c++ - 尝试在 Qt 中使用 boundingRect

Question

Kwadrat 类有什么问题？我有一个错误:

Invalid new-expression of abstract class type 'Kwadrat'

Kwadrat* kwadrat = new Kwadrat(20);

我想要在屏幕上移动一个正方形(当它在 X 坐标上达到 370 或在 Y 坐标上达到 370 时它会回到中间)。

当 Kwadrat 是经典的 QGraphicsRectItem 时，我有一个 (0,0) 坐标的错误。

#include <QApplication>
#include <QGraphicsView>
#include <QGraphicsScene>
#include <QPixmap>
#include "poruszanie.h"
#include <QRectF>
#include <QGraphicsRectItem>

class Kwadrat : public QGraphicsItem
{
    Q_OBJECT

public:
    Kwadrat(int size)
        : QGraphicsItem(NULL) // we could parent, but this may confuse at first
    {
        m_boundingRect = QRectF(0, 0, size, size);
    }

    QRectF boundingRect() const
    {
        return m_boundingRect;
    }

private:
    QRectF m_boundingRect;
};

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    QGraphicsScene*scena=new QGraphicsScene();

   // Poruszanie*kwadrat=new Poruszanie();
   // kwadrat->setRect(0,0,20,20);
   // kwadrat->setBrush(QBrush(Qt::white));
   // scena->addItem(kwadrat);

    Kwadrat*kwadrat=new Kwadrat(20);
    kwadrat->setBrush(QBrush(Qt::white));
    scena->addItem(kwadrat);

    kwadrat->setFlag(QGraphicsItem::ItemIsFocusable);
    kwadrat->setFocus();

    QGraphicsView *widok=new QGraphicsView(scena);
    widok->setBackgroundBrush(QBrush(Qt::yellow));
    widok->setMinimumSize(400,400);
    widok->show();

    return a.exec();
}

Answer 1

To write your own graphics item, you first create a subclass of QGraphicsItem, and then start by implementing its two pure virtual public functions: boundingRect(), which returns an estimate of the area painted by the item, and paint(), which implements the actual painting.

来自 http://doc.qt.io/qt-5/qgraphicsitem.html#details

所以你需要实现纯虚函数paint来绘制你的QGraphicsItem来消除错误，你的编译器应该已经表明，除了告诉你知道你的类是抽象的(因为缺少函数实现)。