U++

I see that by default we don't initialize size value to (0, 0):

    Size_() {}
    Size_(T cx, T cy) : cx(cx), cy(cy) {}

I believe the first constructor should look like this

    Size_() : cx(static_cat<T>(0), static_cast<T>(0)) {}
    Size_(T cx, T cy) : cx(cx), cy(cy) {}

And what I need is to make sure that:

Size size;
if (size.IsEmpty()) {
  // Should be empty by default.
}