c++ - What's wrong with my (pointer based) mergesort? -

no errors, doesn't sort list. worked when used indices instead of pointers directly. feel i'm missing way pointer's should behave... . correct in assuming pointers passed value (copied) recursive calls, or messing them down way?

#include<iostream>  using namespace std;  void merge(int *start, int *pivot, int *end) {     const int n = start - end;     int ret[n];     int i;     (i=0; i<n; ++i) {         if (*start < *pivot) {             ret[i] = *(start++);         }         else {             ret[i] = *(pivot++);         }     }     (i=0;i<n;++i) {         start[i] = ret[i];     } }  void sort1(int* start,int* end) {     int n = end - start;     if (n <= 1) {         return;     }     int* pivot = &start[n/2];     sort1(start,pivot);     sort1(pivot,end);     merge(start,pivot,end); }  int main() {     int x[] = {1,3,6,2,4,5};     sort1(x,x+6);     int i;     (i=0; i<6; ++i) {         cout << x[i] << endl;     } } 

my current output 1 1 3 3 1 1

i think merge @ fault. there no bounds-testing on 2 sub-arrays, reach end of 1 array, you'll taking values other (that have been copied).

it's normal split code merge 3 loops follows:

int *a1 = start; int *a2 = pivot; int *r = &ret[0];  // copy smallest each sub-array while( a1 != pivot && a2 != end ) {     if( *a1 < *a2 ) *r++ = *a1++;     else *r++ = *a2++; }  // copy remaining values first sub-array while( a1 != pivot ) *r++ = *a1++;  // copy remaining values second sub-array while( a2 != end ) *r++ = *a2++;