How to use function pointers in polymorphism? - function

Question

I want to be able to sort an array out using function pointers in polymorphism. Not to mention, am only doing this to see how things work and so forth.

Answer 1

Here's a simple generic sorting interface, an insertion sort implemented through that interface, and some test code that demonstrates its use:
#include <string.h>
#include <stdio.h>
#include <stdlib.h>
struct sort_interface {
// number of elements
size_t nmemb;
// passed through to 'arg' of compare() and swap()
void *arg;
// compares elements at 'i' and 'j'
int (*compare)(void *arg, size_t i, size_t j);
// swaps elements at 'i' and 'j'
void (*swap)(void *arg, size_t i, size_t j);
};
static void insertion_sort (struct sort_interface iface)
{
for (size_t i = 0; i < iface.nmemb; i++) {
size_t j = i;
while (j > 0) {
if (iface.compare(iface.arg, j - 1, j) <= 0) {
break;
}
iface.swap(iface.arg, j - 1, j);
j--;
}
}
}
static int func_comparator (void *arg, size_t i, size_t j)
{
int *arr = arg;
if (arr[i] < arr[j]) {
return -1;
}
if (arr[i] > arr[j]) {
return 1;
}
return 0;
}
static void func_swap (void *arg, size_t i, size_t j)
{
int *arr = arg;
int temp = arr[i];
arr[i] = arr[j];
arr[j] = temp;
}
int main (int argc, char *argv[])
{
int arr[] = {7, 6, 8, 2, 9, 1, 156, 1, 62, 1671, 15};
size_t count = sizeof(arr) / sizeof(arr[0]);
struct sort_interface iface;
iface.nmemb = count;
iface.arg = arr;
iface.compare = func_comparator;
iface.swap = func_swap;
insertion_sort(iface);
for (size_t i = 0; i < count; i++) {
printf("%d ", arr[i]);
}
printf("\n");
return 0;
}
You might also want to take a look at the qsort() function of the C standard library, which too uses a function pointer comparator, but is somewhat limited to compared to the above. In particular, it assumes you're sorting a continuous array, and if you have pointers to elements or their members, those will be broken (but the above interface allows you to fix pointers in swap()).
Here's an example for how to use the qsort() interface, and also an insertion sort implementation that uses the same interface as qsort():
#include <string.h>
#include <stdio.h>
#include <stdlib.h>
static void insertion_sort (void *base, size_t nmemb, size_t size, int(*compar)(const void *, const void *))
{
char temp[size];
for (size_t i = 0; i < nmemb; i++) {
size_t j = i;
while (j > 0) {
char *x = (char *)base + (j - 1) * size;
char *y = (char *)base + j * size;
if (compar(x, y) <= 0) {
break;
}
memcpy(temp, x, size);
memcpy(x, y, size);
memcpy(y, temp, size);
j--;
}
}
}
static int int_comparator (const void *ve1, const void *ve2)
{
const int *e1 = ve1;
const int *e2 = ve2;
if (*e1 < *e2) {
return -1;
}
if (*e1 > *e2) {
return 1;
}
return 0;
}
int main (int argc, char *argv[])
{
int arr[] = {7, 6, 8, 2, 9, 1, 156, 1, 62, 1671, 15};
size_t count = sizeof(arr) / sizeof(arr[0]);
qsort(arr, count, sizeof(arr[0]), int_comparator); // or insertion_sort()
for (size_t i = 0; i < count; i++) {
printf("%d ", arr[i]);
}
printf("\n");
return 0;
}