Need help with quick sort

[mgreider]

So I'm learning c++ and I'm extremely confused by quick sort. I've watched videos and read chapters and I'm still stumped by this assignment. The idea of the assignment is to sort a user input string with quick sort, using a middle pivot and recursion.
The main issue I'm having is the implementation of the for loop that will break the string into two parts (the less than pivot and the greater than pivot.)

This whole assignment has me lost I feel like I'm way off track. Could someone help get me going in the right direction?

 #include<iostream>
#include<string>
using namespace std;

//Here we use quicksort to sort the string of characters
//we sort the string in place without making any copies, so
//for a call to quicksort we pass in the low and high indices
//we sort to.
void QuickSort(string &input, int low, int high)
{
    //initial conditions
    int smallindex = low;
    int index = low;
    char pivot;
    //base case if only one element
    if(input.size()<=1)
        return;
    
    //choose pivot as middle element
    pivot=input[input.size()/2];

    //swap pivot with index
    

    swap(input[index],input[input.size()/2]);

    //execute quicksort loop
    for(int index;index<high;index++)
    {
        if(input.at(index)<pivot)
        {
            smallindex++;
            //swap pivot with smallindex
            swap(input[index],input[smallindex]);
        }
    }
    
    //recursively call each side
    QuickSort(input, low, smallindex - 1);
    QuickSort(input, smallindex + 1, high);
}

//in main we just want to take in a string
int main()
{
    string input;
    cout << "Please input a string:" << endl;
    
    //get the string
    getline(cin, input);
    
    //efficiently sort the list
    QuickSort(input, 0, input.length()-1);
    //print the output
    cout << input << endl;
    
    return 0;
}