The QFont documentation says:

"When a QPainter draws a character from a font the QFont will report whether or not it has the character; if it does not, QPainter will draw an unfilled square."

However, on X11, when adding a Korean character to a QLabel using a font which does not contain Korean glyphs (and font merging is disabled), a square is not always painted:

The fonts Courier and Nimbus Mono L do not contain a square .notdef glyph, but no square is painted.
The fonts Bell MT and Albany AMT do contain a square .notdef glyph, but no square is painted.

The fonts Cumberland AMT, Thorndale AMT, DejaVu Sans and Bitstream Vera Serif do contain a square .notdef glyph, and a square is painted.

OpenOffice Writer, on the other hand, paints a square for all the above fonts which do no result in a square in Qt.