Template Matrix-Matrix multiplication c++

Question

I am trying to do Matrix-Matrix multiplication with template and I keep getting following error. ( I am trying to multiply non-square matrix)

Error 1 error C2593: 'operator *' is ambiguous

Can any one give me an advice on how to fix this?

//Matrix.h
#pragma once
#include <iostream>
#include <vector>
using namespace std;

template<class T, int m, int n>
class Matrix;

template<class T, int m, int n, int l>
Matrix<T, m, n> operator*(const Matrix<T, m, n>&, const Matrix<T, n, l>&);

template<class T, int m, int n>
class Matrix
{
vector<vector<T>> elements;
int nrow;
int ncol;

public:
    Matrix();
    ~Matrix();
    void print();
    template<int l>
    friend Matrix<T, m, l> operator*<>(const Matrix<T, m, n>&, const Matrix<T, n, l>&);

};

template<class T, int m, int n>
Matrix<T, m, n>::Matrix() : nrow(m), ncol(n)
{
    for (int i = 0; i < nrow; i++){
        vector<T> row(ncol, i);
        elements.push_back(row);
    }
}

template<class T, int m, int n>
Matrix<T, m, n>::~Matrix(){}

template<class T, int m, int n>
void Matrix<T, m, n>::print()
{
    for (int i = 0; i < nrow; i++){
        for (int j = 0; j < ncol; j++){
            cout << elements[i][j] << " ";
        }
    cout << endl;
    }
}

template<class T, int m, int n, int l> 
Matrix<T, m, l> operator*(const Matrix<T, m, n>& m1, const Matrix<T, n, l>& m2){
    int nrow = m1.nrow;
    int ncol = m2.ncol;
    Matrix<T, m, l> m3;
    for (int i = 0; i < nrow; ++i){
        for (int j = 0; j < ncol; ++j){
            m3.elements[i][j] = 0;
            for (int k = 0; k < m1.ncol; k++){
                T temp = m1.elements[i][k] * m2.elements[k][j];
                m3.elements[i][j] = temp + m3.elements[i][j];
            }
        }
    }
return m3;
}

//main.cpp
#include "Matrix.h"
using namespace std;

int main()
{
Matrix<int, 3, 2> a;
Matrix<int, 2, 1> b;
Matrix<int, 3, 1> c;
c = a*b;

c.print();
}

The problem occurs in the matrix multiplication possibly due to the coding error in the template.

Answer 1

The error is here:

./matrix.cpp:48:28: error: function template partial specialization is not allowed
    friend Matrix<T, m, l> operator*<>(const Matrix<T, m, n>&, const Matrix<T, n, l>&);
                           ^        ~~
1 error generated.

change it to this:

friend Matrix<T, m, l> operator*(const Matrix<T, m, n>&, const Matrix<T, n, l>&);

Answer 2

I had to change what Richard changed, but I also had to change the declaration of operator* and the friend as follows:

template<class T, int m, int n, int l>
Matrix<T, m, l> operator*(const Matrix<T, m, n>&, const Matrix<T, n, l>&);
          // ^ here

and:

template<class _T, int _m, int _n, int l>
friend Matrix<_T, _m, l> operator*(const Matrix<_T, _m, _n>&, const Matrix<_T, _n, l>&);

I got the " operator* is ambiguous" error if I didn't change the first of these two, since the forward declaration doesn't match the instantiation further down.

It's currently outputting:

0
1
2

That doesn't seem quite right, but I'm not awake enough to debug further.