passing a QPainter object to widgets

Hello,
I'm writing a program that acts this way:
- I have a principal QWidget over which I create a QPainter,
- I draw on the widget,
- I pass the Painter to other widgets so I can use those widgets to draw on the principal one,
- Finally,in the principal widget,I call end() on the painter.

The program works properly but ends with a segmentation fault. I runned gdb and it sais that the problem was that it doesn't found qlist.h (quite strange I think).

However the output has no apparent problem:I can display the result of my painting on video and I can eventually draw it on a file. The problem arises when I try to convert the .ps image created with my program to a .pdf,in fact I obtain this error using dvipdf ( in fact I use the .ps file I create as an image in a latex text):

/usr/bin/dvipdf: line 40: 20896 Broken pipe exec dvips -q -f "$infile"
20897 Segmentation fault | gs $OPTIONS -q -dNOPAUSE -dBATCH -sDEVICE=pdfwrite -sOutputFile="$outfile" $OPTIONS -c .setpdfwrite -

I'm sure it's not a problem of the latex file or of my dvipdf because if I do the same things but without passing the QPainter to other widgets,I have no problem.

What can I do?

thanks!

Hi,

You can try to do something like

Code:

---

// Principal widget
void ChildWidget::myFunc(QPainter* painter)
{
    painter->begin(parentWidget());
    // Drawing
    painter->end();
}

---

PS. If you use Qt 4.1 you can print to a PDF file directly using

Code:

---

printer.setPrinterMode(QPrinter::HighResolution);
printer.setOutputFormat(QPrinter::PdfFormat);

---

PPS. Sorry for my very bad English.

PPPS. For vratojr: I'm italian like you. If you like you can write me in private post in Italian

Quote:

---

Originally Posted by vratojr

- I draw on the widget,
- I pass the Painter to other widgets so I can use those widgets to draw on the principal one,

---

QCanvas does similar thing, so there shouldn't be any problems with this approach.

How do those widget draw themselves?

Quote:

---

Originally Posted by vratojr

The program works properly but ends with a segmentation fault. I runned gdb and it sais that the problem was that it doesn't found qlist.h (quite strange I think).

---

Make sure you compile your application in debug mode (you should have "CONFIG += debug" in .pro file). Then run your application from gdb. When is crashes issue "bt" command to print the backtrace and post it here.

Quote:

---

How do those widget draw themselves?

---

Here is a brief description:

Code:

---

//Inside the Main widget's paintEvent:
QPainter painter;
painter.begin( graficoPaintDevice );
//drawing
assex = new AsseNumerico (/*options*/,painter,this);
assex.draw();
painter.end();
 
//stucture of the second widget
class AsseNumerico,public QWidget{
 public:
  AsseNumerico(....);
  void draw(QPainter &painter);
private:
 QPainter p;
};
 
AsseNumerico::AsseNumerico(/*options*/,QPainter &painter,QWidget *parent):QWidget(parent),p(painter){};
 
void AsseNumerico::draw(Qpainter &painter){
 p.drawSomething();
}

---

Quote:

---

Make sure you compile your application in debug mode (you should have "CONFIG += debug" in .pro file). Then run your application from gdb. When is crashes issue "bt" command to print the backtrace and post it here.

---

Well, first. I will explain better and briefly how the program works. I do some calculation to draw an istogram on screen and afterward I give the opportunity to print it.
If I display the istogram on screen WITHOUT trying to print it I got the the error:

*** glibc detected *** double free or corruption (!prev): 0x0825f648 ***

And, if i use the file in the .ps and then use dvipdf I got the error:
/usr/bin/dvipdf: line 40: 28347 Broken pipe exec dvips -q -f "$infile"
28348 Segmentation fault | gs $OPTIONS -q -dNOPAUSE -dBATCH -sDEVICE=pdfwrite -sOutputFile="$outfile" $OPTIONS -c .setpdfwrite -
Segmentation fault

And this is the ouput from gdb:

*** glibc detected *** double free or corruption (!prev): 0x08252e38 ***

Program received signal SIGABRT, Aborted.
[Switching to Thread -1221314880 (LWP 31932)]
0xffffe410 in ?? ()
(gdb) bt
#0 0xffffe410 in ?? ()
#1 0xbfffed94 in ?? ()
#2 0x00000006 in ?? ()
#3 0x00007cbc in ?? ()
#4 0xb738c6e5 in raise () from /lib/tls/libc.so.6
#5 0xb738e049 in abort () from /lib/tls/libc.so.6
#6 0xb73c07ba in __fsetlocking () from /lib/tls/libc.so.6
#7 0xb73c6d9c in malloc_trim () from /lib/tls/libc.so.6
#8 0xb73c737a in free () from /lib/tls/libc.so.6
#9 0xb756a6f1 in operator delete () from /usr/lib/libstdc++.so.6
#10 0xb7bca5d7 in ~QPainterPrivate (this=0x82644b0) at qpainter_p.h:120
#11 0xb7bbe78a in ~QPainter (this=0x819629c) at painting/qpainter.cpp:635
#12 0x08081a4b in ~AsseRiferimento (this=0x8196238) at .moc/moc_assenumerico.cpp:77

Quote:

---

Originally Posted by vratojr

*** glibc detected *** double free or corruption (!prev): 0x08252e38 ***

Program received signal SIGABRT, Aborted.
[...]
#12 0x08081a4b in ~AsseRiferimento (this=0x8196238) at .moc/moc_assenumerico.cpp:77

---

It seems that you delete memory twice. What happens in that AsseRiferimento destructor?

Quote:

---

Originally Posted by vratojr

Here is a brief description:

Code:

---

//Inside the Main widget's paintEvent:
QPainter painter;
painter.begin( graficoPaintDevice );
//drawing
assex = new AsseNumerico (/*options*/,painter,this);
assex.draw();
painter.end();
 
//stucture of the second widget
class AsseNumerico,public QWidget{
 public:
  AsseNumerico(....);
  void draw(QPainter &painter);
private:
 QPainter p;
};
 
AsseNumerico::AsseNumerico(/*options*/,QPainter &painter,QWidget *parent):QWidget(parent),p(painter){};
 
void AsseNumerico::draw(Qpainter &painter){
 p.drawSomething();
}

---

---

Try to use QPainter pointer as function parameter instead o reference

Code:

---

AsseNumerico::AsseNumerico(/*options*/,QPainter* painter,QWidget *parent):QWidget(parent),p(painter){};

---

Using copy constructor you force the creation of a copy object and destruction of it. Passing pointer you don't.

Code:

---

It seems that you delete memory twice. What happens in that AsseRiferimento destructor?

---

Nothing.
I didn't wrote code for the destructor.
should I?

Quote:

---

Originally Posted by vratojr

Nothing.
I didn't wrote code for the destructor.
should I?

---

No, you don't have to write it. So the problem is probably because you create a copy that QPainter. Try what mcosta suggests.

Quote:

---

Originally Posted by jacek

So the problem is probably because you create a copy that QPainter.

---

Qt Official Documentation says

Quote:

---

QPainter::QPainter ( QPaintDevice * pd )
Constructs a painter that begins painting the paint device pd immediately.
This constructor is convenient for short-lived painters, e.g. in a paint event and should be used only once.

The constructor calls begin() for you and the QPainter destructor automatically calls end().

---

So, you must use QPainter pointer.

You were right, I resolved doing that way.

Thank you very much boys!