ITERATION(x))
    {
        printf("%d\n", NEXT(x));
    }

    return EXIT_SUCCESS;
}