定义了一个 Matrix 结构体

struct Matrix {
    int v[N][N], n, m;
    Matrix(int n = 0, int m = 0) {
    	memset(v, 0, sizeof v); 
        this->n = n; this->m = m;
    }
}

如何重载运算符 >>，使 cin >> x 与下列代码等价？（有 Matrix x(n, m); ）

for (int i = 1; i <= n; ++i)
    for (int j = 1; j <= m; ++j)
        cin >> x.v[i][j];