app crash when slider connect to paintEvent

pantat merah

hi everyone, i need help, i'm trying to create donut chart with paintEvent and i will connecting that donut bar with slider to simulate.
when i sliding the slider the run app crash, anyone tell me what's wrong
i connecting slider with this

connect(ui->horizontalSlider,SIGNAL(valueChanged(int)),this,SLOT(setvar(int)));

this is my script in mainwindow.h

#ifndef MAINWINDOW_H
#define MAINWINDOW_H
#include <QMainWindow>
#include <QWidget>
#include "donutr.h"

namespace Ui {
class MainWindow;
}

class MainWindow : public QMainWindow
{
    Q_OBJECT

private:
    donutr *r;
    Ui::MainWindow *ui;

public slots:
    void setvar(int val){r->setdonut(val);}
};

#endif // MAINWINDOW_H

and this is my script in promote widget donutr.h

#ifndef DONUTR_H
#define DONUTR_H
#include <QWidget>

class donutr : public QWidget
{
    Q_OBJECT

public:
    donutr(QWidget *parent);
    int setdonut(int val) {
    int data = val;
    }

private:
   int data;
protected:
    void paintEvent(QPaintEvent *event);
};

#endif // DONUT_H

and this is the donutr.cpp

#include "mainwindow.h"
#include <QPainter>

donutr::donutr(QWidget *parent) : QWidget(parent)
{
}

void donutr::paintEvent(QPaintEvent *event)
{  
    Q_UNUSED(event);
    QPainter painter(this);
    QColor warna1 = QColor(230, 230, 230);
    QRectF size1 = QRectF(0,30,300,300);
    QRectF size2 = QRectF(26,56,250,250);
    painter.setBrush(QColor(255, 143, 145,255));
    painter.setPen(QColor(255, 143, 145,255));
    painter.drawPie(size1,0*16, data);
    painter.setBrush(warna1);
    painter.setPen(warna1);
    painter.drawPie(size2,0,360*16);
}

mrjj

Hi and welcome
You have the widget as
donutr *r;

do you create an instance ?
Like
r= new donutr(this);

somewhere?

Else you will crash here
void setvar(int val){
r->setdonut(val); <<<<< is r a valid object here?
}

pantat merah

@mrjj no, i don't create
r= new donutr(this);

i try to create
r= new donutr(this);
in mainwindow.h but some error show, i trying to move that script to mainwindow.cpp the error don't show agan and my app don't close
thx for your info.

now my script works

mrjj

Hi
Ok. A good place to create instance would been
just after the setupUI in constructor of mainwindow.

Just as note
when you write
donutr *r;

You just declare it. A pointer to a donutr . but its not set to point to anything.
So if u use it - without setting it to point to valid object. it will crash.