The classes QTextLayout, QFontMetrics, QFontMetricsF and QFont each offer a constructor where you can provide a pointer to a QPaintDevice. In the end, all they ever do is read the dpi settings from the paint device.

When writing code that performs layout calculations, e.g. for printing, you are forced to keep a lot of methods non-const for the sole reason that the paint device is required as a non-const parameter. Using const_cast is possible, but ugly (triggers warnings in static code analysis, etc.)

Suggestion: Make these QPaintDevice constructor parameters const