Quick Sort in C++ not working when middle element chosen as pivot

I have tried an implementation of quick sort in c++. I am facing an issue. If I arbitrarily select the pivot as the first or the last element, the program runs fine, but if I select the middle element as the pivot ( (beg + end)/2 ), then the output is not perfectly correct. Most elements are in sorted order, only some are in random, incorrect places. The following is my code:

#include <iostream>
#include <cstdlib>
#include <chrono>
#include <fstream>

using namespace std;
using namespace std::chrono;

void quickSort(int[], int, int);
void print(int);

void print(int n) //prints 50 random numbers in a file
{
    ofstream of("List.txt");
    int i;
    for (i = 0; i < n; i++)
    {
        int x = rand() % 50 + 1;
        of << x << endl;
    }
    of.close();
}

int sortf() //calls the quicksort function and sends it array of elements which
{           //were previously stored in the file and outputs sorted values to another file
    int arr[50];
    int n = 50;
    ifstream f("List.txt");
    int counter = 0;
    int i;
    while (!f.eof() && counter < 50)
    {
        f >> arr[counter];
        counter++;
    }
    quickSort(arr, 0, 49);
    ofstream of("ListOut.txt");
    for (i = 0; i < n; i++)
    {
        of << arr[i] << endl;
    }
}

void quickSort(int arr[], int start, int end) //applies quicksort algorithm
{
    if (start < end)
    {
        int a = start;
        int b = end;
        int p = arr[(a + b) / 2];
        int x = (a + b) / 2;
        int temp;
        while (a < b)
        {
            while (arr[b] > p)
            {
                b--;
            }
            while (arr[a] <= p && a <= b)
            {
                a++;
            }
            if (a < b)
            {
                temp = arr[b]; //swapping left and right position elements
                arr[b] = arr[a];
                arr[a] = temp;
            }
        }

        temp = arr[b]; //bringing pivot in the middle, so that
        arr[b] = arr[x]; //elements smaller than pivot are to the left
        arr[x] = temp; //and elements greater than pivot are to the right
        quickSort(arr, start, b - 1);
        quickSort(arr, b + 1, end);
    }
}

int main()
{
    print(50); //printing 50 numbers in the file
    high_resolution_clock::time_point t1 = high_resolution_clock::now();
    sortf();
    high_resolution_clock::time_point t2 = high_resolution_clock::now();
    auto duration = duration_cast<nanoseconds>(t2 - t1).count();
    cout << "\nTime taken:\n" << duration; //outputs time taken for input, sorting and output.
}

The output file after execution contains the following list of elements:

3 16 7 9 25 12 10 12 13 14 24 18 13 16 18 18 21 19 20 20 22 22 23 23 24 27 27 28 30 28 30 30 31 33 34 35 36 41 36 37 37 37 38 41 43 43
44 44 49 50

Please help me in correcting my code as I have wasted quite a lot of hours on this little problem.