试图在QLabel上绘画失败（无法在没有对象的情况下调用成员函数'virtual void QLabel :: paintEvent（QPaintEvent *）'）

Question

I have seen many examples on how to use a paintevent, but I just cannot get it to work.

I have in my .ui file a label named 'image' and I am trying to paint inside it. I fail miserably. In most of the examples I've seen they use the

QLabel::paintEvent(e)

but I cannot use this, I get:

error: cannot call member function 'virtual void QLabel::paintEvent(QPaintEvent*)' without object

And, when I use

ui->image->paintEvent(e);

I get

/usr/include/qt4/QtGui/qlabel.h:141: error: 'virtual void QLabel::paintEvent(QPaintEvent*)' is protected

I seem to be missing something... That's the part of my code that I try to implement this:

void crop_my_image::paintEvent(QPaintEvent *e)
{
    ui->image->paintEvent(e);
    QPainter painter(ui->image);
    painter.setPen(QPen(QBrush(QColor(0,0,0,180)),1,Qt::DashLine));
    painter.setBrush(QBrush(QColor(255,255,255,120)));
    painter.drawRect(selectionRect);
}

crop_my_image is of QDialog type!

PS: If, instead of ui->image->paintEvent(e); I use QDialog::paintEvent(e); I can successfully draw on my dialog, so I should be in the right path!

Thanks in advance for any answers!

Answer 1

You have to do exactly the same with label what you did with QDialog, which is create class which will inherit from QLabel and implement paintEvent function. Example:

//MyLabel.h
class MyLabel : public QLabel
{
    Q_OBJECT
    public:
        MyLabel(QWidget *parent = 0);
    private:
        void paintEvent(QPaintEvent *);
};

//MyLabel.cpp

MyLabel::MyLabel(QWidget *parent)
    : QLabel(parent)
{
    /*...*/

}

void MyLabel::paintEvent(QPaintEvent *)
{
    /* paint somehting on your label*/
}

And than you will be able to do:

void crop_my_image::paintEvent(QPaintEvent *e)
{
    myLabelObject->paintEvent(e);
    QPainter painter(ui->image);
    painter.setPen(QPen(QBrush(QColor(0,0,0,180)),1,Qt::DashLine));
    painter.setBrush(QBrush(QColor(255,255,255,120)));
    painter.drawRect(selectionRect);
}

But firstly, why event you want to invoke this method?