simple QString comment

[high_flyer]

calling clear() on is makes it both empty and null.

Should not.
From the docs:

void QString::clear ()

Clears the contents of the string and makes it empty.

bool QString::isNull () const

Returns true if this string is null; otherwise returns false.

Example:

QString().isNull(); // returns true
QString("").isNull(); // returns false
QString("abc").isNull(); // returns false

Qt makes a distinction between null strings and empty strings for historical reasons. For most applications, what matters is whether or not a string contains any data, and this can be determined using isEmpty().

See also isEmpty().