class QtWidgetsApplication5 : public QDialog
{
    Q_OBJECT

public:
    QtWidgetsApplication5(QWidget *parent = Q_NULLPTR)
        : QDialog(parent)
    {
        ui.setupUi(this);
        this->setAttribute(Qt::WA_TranslucentBackground);//设置窗口背景透明
        this->setWindowFlags(Qt::FramelessWindowHint);   //设置无边框窗口
    }

    void paintEvent(QPaintEvent *event) override
    {
        QPainter painter(this);
        painter.setRenderHint(QPainter::Antialiasing);  // 反锯齿;
        painter.setBrush(QBrush(Qt::red));
        painter.setPen(Qt::black);
        QRect rect = this->rect();
        rect.setWidth(rect.width() - 1);
        rect.setHeight(rect.height() - 1);
        painter.drawRoundedRect(rect, 15, 15);
        
       /* QStyleOption opt;
        opt.init(this);
        QPainter p(this);
        style()->drawPrimitive(QStyle::PE_Widget, &opt, &p, this);

        this->setStyleSheet("{background-color:transparent; }");*/
        QDialog::paintEvent(event);


    }

private:
    Ui::QtWidgetsApplication5Class ui;
};