C++ 程序重载加法运算符以相加两个矩阵

#include <iostream>
#include <vector>
using namespace std;
class Matrix {
  public:
    Matrix() {}
    Matrix(const Matrix& x) : a(x.a) {}
    Matrix(const vector<vector<int>>& v) : a(v) {}
    Matrix operator+(const Matrix&);
    vector<vector<int>> a;
    void display(){
        for(int i = 0; i<a.size(); i++){
            for(int j = 0; j<a[i].size(); j++){
                cout << a[i][j] << " ";
            }    
            cout << endl;
        }
    }
};
Matrix Matrix::operator+(const Matrix& m){
    vector<vector<int>> vv = a;
    for (int i=0; i<vv.size(); i++){
        for (int j=0; j<vv[0].size(); j++){
            vv[i][j] += m.a[i][j];
        }
    }
    return Matrix(vv);
}
int main(){
   vector<vector<int>> mat1 = {{5,8},{9,6},{7,9}};
   vector<vector<int>> mat2 = {{8,3},{4,7},{6,3}};
   int r = mat1.size();
   int c = mat1[0].size();
   Matrix m1(mat1), m2(mat2), res;
   res = m1 + m2;
   res.display();
}