Simply benchmark to measure the cost of virtual functions.

gistfile1.cpp

#include <stdlib.h>

#define N 1000000
#define M 100

int cmp(const void *a, const void *b) { return *(int*)b - *(int*)a; }

class Compare {
 public:
  virtual int compare(const void *a, const void *b) { return cmp(a, b); }
};

Compare* c;
int vcmp(const void *a, const void *b) { return c->compare(a, b); }

int main() {
  int *array = new int[N];
  c = new Compare();
  for (int i = 0; i < M; i++) {
    // Switch between "cmp" and "vcmp" and measure the difference.
    // I measure a 20% slowdown with vcmp.
    qsort(array, N, sizeof(int), cmp);
  }
}