关于在类体内实现函数的错误

class NoFocusFrameDelegate : public QStyledItemDelegate
{
public:
    NoFocusFrameDelegate(QObject *parent) :QStyledItemDelegate(parent)
    {

    }

    void NoFocusFrameDelegate::paint(QPainter *painter, const QStyleOptionViewItem &option, const QModelIndex &index) const
    {
        QStyleOptionViewItem itemOption(option); // remove the focus state
        if (itemOption.state & QStyle::State_HasFocus)
        {
            itemOption.state ^= QStyle::State_HasFocus;
        }
        QStyledItemDelegate::paint(painter, itemOption, index);
    }
};

红色行会报错：error: extra qualification 'NoFocusFrameDelegate::' on member 'paint' [-fpermissive]
     void NoFocusFrameDelegate::paint(QPainter *painter, const QStyleOptionViewItem &option, const QModelIndex &index) const

解决方法是去掉
NoFocusFrameDelegate::