QPainter doesn't paint

[Blitzor DDD]

Hello!

I am trying to paint a simple plot (sin) on a screen.
Maybe my method is a bit wierd I don't know, this is my first paint-expirience.
Hower, I encountered a problem.

Here is my code:

.h

#ifndef WIDGET_H
#define WIDGET_H
 
#include <QWidget>
#include <QLabel>
#include <QPainter>
 
 
 
class Widget : public QWidget
{
    Q_OBJECT
const double pi = 3.14159;
public:
    Widget(QWidget *parent = 0);
    ~Widget();
 
private:
    QLabel *Hi;
    QPainter *paint;
  virtual  void paintEvent(QPaintEvent * );
};
 
#endif // WIDGET_H

#ifndef WIDGET_H
#define WIDGET_H

#include <QWidget>
#include <QLabel>
#include <QPainter>



class Widget : public QWidget
{
    Q_OBJECT
const double pi = 3.14159;
public:
    Widget(QWidget *parent = 0);
    ~Widget();

private:
    QLabel *Hi;
    QPainter *paint;
  virtual  void paintEvent(QPaintEvent * );
};

#endif // WIDGET_H

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.cpp

#include "widget.h"
#include <QWidget>
#include <cmath>
 
Widget::Widget(QWidget *parent)
    : QWidget(parent)
{
     this->setGeometry(100,100,800,500);
     paint = new QPainter(this);
}
 
Widget::~Widget()
{
 
}
 
 void Widget::paintEvent(QPaintEvent * )
{
    paint-> setPen(QPen(Qt::red));
 
    for(int i=0;i<1000;i++)
    {
        qreal x=sin(pi/6+i);
        qreal y=sin(pi/6+i);
        paint->drawPoint(QPointF(x,y));
    }
}

#include "widget.h"
#include <QWidget>
#include <cmath>

Widget::Widget(QWidget *parent)
    : QWidget(parent)
{
     this->setGeometry(100,100,800,500);
     paint = new QPainter(this);
}

Widget::~Widget()
{

}

 void Widget::paintEvent(QPaintEvent * )
{
    paint-> setPen(QPen(Qt::red));

    for(int i=0;i<1000;i++)
    {
        qreal x=sin(pi/6+i);
        qreal y=sin(pi/6+i);
        paint->drawPoint(QPointF(x,y));
    }
}

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At first, I just tried to paint in .cpp without void Widget::paintEvent(QPaintEvent * ) like this:

#include "widget.h"
#include <QWidget>
#include <cmath>
 
Widget::Widget(QWidget *parent)
    : QWidget(parent)
{
     this->setGeometry(100,100,800,500);
     paint = new QPainter(this);
     paint-> setPen(QPen(Qt::red));
 
     for(int i=0;i<1000;i++)
     {
         qreal x=sin(pi/6+i);
         qreal y=sin(pi/6+i);
         paint->drawPoint(QPointF(x,y));
     }
}
 
Widget::~Widget()
{
 
}

#include "widget.h"
#include <QWidget>
#include <cmath>

Widget::Widget(QWidget *parent)
    : QWidget(parent)
{
     this->setGeometry(100,100,800,500);
     paint = new QPainter(this);
     paint-> setPen(QPen(Qt::red));

     for(int i=0;i<1000;i++)
     {
         qreal x=sin(pi/6+i);
         qreal y=sin(pi/6+i);
         paint->drawPoint(QPointF(x,y));
     }
}

Widget::~Widget()
{

}

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It didn't work. Then I looked throught the Internet and found out that one can paint only in void Widget::paintEvent(QPaintEvent * ), and besides this method must be virtual. I have changed this, it didn't help either.

then I found that paint(this) should be changed to this paint.begin(viewport());
done and viewport() "no matching fucntion to call"

What seems to be the problem here? and could someone help me to fix that?
Thank you in advance!

[ChrisW67]

Typically you allocate a QPainter on the stack inside paintEvent(). As it goes out of scope QPainter::end() will ensure all painting is completed and resources freed.
All 1000 of your x,y coordinates has a value between 0 and 1: they all map to a single pixel.
The paintEvent() function is normally protected.

[anda_skoa]

Typically you allocate a QPainter on the stack inside paintEvent().

That is actually the only supported option for painting onto a widget, due to how the widget buffering is implemented internally.

This code should even have generated a warning at runtime (creating a QPainter on the widget outside of paintEvent()).

Creating a painter on viewport() is for painting in a QAbstractScrollArea derived class, in normal widgets the paint device is "this".

Cheers,
_