Error when trying to overload output operator

Question

I'm learning C++, and I'm running into a linking error when building my project.

The error is associated with my code that is overloading the output << operator.

This is the error that I am getting:

1>lab4.obj : error LNK2019: unresolved external symbol "class std::basic_ostream<char,struct std::char_traits<char> > & __cdecl operator<<(class std::basic_ostream<char,struct std::char_traits<char> > &,class Matrix<2,2> const &)" (??6@YAAAV?$basic_ostream@DU?$char_traits@D@std@@@std@@AAV01@ABV?$Matrix@$01$01@@@Z) referenced in function _main
1>c:\users\matt\documents\visual studio 2010\Projects\lab4\Debug\lab4.exe : fatal error LNK1120: 1 unresolved externals

How do I fix this error? I have attached my code below

#include <iostream>
#include <string>
#include <stdio.h>
using namespace std;

template <int m, int n> 
class Matrix{
public:
    int data[m][n];
public:
    Matrix(){};

    void setup(){
        for(int i=0;i<m;i++){
            for(int j=0;j<n;j++){
                data[i][j] = 0;
            }
        }
    }

    void setAll(int integer){
        for(int i=0;i<m;i++){
            for(int j=0;j<n;j++){
                data[i][j] = integer;
            }
        }
    }

    void output(){
        for(int i=0;i<m;i++){
            cout << "[ ";
            for(int j=0;j<n;j++){
                cout << data[i][j] << " ";
            }
            cout<<"]"<<endl;
        }
    }

    Matrix<m,n> operator+ (const Matrix<m,n> &rhs)
    {
        Matrix<m,n> result;

        for(int i=0;i<m;i++){
            for(int j=0;j<n;j++){
                result.data[i][j] = this->data[i][j] + rhs.data[i][j];
            }
        }

        return result;
    }

    Matrix<m,n> operator- (const Matrix<m,n> &rhs)
    {
        Matrix<m,n> result;

        for(int i=0;i<m;i++){
            for(int j=0;j<n;j++){
                result.data[i][j] = this->data[i][j] - rhs.data[i][j];
            }
        }

        return result;
    }

    template <int l>
    Matrix <m,l> operator*(const Matrix<n,l> &rhs) //const
    {
        //only if number of columns in the first matrix equals the number of rows in the second matrix. 
        Matrix<m,l> result;
        for(int i = 0; i < m; i++){
            for(int j = 0 ; j < l ; j++){
                result.data[i][j] = 0;
                for(int k = 0; k < n; k++){
                    result.data[i][j] += this->data[i][k] * rhs.data[k][j];
                }
            }
        }
        return result;
    };

    Matrix <m,n> operator*(int scalar){
        Matrix <m,n> result;
        for(int i= 0; i < m; i++){
            for(int j = 0; j < n; j++){
                result.data[i][j] = this->data[i][j] * scalar;
            }
        }
        return result;
    };

    friend ostream& operator<<(ostream &os, const Matrix<m,n> &rhs);
    friend Matrix <m,n> operator++(const Matrix<m,n> &rhs, int);
    friend Matrix <m,n> operator--(const Matrix<m,n> &rhs, int);
};

template <int m, int n> 
ostream& operator<< (ostream &os, const Matrix<m,n> &rhs){
    for(int i=0;i<m;i++){
        os << "[ ";
        for(int j=0;j<n;j++){
            os << rhs.data[i][j] << " ";
        }
        os<<"]"<<endl;
    }
    return os;
}

template <int m, int n> 
 Matrix <m,n> operator++(const Matrix<m,n> &rhs, int){
    for(int i=0;i<m;i++){
        for(int j=0;j<n;j++){
            rhs.data[i][j]++;
        }
    }
}

template <int m, int n> 
 Matrix <m,n> operator--(const Matrix<m,n> &rhs, int){
    for(int i=0;i<m;i++){
        for(int j=0;j<n;j++){
            rhs.data[i][j]--;
        }
    }
}

int main(){
    Matrix<2,2> m = Matrix<2,2>();
    m.setup();
    //m.output();

    Matrix<2,2> p = Matrix<2,2>();
    p.setup();

    Matrix<2,2> z = Matrix<2,2>();
    z = m + p;
    //z.output();

    z = z * z;
    cout << z;
    string stopper;
    getline(cin, stopper);
}

Answer 1

Is your whole source located really in a single file, as posted here? Are you aware, that you have to define function-members of a template class in its header? (actually, it's just the simplest solution, there are some workarounds)

Ref.: http://www.parashift.com/c++-faq/separate-template-fn-defn-from-decl.html

If not, it can be the reason of an error.

BTW, this source is erroneous:

template <int m, int n> 
ostream& operator<< (ostream &os, const Matrix<m,n> &rhs){
    for(int i=0;i<m;i++){
        os << "[ ";
        for(int j=0;j<n;j++){
            os << data[i][j] << " ";
        }
        os<<"]"<<endl;
    }
    return os;
}

data here is not declared, it must be referenced as rhs.data, since this operator<< is not a member of the class matrix.

Upd.

You have to declare all your friend functions inside the class as

template <int f_m, int f_n> 
friend ostream& operator<<(ostream &os, const Matrix<f_m,f_n> &rhs);

They're separate templates, their template args must be separate. It's strange, that MSVC++ does not issue that error. gcc says:

matrix.cc:91:67: warning: friend declaration «std::ostream& operator<<(std::ostream&, const Matrix<m, n>&)» declares a non-template function [-Wnon-template-friend]
matrix.cc:91:67: note: (if this is not what you intended, make sure the function template has already been declared and add <> after the function name here)