xylo

Nothing to it!

Actually there is and the solutions maybe a little brittle.

Firstly, let's actually derive

class MyString: public QString {

};

Secondly go and look in QString and copy all of the constructors out, you will need them.

Now you can add your code too.

    String() : QString() { }
//    explicit String(const QString & value) : QString(value) { }
    explicit String(const QChar *unicode, int size = -1) : QString(unicode, size) { }
    String(QChar c) : QString(c) { }
    String(int size, QChar c) : QString(size, c) { }
    // this should pick up const char * surely tbh i need improvements here, my source code is utf8 for a start
    String(const char * _utf8) : QString(QLatin1String(_utf8)) { }
    inline String(QLatin1String latin1) : QString(latin1) { }

    inline String(const QString & _qstring) Q_DECL_NOTHROW : QString(_qstring) { }

    // might need these too. should be able to assign a QString to a String (should be easy)
#if 0
    QString &operator=(QChar c);
    QString &operator=(const QString &) Q_DECL_NOTHROW;
    inline QString &operator=(QLatin1String latin1);
#endif
#if 0
    // might needs this
#ifdef Q_COMPILER_RVALUE_REFS
    inline QString(QString && other) Q_DECL_NOTHROW : QString(other) { }
    // how the hell do you pass this on?
    inline QString &operator=(QString &&other) Q_DECL_NOTHROW
    { qSwap(d, other.d); return *this; }
#endif
#endif
    // default value or not?
    String(const QJsonValue & json) {
        // is this utter overkill in syntax terms.
        *((QString *) (this)) = json.toString();
    }

    operator QJsonValue() const {
        return QJsonValue(*this);
    }

Does it work? Yes, but if QString's ctors change between revisions you'll need to update your derivation.