Tak robię

class MyClass : public QObject
{
    Q_OBJECT
public:
    explicit MyClass(qint32 arg1, QObject *parent = 0);
    const qint32 atr1;
    qint32 get_atr1(qint32 arg1) {
        qint32 to_be_atr1;
        //do sth
        return to_be_atr1;
    }
};

MyClass::MyClass(qint32 arg1, QObject *parent) :
    atr1 { get_atr1(arg1) },
    QObject(parent)
{
}