    //=============================================================================
    //  MuseScore
    //  Music Composition & Notation
    //
    //  Copyright (C) 2016 Werner Schweer
    //
    //  This program is free software; you can redistribute it and/or modify
    //  it under the terms of the GNU General Public License version 2
    //  as published by the Free Software Foundation and appearing in
    //  the file LICENCE.GPL
    //=============================================================================

    #ifndef __STAFFTYPECHANGE_H__
    #define __STAFFTYPECHANGE_H__

    #include "element.h"

    namespace Ms {

    class StaffType;

    //---------------------------------------------------------
    //   @@ StaffTypeChange
    //---------------------------------------------------------

    class StaffTypeChange : public Element {
          Q_GADGET

          StaffType* _staffType { 0 };
          qreal lw;

          virtual void layout() override;
          virtual void spatiumChanged(qreal oldValue, qreal newValue) override;
          virtual void draw(QPainter*) const override;

       public:
          StaffTypeChange(Score* = 0);
          StaffTypeChange(const StaffTypeChange&);
          virtual StaffTypeChange* clone() const override { return new StaffTypeChange(*this); }

          virtual ElementType type() const override { return ElementType::STAFFTYPE_CHANGE; }
          virtual bool systemFlag() const override    { return false;  }

          virtual void write(XmlWriter&) const override;
          virtual void read(XmlReader&) override;

          StaffType* staffType() const     { return _staffType; }
          void setStaffType(StaffType* st) { _staffType = st; }

          Measure* measure() const            { return (Measure*)parent();   }

          virtual QVariant getProperty(P_ID propertyId) const override;
          virtual bool setProperty(P_ID propertyId, const QVariant&) override;
          virtual QVariant propertyDefault(P_ID) const override;
          };

    }     // namespace Ms

    #endif

↑ V690 The 'StaffTypeChange' class implements a copy constructor, but lacks the '=' operator. It is dangerous to use such a class.