Save a list of QPixmaps to .ico file - c++

Question

I'm interested in creating a single .ico file (with transparency) from a list of QPixmap images (with sizes 16x16, 32x32, 48x48...). I haven't seen any related method in Qt's documentation: QPixmap, QImage, QIcon (which is for storing images for UI states, not related to the file format)...
Does Qt has such functionality? How can I save such file? May be mixing with Windows API?
PS: A low-level solution would be to directly write the .ico file, but I'm more interested in not reinventing the wheel if possible.

Answer 1

It seems that there is no built in support in Qt for writing ICO files, so here I'm publishing a code snippet to generate one from a list of pixmaps. Hope it may be useful for somebody else.
template<typename T>
void write(QFile& f, const T t)
{
f.write((const char*)&t, sizeof(t));
}
bool savePixmapsToICO(const QList<QPixmap>& pixmaps, const QString& path)
{
static_assert(sizeof(short) == 2, "short int is not 2 bytes");
static_assert(sizeof(int) == 4, "int is not 4 bytes");
QFile f(path);
if (!f.open(QFile::OpenModeFlag::WriteOnly)) return false;
// Header
write<short>(f, 0);
write<short>(f, 1);
write<short>(f, pixmaps.count());
// Compute size of individual images
QList<int> images_size;
for (int ii = 0; ii < pixmaps.count(); ++ii) {
QTemporaryFile temp;
temp.setAutoRemove(true);
if (!temp.open()) return false;
const auto& pixmap = pixmaps[ii];
pixmap.save(&temp, "PNG");
temp.close();
images_size.push_back(QFileInfo(temp).size());
}
// Images directory
constexpr unsigned int entry_size = sizeof(char) + sizeof(char) + sizeof(char) + sizeof(char) + sizeof(short) + sizeof(short) + sizeof(unsigned int) + sizeof(unsigned int);
static_assert(entry_size == 16, "wrong entry size");
unsigned int offset = 3 * sizeof(short) + pixmaps.count() * entry_size;
for (int ii = 0; ii < pixmaps.count(); ++ii) {
const auto& pixmap = pixmaps[ii];
if (pixmap.width() > 256 || pixmap.height() > 256) continue;
write<char>(f, pixmap.width() == 256 ? 0 : pixmap.width());
write<char>(f, pixmap.height() == 256 ? 0 : pixmap.height());
write<char>(f, 0); // palette size
write<char>(f, 0); // reserved
write<short>(f, 1); // color planes
write<short>(f, pixmap.depth()); // bits-per-pixel
write<unsigned int>(f, images_size[ii]); // size of image in bytes
write<unsigned int>(f, offset); // offset
offset += images_size[ii];
}
for (int ii = 0; ii < pixmaps.count(); ++ii) {
const auto& pixmap = pixmaps[ii];
if (pixmap.width() > 256 || pixmap.height() > 256) continue;
pixmap.save(&f, "PNG");
}
return true;
}
Code also available in GitHub.