why QPixmap along with QScale is causing segmentation fault on qt 5.9 ?

nikhil_belani

i have executed below code and it’s giving segmentation fault

#include <QApplication>
#include <QLabel>
#include <QPicture>
#include <QPainter>
#include <QPixmap>

int main(int argc, char *argv[])
{
       QApplication a(argc, argv);
       const QSize qsize (100,100);
       QPixmap *bg = new QPixmap(200,200);
       QPainter* p = new QPainter(bg);
      *bg = bg->scaled(qsize)
       bg->fill(QColor(180, 192, 201));
       p->setPen(QPen(QColor(180, 192, 201),1, Qt::SolidLine));
       p->drawRect(0, 0, 200, 200);
 
       return a.exec();
}
 (added tags : mrjj)

if we comment *bg = bg->scaled(qsize) ; line no segmentation fault is found .
issue is found on Qt 5.9 version

JKSH

@nikhil_belani said in why QPixmap along with QScale is causing segmentation fault on qt 5.9 ?:

   QPixmap *bg = new QPixmap(200,200);
   QPainter* p = new QPainter(bg);

Don't use new here. Just allocate QPainter and QPixmap on the stack.

QPixmap bg1(200, 200);
QPixmap bg2 = bg1.scaled(qsize);

See http://doc.qt.io/qt-5/implicit-sharing.html

nikhil_belani

thanks same was code was working in 4.7 Qt version .
why its not working with qt 5.9 ??

JKSH

@nikhil_belani said in why QPixmap along with QScale is causing segmentation fault on qt 5.9 ?:

thanks

You're welcome. Does your updated code work now?

same was code was working in 4.7 Qt version .
why its not working with qt 5.9 ??

I don't know the exact cause, but lots of internal implementations changed between Qt 4.8 and Qt 5.0.

Even in Qt 4, the recommended way to use QPixmap was by-value, not by-pointer.

mrjj

Hi
It crashes in the painter that have a reference to the BG and i assume
its due to implicit sharing being fooled with *b = implicit shared copy of scaled self

if you flip the order to
QPixmap *bg = new QPixmap(200,200);
bg=bg->scaled(qsize);
QPainter p = new QPainter(bg);
it wont crash.

However as @JKSH points out. QPixmaps are meant to be used as non pointers.