QList<double> => QVariant => QList<double>

[anda_skoa]

You problem is here:

QVariant x2;
    x2.fromValue<QList<double> >(x1);

QVariant x2;
    x2.fromValue<QList<double> >(x1);

To copy to clipboard, switch view to plain text mode 

QVariant::fromValue() returns a QVariant. It is a static method, it does not (cannot) change x2.

QVariant x2 = QVariant::fromValue(x1);

QVariant x2 = QVariant::fromValue(x1);

To copy to clipboard, switch view to plain text mode 

Cheers,
_