junk.cpp

// the what has been

Matrix &Matrix::operator*=(const Matrix &m)
{
  return *this = *this * m; // observe that, due to the copy-and-swap implementation
                            // of the assignment operator, there are no redundant copies created
}

Matrix Matrix::operator*(const Matrix &m) const
{
  Matrix res(_rows, m._cols);
  for (std::size_t i = 0; i < _rows; i++)
  {
    for (std::size_t j = 0; j < m._cols; j++)
    {
      for (std::size_t k = 0; k < _cols; k++)
      {
        res.get(i, j) += get(i, k) * m.get(k, j);
      }
    }
  }
  return res;
}


// the suggested

Matrix &Matrix::operator*=(const Matrix &m)
{
  Matrix res(_rows, m._cols);
  for (std::size_t i = 0; i < _rows; i++)
    for (std::size_t j = 0; j < m._cols; j++)
      for (std::size_t k = 0; k < _cols; k++)
      {
        res.get(i, j) += get(i, k) * m.get(k, j);
      }
  *this = res; // res gets copied, O(n)
  return *this;
}

Matrix Matrix::operator*(const Matrix &m) const
{
  Matrix this_copy = *this; // a copy of *this, O(n)
  return this_copy *= m;
}