qt如何实现画图板的各项功能（如：新建画布，打开图片，选择画笔颜色和粗细，橡皮擦，填充颜色等）

在 Qt 中实现一个画图板的功能可以通过使用 QWidget 或者 QGraphicsView 进行绘图，以及使用各种控件和信号槽来实现不同的功能。下面是一个简单的示例，展示了如何实现画图板的一些基本功能：

#include <QtWidgets>

class DrawingBoard : public QWidget {
public:
    DrawingBoard(QWidget *parent = nullptr) : QWidget(parent) {
        setAttribute(Qt::WA_StaticContents);
        setSizePolicy(QSizePolicy::Expanding, QSizePolicy::Expanding);
        setMouseTracking(true);

        m_penColor = Qt::black;
        m_penWidth = 2;
        m_brushColor = Qt::white;
        m_eraserMode = false;
    }

protected:
    void paintEvent(QPaintEvent *event) override {
        QPainter painter(this);
        painter.setRenderHint(QPainter::Antialiasing);
        painter.drawImage(rect(), m_image, rect());
    }

    void mousePressEvent(QMouseEvent *event) override {
        if (event->button() == Qt::LeftButton) {
            m_lastPoint = event->pos();
        }
    }

    void mouseMoveEvent(QMouseEvent *event) override {
        if (!(event->buttons() & Qt::LeftButton))
            return;

        QPainter painter(&m_image);
        painter.setPen(QPen(m_penColor, m_penWidth));

        if (m_eraserMode) {
            painter.setCompositionMode(QPainter::CompositionMode_Clear);
            painter.drawPoint(event->pos());
        } else {
            painter.setCompositionMode(QPainter::CompositionMode_SourceOver);
            painter.drawLine(m_lastPoint, event->pos());
            m_lastPoint = event->pos();
        }

        update();
    }

    void resizeEvent(QResizeEvent *event) override {
        if (width() > m_image.width() || height() > m_image.height()) {
            int newWidth = qMax(width() + 128, m_image.width());
            int newHeight = qMax(height() + 128, m_image.height());
            resizeImage(&m_image, QSize(newWidth, newHeight));
            update();
        }

        QWidget::resizeEvent(event);
    }

    void resizeImage(QImage *image, const QSize &newSize) {
        if (image->size() == newSize)
            return;

        QImage newImage(newSize, QImage::Format_RGB32);
        newImage.fill(m_brushColor);
        QPainter painter(&newImage);
        painter.drawImage(QPoint(0, 0), *image);
        *image = newImage;
    }

private:
    QImage m_image;
    QPoint m_lastPoint;
    QColor m_penColor;
    int m_penWidth;
    QColor m_brushColor;
    bool m_eraserMode;
};

int main(int argc, char *argv[]) {
    QApplication app(argc, argv);

    QWidget window;
    QVBoxLayout layout(&window);

    DrawingBoard drawingBoard;
    layout.addWidget(&drawingBoard);

    QPushButton newButton("New");＊＊＊