MergeSort C++ using vectors

Question

I have been trying to fix my code for a while now, and while it does print out the numbers, it only prints them out in the order that I gave them in. It's not really doing the merge sort as it was intended to, don't know if I missed typed something but would really appreciate the feedback and help. Also the part where it says static void ordena(typename std::vector<T>::iterator &&b, typename std::vector<T>::iterator &&e) that can't be changed whatsoever since it's required for me to have it to turn it in.

#include <vector>
#include <algorithm>
#include <iostream>

using namespace std;

template<typename T>
class mergeSort {
public:

    static void ordena(typename std::vector<T>::iterator &&b, typename std::vector<T>::iterator &&e) {
        if (b-e+1 <= 1)
            return;

        auto iter = b + (b-e+1) / 2;
        vector<T> v1(b, iter);
        vector<T> v2(iter, e);

        ordena(v1.begin(),v1.end());
        ordena(v2.begin(),v2.end());

        vector<T> v3;
        merge(v3, v1, v2);
    }

    static void merge(vector<T> &v3, vector<T> &v1, vector<T> &v2){
        auto siz1 = v1.size();
        auto siz2 = v2.size();
        size_t p1 = 0;
        size_t p2 = 0;

        while (p1 < siz1 && p2 < siz2) {
            if (v1.at(p1) < v2.at(p2))
                v3.push_back(v1.at(p1++));
            else
                v3.push_back(v2.at(p2++));
        }

        while (p1 < siz1) v3.push_back(v1.at(p1++));
        while (p2 < siz2) v3.push_back(v2.at(p2++));
    }
};

template<typename T>
std::string to_string(const std::vector<T> &elements){
    std::string str{};
    for(T e : elements) str += std::to_string(e) + ", ";
    return str.substr(0,str.length()-2);
}

int main()
{
    vector<int> elements;

    elements.push_back(100);
    elements.push_back(10);
    elements.push_back(80);
    elements.push_back(30);
    elements.push_back(60);
    elements.push_back(50);
    elements.push_back(40);
    elements.push_back(70);
    elements.push_back(20);
    elements.push_back(90);

    mergeSort<int>::ordena(elements.begin(), elements.end());
    std::cout << "First ordered sequence : " << to_string(elements) << std::endl;
}

Answer 1

After merge(v3, v1, v2), copy v3 to *b to *(e-1). This is not an efficient way to do this, but it should work.

// only single reference is needed
static void ordena(typename std::vector<T>::iterator &b, typename std::vector<T>::iterator &e) {
    // ...
    merge(v3, v1, v2);
    std::copy(v3.begin(), v3.end(), b);

---

You could also change merge to take an iterator as the first parameter

    merge(b, v1, v2);