// Creating a simple empty cube
GUIElem img(0, 0, 40, 40);
// Creating an interpolation for img, to rotate it to 90°
RotationInterpolation inter(&img, 90);

// Inside the loop
// This calculates the interpolations from 0° to 90° and updates the image
inter.update(delta_time);
// This draw the image
img.draw();