How to make generic function using void * in c?

Question

I have an incr function to increment the value by 1 I want to make it generic,because I don\'t want to make different functions for the same functi

Answer 1

You can't do exactly what you're asking - operators like increment need to work with a specific type. So, you could do something like this:

enum type { 
    TYPE_CHAR,
    TYPE_INT,
    TYPE_FLOAT
};

void incr(enum type t, void *vp)
{
    switch (t) {
        case TYPE_CHAR:
        (*(char *)vp)++;
        break;

        case TYPE_INT:
        (*(int *)vp)++;
        break;

        case TYPE_FLOAT:
        (*(float *)vp)++;
        break;
    }
}

Then you'd call it like:

int i=5;
float f=5.6f;
char c='a';

incr(TYPE_INT, &i);
incr(TYPE_FLOAT, &f);
incr(TYPE_CHAR, &c);

Of course, this doesn't really give you anything over just defining separate incr_int(), incr_float() and incr_char() functions - this isn't the purpose of void *.

The purpose of void * is realised when the algorithm you're writing doesn't care about the real type of the objects. A good example is the standard sorting function qsort(), which is declared as:

void qsort(void *base, size_t nmemb, size_t size, int(*compar)(const void *, const void *));

This can be used to sort arrays of any type of object - the caller just needs to supply a comparison function that can compare two objects.

Both your swap() and sort() functions fall into this category. swap() is even easier - the algorithm doesn't need to know anything other than the size of the objects to swap them:

void swap(void *a, void *b, size_t size)
{
    unsigned char *ap = a;
    unsigned char *bp = b;
    size_t i;

    for (i = 0; i < size; i++) {
        unsigned char tmp = ap[i];

        ap[i] = bp[i];
        bp[i] = tmp;
    }
}

Now given any array you can swap two items in that array:

int ai[];
double ad[];

swap(&ai[x], &ai[y], sizeof(int));
swap(&di[x], &di[y], sizeof(double));