friend istream& operator >> (istream &in, Matrix &mas)
{
    for (int i = 0; i < mas.n; i++)
    {
        for (int j = 0; j < mas.m; j++)
        {
            in >> mas.mas[i][j];

        }
    }
    return in;
}

class Matrix
{private:
int n, m, **mas;
public:
  Matrix (int n, int m)
{
    int**mas = new int*[n];
    for (int i = 0; i < n; i++)
    {
        mas[i] = new int[m];
    }
    for (int i = 0; i < n; i++)
        for (int j = 0; j < m; j++)
            mas[i][j] = 0;
}

Matrix (int n, int m) : n(n), m(m)
{
  mas = new int*[n];
  ...