Saving 64-bit integer with QSettings

Question

Is there any neat way, short of converting number to QByteArray, to save quint64 with QSettings? The problem is QVariant doesn't accept qint64 nor quint64.

Answer 1

QVariant supports qlonglong and qulonglong. As the documentation says, these are the same as qint64 and quint64. So you can just use QVariant::QVariant(qlonglong) and QVariant::toLongLong.

Answer 2

What if you store qint64 as a string. QString supports such conversion: QString::number(qlonglong n, int base), where qlonglong is the same as qint64. The same for quint64 - use QString::number(qulonglong n, int base), where qulonglong is the same as quint64.

QSettings settings("config.ini", QSettings::IniFormat);
[..]
qint64 largeNumber = Q_INT64_C(932838457459459);
settings.setValue("LargeNumber", QString::number(largeNumber));
[..]

Answer 3

Another solution is to realize that IEEE 754 double format has a 53 bit fraction (don't forget the implicit 53rd bit!) and a sign bit. This allows you to store unsigned 53 bit integers without loss of precision, or signed 54 bit integers. You can store if:

• your qint64's absolute value is smaller than 2^55, or
• your quint64 is smaller than 2^54.