处理矩阵(数组）的分段错误错误

我正在尝试引用我构建的 2D 数组对象，但由于某种原因，我遇到了分割错误，并且无法找出原因。到目前为止，我已经推断出错误在于我的operator<<函数，但我无法弄清楚要更改什么。

主.cpp：

int main() {    
My_matrix m1(3, 2);
cout << "Before" << endl;
m1(0,0) = 1; //causes segfault
cout << "After" << endl;
m1(0,1) = 2; //causes segfault
m1(1,0) = 3; //causes segfault
m1(1,1) = 4; //causes segfault
m1(2,0) = 5; //causes segfault
m1(2,1) = 6; //causes segfault
cout << "Checkpoint 1" << endl;
My_matrix m2(); //works
cout << "Checkpoint 2" << endl;
cout << m1.number_of_rows() << endl; //works
cout << "Checkpoint 3" << endl;
cout << m1; //Segfault
cout << "Checkpoint 4" << endl;
cout << m1; //Segfault
cout << "Checkpoint 5" << endl;
cout << m2; //Segfault
cout << "Checkpoint 6" << endl;
}

My_matrix.cpp：

#include "My_matrix.h"
#include <stdexcept>
My_matrix::My_matrix()
{
  n = 0;
  m = 0;
  ptr = nullptr;
}
My_matrix::My_matrix(int n1, int m1)
{
  n = n1;
  m = m1;
  ptr = new int*[n];
  for(int i = 0; i < n; i++) {
      ptr[i] = new int[m];
  }
  for (int i = 0; i < n; i++) {
      for (int j = 0; j < m; j++) {
          ptr[i][j] = 0;
      }
  }
}
My_matrix::My_matrix(const My_matrix& mat)
{
  this->n = mat.n;
  this->m = mat.m;
  ptr = new int*[n];
  for(int i = 0; i < n; i++) {
      ptr[i] = new int[m];
  }
  //----- copy elements over to new matrix -----//
  for (int i = 0; i < mat.n; i++) {
      for (int j = 0; j < mat.m; j++) {
          this->ptr[n][m] = mat.ptr[n][m];
      }
  }
}
My_matrix::~My_matrix()
{
  clear();
}
My_matrix& My_matrix::operator=(const My_matrix& mat)
{
  clear();
  this->n = mat.n;
  this->m = mat.m;
  ptr = new int*[n];
  for(int i = 0; i < n; i++) {
      ptr[i] = new int[m];
  }
  //----- copy elements over -----//
  for (int i = 0; i < mat.n; i++) {
      for (int j = 0; j < mat.m; j++) {
          this->ptr[n][m] = mat.ptr[n][m];
      }
  }
}
int My_matrix::number_of_rows() const
{
  return n;
}
int My_matrix::number_of_columns() const
{
  return m;
}
int* My_matrix::operator()(int i) const
{
  return ptr[i];
}
int& My_matrix::operator()(int i, int j) const
{
  return ptr[n][m];
}
int& My_matrix::operator()(int i, int j)
{
  return ptr[n][m];
}
int& My_matrix::elem(int i, int j) const
{
  if (i < 0 || i >= n) throw out_of_range("Out of range");
  if (j < 0 || j >= m) throw out_of_range("Out of range");
  return ptr[n][m];
}
int& My_matrix::elem(int i, int j)
{
  if (i < 0 || i >= n) throw out_of_range("Out of range");
  if (j < 0 || j >= m) throw out_of_range("Out of range");
  return ptr[n][m];
}
void My_matrix::clear()
{
    for (int i = 0; i < n; i++) {
        delete[] ptr[i];
    }
    delete[] ptr;
    n = 0;
    m = 0;
}
ostream& operator<<(ostream& out, const My_matrix& mat)
{
  for (int i = 0; i < mat.number_of_rows(); i++) {
      for (int j = 0; j < mat.number_of_columns(); j++) {
          out << mat(i, j) << " "; //This line is the culprit ****
      }
      out << endl;
  }
  return out;
}