c++ qt继承QObject和QGraphicsItem画十字点、画矩形、画多边形、画直线卡尺，在同一个类中,而且可以通过鼠标滚轮缩放

可以通过以下代码实现：

#include <QtGui>

class MyGraphicsItem : public QObject, public QGraphicsItem
{
public:
    MyGraphicsItem(QGraphicsItem *parent = nullptr) : QObject(parent), QGraphicsItem(parent) {}

    QRectF boundingRect() const override
    {
        return QRectF(-50, -50, 100, 100);
    }

    void paint(QPainter *painter, const QStyleOptionGraphicsItem *option, QWidget *widget) override
    {
        painter->setRenderHint(QPainter::Antialiasing);
        painter->setPen(QPen(Qt::black, 2));
        painter->drawLine(-50, 0, 50, 0);
        painter->drawLine(0, -50, 0, 50);
        painter->drawRect(-25, -25, 50, 50);

        QPolygonF polygon;
        polygon << QPointF(-30, 30) << QPointF(0, -30) << QPointF(30, 30);
        painter->drawPolygon(polygon);

        painter->setPen(QPen(Qt::red, 1, Qt::DashLine));
        painter->drawLine(-50, 0, 50, 0);
        painter->drawLine(0, -50, 0, 50);
    }

    void mousePressEvent(QGraphicsSceneMouseEvent *event) override
    {
        if (event->button() == Qt::LeftButton) {
            m_lastPos = event->pos();
            m_isDragging = true;
        }
    }

    void mouseMoveEvent(QGraphicsSceneMouseEvent *event) override
    {
        if (m_isDragging) {
            QPointF delta = event->pos() - m_lastPos;
            setPos(pos() + delta);
            m_lastPos = event->pos();
        }
    }

    void mouseReleaseEvent(QGraphicsSceneMouseEvent *event) override
    {
        if (event->button() == Qt::LeftButton) {
            m_isDragging = false;
        }
    }

    void wheelEvent(QGraphicsSceneWheelEvent *event) override
    {
        qreal factor = qPow(1.2, event->delta() / 240.0);
        setScale(scale() * factor);
    }

private:
    QPointF m_lastPos;
    bool m_isDragging = false;
};

在这个类中，我们继承了 QObject 和 QGraphicsItem，实现了 boundingRect() 和 paint() 函数来绘制图形，以及 mousePressEvent()、mouseMoveEvent()、mouseReleaseEvent() 和 wheelEvent() 函数来处理鼠标事件。在 paint() 函数中，我们绘制了十字点、矩形、多边形和直线卡尺，并使用 setPen() 函数设置了画笔的颜色、宽度和样式。在 mousePressEvent()、mouseMoveEvent() 和 mouseReleaseEvent() 函数中，我们处理了鼠标拖动事件，使图形可以被拖动。在 wheelEvent() 函数中，我们处理了鼠标滚轮事件，使图形可以被缩放。