Polymorphic pointers-to-functions whose parameters are void pointers as arguments in C

Question

My question is about this sort of table that's simplified here. Often, I find myself putting this element, (int (*)(const void *, const void *))&strcmp for bsearch. I go between prototyping with void pointers, creating dummy functions, and casting, but none is really a satisfactory, especially when the functions are complex.

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

static void number(int *const);
static void letter(char *const);

static void (*const action[])(void *const) = {
    (void (*)(void *))&number,
    (void (*)(void *))&letter
};

/* corresponds to the 'action' index */
enum Type { NUMBER = 0, LETTER = 1 };

struct Thing {
    enum Type type;
    void *data;
};

int main(void) {
    const char aah  = 'a';
    const int  five = 5;
    const int  six  = 6;
    const struct Thing things[] = {
        { LETTER, (void *)&aah },
        { NUMBER, (void *)&five },
        { NUMBER, (void *)&six }
    };
    const int things_size = sizeof things / sizeof(struct Thing);
    const struct Thing *t;

    /* prints a 5 6 */
    for(t = things; t < things + things_size; t++) {
        action[t->type](t->data);
    }

    return EXIT_SUCCESS;
}

static void number(int *const dp) {
    printf("%d\n", *dp);
}

static void letter(char *const cp) {
    printf("%c\n", *cp);
}

This https://stackoverflow.com/questions/559581/casting-a-function-pointer-to-another-type/559671#559671 seems to suggest that it's dubious to do this. What is the cleanest, most correct way to achieve this sort of polymorphism?

Answer 1

This doesn't really answer your question, but when using function pointers it is usually easier to use a typedef:

typedef void (*Action) (void * const);
static Action action[] = { (Action) number, (Action) letter };

Also the calling of the action would be better expressed as

struct Thing {
    Action action;
    void *data;
};
...
const struct Thing things[] = {
    { (Action) letter, (void *)&aah },
    { (Action) number, (void *)&five },
    { (Action) number, (void *)&six }
};
for(t = things; t < things + things_size; t++) {
    t->action(t->data);
}