manual access/construction to OpenCV Mat Data - c++

Question

I'm new to OpenCV, I have created a simple program for learning purposes, here is the code:
cv::Mat image;
std::string imageName = "test.jpg";
image = cv::imread(imageName, cv::IMREAD_COLOR); // Read the file
if(! image.data ) // Check for invalid input
{
std::cout << "Could not open or find the image" << std::endl ;
return false;
}
uchar* mData = image.data;//input buffer data
uchar* nData = (uchar*)malloc(image.rows * image.cols * image.channels() * sizeof(uchar));//output buffer data
std::cout << image.rows << " " << image.cols << " " << image.channels() << " " << sizeof(uchar) << std::endl;
std::cout << image.step << std::endl;
//int r,g,b;
for(int i = 0;i < image.rows ;i++)
{
for(int j = 0;j < image.step ;j++)
{
nData[image.step * i + j ] = mData[image.step * i + j ] / 3;
nData[image.step * i + j + 1] = mData[image.step * i + j + 1] / 3;
nData[image.step * i + j + 2] = mData[image.step * i + j + 2] / 3;
}
}
cv::Mat outputImage(image.rows, image.cols, CV_8UC3, (void*)nData);
if(! outputImage.data ) // Check
{
std::cout << "output mat failure" << std::endl ;
return false;
}
cv::namedWindow( "Input image window", cv::WINDOW_AUTOSIZE ); // Create a window for display.
cv::imshow( "Input image window", image ); // Show our image inside it.
cv::namedWindow( "Output image window", cv::WINDOW_AUTOSIZE ); // Create a window for display.
cv::imshow( "Output image window", outputImage ); // Show our image inside it.
cv::waitKey(0); // Wait for a keystroke in the window
free(nData);
1- is it safe to access the input Mat Data like this? what is the need of isContinuous()?
2- is this the correct way to do such a manual copy?
3- free(nData) do I have to manually handle my own memory?
4- I will use this with GPU, so I need to make sure that the input pointer "is safe" so I can use memcpy without worrying