Template Matrix scalar multiplication c++ - c++

Question

When I try to compile the following coding, I have get an error saying
Error 1 error LNK2019: unresolved external symbol "class Matrix __cdecl operator*(int const &,class Matrix &)" (??D#YA?AV?$Matrix#H$02$01##ABHAAV0##Z) referenced in function _main C:\Users\pc\Documents\Visual Studio 2013\Projects\Homework4\Homework4\main.obj Homework4
This error is occurred from the scalar multiplication on matrix and I don't know how to fix it. Can anyone give me an advice on this?
//Matrix.h
#pragma once
#include <iostream>
#include <vector>
using namespace std;
template<class T, int m, int n>
class Matrix
{
vector<vector<T>> elements;
int nrow;
int ncol;
public:
Matrix();
~Matrix();
void print();
vector<T> row(int);
vector<T> col(int);
friend Matrix<T, m, n> operator+<>(const Matrix<T, m, n>&, const Matrix<T, m, n>&);
friend Matrix<T, m, n> operator-<>(const Matrix<T, m, n>&, const Matrix<T, m, n>&);
friend Matrix<T, m, n> operator*(const T&, Matrix<T, m, n>&);
};
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>
vector<T> Matrix<T, m, n>::row(int i)
{
return elements[i];
}
template<class T, int m, int n>
vector<T> Matrix<T, m, n>::col(int j)
{
vector<T> col;
for (int i = 0; i < ncol; i++)
{
col.push_back(elements[i][j]);
}
return col;
}
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>
Matrix<T, m, n> operator+(const Matrix<T, m, n>& m1, const Matrix<T, m, n>& m2){
Matrix<T, m, n> m3;
int nrow = m1.nrow;
int ncol = m1.ncol;
for (int i = 0; i < nrow; ++i){
for (int j = 0; j < ncol; ++j){
m3.elements[i][j] = m1.elements[i][j] + m2.elements[i][j];
}
}
return m3;
}
template<class T, int m, int n>
Matrix<T, m, n> operator-(const Matrix<T, m, n>& m1, const Matrix<T, m, n>& m2){
Matrix<T, m, n> m3;
int nrow = m1.nrow;
int ncol = m1.ncol;
for (int i = 0; i < nrow; ++i){
for (int j = 0; j < ncol; ++j){
m3.elements[i][j] = m1.elements[i][j] - m2.elements[i][j];
}
}
return m3;
}
template<class T, int m, int n>
Matrix<T, m, n> operator*(const T& c, const Matrix<T, m, n>& m1){
int nrow = m1.nrow;
int ncol = m1.ncol;
Matrix<T, m, n> m2;
for (int i = 0; i < nrow; ++i){
for (int j = 0; j < ncol; ++j){
m2.elements[i][j] = c*m1.elements[i][j];
}
}
return m2;
}
//main.cpp
#include "Matrix.h"
using namespace std;
int main()
{
Matrix<int, 3, 2> a;
Matrix<int, 3, 2> b;
Matrix<int, 3, 2> c;
c = 2*b;
c.print();
}
I think the error comes from the friend function (operation *) in the body of the class, but I don't know how to fix it...

Answer 1

The error occurs because
a) operator* is declared as non-template (you should use <>)
b) if you use <> syntax, you must provide the declaration of the template.
Fix
(Add before Matrix class):
template<class T, int m, int n>
class Matrix;
template<class T, int m, int n>
Matrix<T, m, n> operator*(T&, const Matrix<T, m, n>&);
//class matrix definition here
Add <> to operator* friend declaration:
friend Matrix<T, m, n> operator*<>(T&, const Matrix<T, m, n>&);