Saving image with libpng - const object - c++

Question

I have a class which manages a grayscale image. I want to save it with libpng. To do that I want to use a const member function like this:
void GrayscaleImage::SavePNG(std::string filename) const
{
// ...
png_bytep* row_pointers = new png_bytep[m_height];
for (int i = 0; i < height_; i++) {
row_pointers[i] = const_cast<png_bytep>(m_data.data()) + i * m_width * sizeof(uint8_t);
}
png_set_rows(png_ptr, info_ptr, row_pointers);
// ...
}
The problem is that the third argument of png_set_rows is non-const, so I have to use const_cast at some point, if I want the member function GrayscaleImage::SavePNG to be const. I'm wondering, is it safe to do this?

Answer 1

libpng provides an API to cause it to free the row_pointers and the stuff they point to; png_data_freer. That's the default on read (where png_set_rows can currently be called but the call gets ignored).
What you did is safe, so long as you don't call png_data_freer. None of the write APIs modify the input data.
The problem exists in png_write_image, the API png_write_png calls, as well, and it exists in png_write_rows. It used to exist in png_write_row which is the lowest level API but that was fixed in libpng 1.5; it's a quiet API change there because it doesn't change the type compatibility of the argument. Changing it any higher would cause existing applications to fail to compile because of type errors.
You're not likely to see changes soon; changing the API in ways that require applications to rewrite existing code is unlikely to happen until libpng 2.0 in my opinion.

Answer 2

It's not safe. Without the third parameter of png_set_rows() declared const, you have no guarantee that it will not modify the input data.
You will always have to deal with libraries that don't declare parameters const even if they could. That is why const_cast exists. You should use it with extreme caution. But judging from the description, it's unlikely that png_set_rows() will modify your data.
EDIT: here is the source code. You can see it doesn't modify row_pointers. (But it definitely modifies the other two arguments!)
void PNGAPI
png_set_rows(png_structp png_ptr, png_infop info_ptr, png_bytepp row_pointers)
{
png_debug1(1, "in %s storage function", "rows");
if (png_ptr == NULL || info_ptr == NULL)
return;
if (info_ptr->row_pointers && (info_ptr->row_pointers != row_pointers))
png_free_data(png_ptr, info_ptr, PNG_FREE_ROWS, 0);
info_ptr->row_pointers = row_pointers;
if (row_pointers)
info_ptr->valid |= PNG_INFO_IDAT;
}

Answer 3

The const in your function definition just says that your instance shouldn't change. Saving to a file shouldn't change your instance so it's alright.
Of course the third parameter of png_set_rows isn't const because it gets set.
It doesn't matter if you create, destroy or change things in a const function as long as they don't belong to your class. Your code doesn't change any instance of GrayscaleImage.