一关于矩阵类和重载运算符的题，求帮助不知道哪里错了

#include <iostream>
#define M 3
#define N 3
using namespace std;
class Matrix
{
public:
    Matrix(){}
    Matrix(int n[M][N])
    {
        int i, j;
        for (i = 0; i < M; i++)
        {
            for (j = 0; j < N; j++)
            {
                m[i][j] = n[i][j];
            }
        }
    }
    ~Matrix(){}
    Matrix operator+ (Matrix &m1)
    {
        Matrix m2,m3;
        int i, j;
        for (i = 0; i < M; i++)
        {
            for (j = 0; j < N; j++)
            {
                m3.m[i][j] =m2. m[i][j] + m1.m[i][j];
            }
        }
        return m3;
    }
    void show()
    {
        for (int i = 0; i < M; i++)
        {
            for (int j = 0; j < N; j++)
            {
                cout << m[i][j] <<" "<< endl;
            }
            cout << endl;
        }
    }
private:
    int m[M][N];
};


int main()
{
    int i, j;
    int a[M][N], b[M][N],c[M][N];
    cout << "a is : " << endl;
    for (i = 0; i < M; i++)
    {
        for (j = 0; j < N; j++)
        {
            cin >> a[i][j];
        }
    }
    cout << "b is :" << endl;
    for (i = 0; i < M; i++)
    {
        for (j = 0; j < N; j++)
        {
            cin >> b[i][j];
        }
    }
    Matrix m1(a), m2(b),m3(c);
    m3=m1.operator+(m2);
    m3.show();
    system("pause");
}