Having trouble clearing a QTreeWidget.

[Nyphel]

Hello,

I've re-configured and re-built Qt in debug mode.

I've tested my code under GDB and I got the same backtrace as before (the one in the above message).

I've tested your code under GDB and I got the following backtrace :

D:\Applications\Qt\4.3.1\Workspace\Listeur>gdb debug/Listeur
GNU gdb 6.3
Copyright 2004 Free Software Foundation, Inc.
GDB is free software, covered by the GNU General Public License, and you are
welcome to change it and/or distribute copies of it under certain conditions.
Type "show copying" to see the conditions.
There is absolutely no warranty for GDB.  Type "show warranty" for details.
This GDB was configured as "i686-pc-mingw32"...
(gdb) run
Starting program: D:\Applications\Qt\4.3.1\Workspace\Listeur/debug/Listeur.exe
warning: ITEMS SELECTED: 1
 
warning: ITEMS SELECTED: 0

D:\Applications\Qt\4.3.1\Workspace\Listeur>gdb debug/Listeur
GNU gdb 6.3
Copyright 2004 Free Software Foundation, Inc.
GDB is free software, covered by the GNU General Public License, and you are
welcome to change it and/or distribute copies of it under certain conditions.
Type "show copying" to see the conditions.
There is absolutely no warranty for GDB.  Type "show warranty" for details.
This GDB was configured as "i686-pc-mingw32"...
(gdb) run
Starting program: D:\Applications\Qt\4.3.1\Workspace\Listeur/debug/Listeur.exe
warning: ITEMS SELECTED: 1

warning: ITEMS SELECTED: 0

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At this step the QTeeWidget is cleared and the application crashed.

[wysota]

So as you see there are no items selected so you can't call at(0). You have to redesign the method.

[Nyphel]

Yes, but I still don't understand the problem.

void QTreeWidget::itemSelectionChanged () [signal]
This signal is emitted when the selection changes in the tree widget. The current selection can be found with selectedItems()

It appears that the signal itemSelectionChanged() has been emited 2 times. This is strange... The first time we have 1 item selected : that's OK, I select 1 item in the QTreeWidget. But why is the list empty ?! It should contains the item, like it is described in the Assistant :

QList<QTreeWidgetItem *> QTreeWidget::selectedItems () const
Returns a list of all selected non-hidden items.
See also itemSelectionChanged().

So I can't understand :
- Why the signal has been emited 2 times (1 just clic 1 item, 1 time, with 1 clic)
- Why the function isEmpty() returns "true", where as count() returns "1".

Have you got an idea ?

[wysota]

If isEmpty() returns true, count() has to return 0. There is no other way, you must be messing something up. I can't say why the selection is empty in your case - maybe you are mixing the selected and the current item? Current item doesn't have to be selected...

[Nyphel]

With this code for my SLOT :

void interface_impl::SLOT_selection_changed()
{
	  QString item_name;
 
	  QList<QTreeWidgetItem*> sel = tree->selectedItems();
	  qDebug("ITEMS SELECTED: %d", sel.count());
 
	  if(!sel.isEmpty())
	  {
	    item_name = sel.at(0)->text(0);
	  } 
}

void interface_impl::SLOT_selection_changed()
{
	  QString item_name;
	
	  QList<QTreeWidgetItem*> sel = tree->selectedItems();
	  qDebug("ITEMS SELECTED: %d", sel.count());
	
	  if(!sel.isEmpty())
	  {
	    item_name = sel.at(0)->text(0);
	  } 
}

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I get the following backtrace :

D:\Applications\Qt\4.3.1\Workspace\Listeur>gdb debug/Listeur
GNU gdb 6.3
Copyright 2004 Free Software Foundation, Inc.
GDB is free software, covered by the GNU General Public License, and you are
welcome to change it and/or distribute copies of it under certain conditions.
Type "show copying" to see the conditions.
There is absolutely no warranty for GDB.  Type "show warranty" for details.
This GDB was configured as "i686-pc-mingw32"...
(gdb) run
Starting program: D:\Applications\Qt\4.3.1\Workspace\Listeur/debug/Listeur.exe
warning: ITEMS SELECTED: 1
 
 
Program exited normally.
(gdb)

D:\Applications\Qt\4.3.1\Workspace\Listeur>gdb debug/Listeur
GNU gdb 6.3
Copyright 2004 Free Software Foundation, Inc.
GDB is free software, covered by the GNU General Public License, and you are
welcome to change it and/or distribute copies of it under certain conditions.
Type "show copying" to see the conditions.
There is absolutely no warranty for GDB.  Type "show warranty" for details.
This GDB was configured as "i686-pc-mingw32"...
(gdb) run
Starting program: D:\Applications\Qt\4.3.1\Workspace\Listeur/debug/Listeur.exe
warning: ITEMS SELECTED: 1


Program exited normally.
(gdb)

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So now the QTreeWidget isn't cleared and the signal isn't emited a second time. That's OK.
And, if I'm right :
- 1 select an item : the signal is emited and the slo is performed
- "tree->selectedItems();" returns 1 QTreeWidgetItem* in the list
- "count()" confirms that there is 1 item in the list
- "isEmpty()" returns "TRUE", indicating that the list is empty

Like "tree->selectedItems();" returns a QList of selected items, I suppose I never take care about current item :s

[wysota]

It is "!isEmpty()", so it negates the result.

[Nyphel]

Yes of course
Like "isEmpty()" returns TRUE, "!isEmpty()" returns FALSE and the instruction "item_name = sel.at(0)->text(0);" is performed, like shown on the backtrace... This is my problem.

I can't understand how this QList can be empty with an item inside of it...

[Nyphel]

Oh I'm so stupid... What am I saying... All works well...
Sorry Wysota ^_^

To conclude, here is my last backtrace showing that there is no more problem.

void interface_impl::SLOT_selection_changed()
{
   QString item_name;
 
   QList<QTreeWidgetItem*> sel = tree->selectedItems();
   qDebug("COUNT: %d", sel.count());
   qDebug("SIZE: %d", sel.size());
   qDebug("EMPTY: %d", sel.isEmpty());
 
   if(sel.isEmpty() == false)
   {
      qDebug("item_name : before acces");
      item_name = sel.at(0)->text(0);
      qDebug("item_name : after acces");
   } 	
 
   qDebug("tree : before clear");
   tree->clear();
   qDebug("tree : after clear");
}

void interface_impl::SLOT_selection_changed()
{
   QString item_name;
   
   QList<QTreeWidgetItem*> sel = tree->selectedItems();
   qDebug("COUNT: %d", sel.count());
   qDebug("SIZE: %d", sel.size());
   qDebug("EMPTY: %d", sel.isEmpty());
	
   if(sel.isEmpty() == false)
   {
      qDebug("item_name : before acces");
      item_name = sel.at(0)->text(0);
      qDebug("item_name : after acces");
   } 	
	
   qDebug("tree : before clear");
   tree->clear();
   qDebug("tree : after clear");
}

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D:\Applications\Qt\4.3.1\Workspace\Listeur>gdb debug/Listeur
GNU gdb 6.3
Copyright 2004 Free Software Foundation, Inc.
GDB is free software, covered by the GNU General Public License, and you are
welcome to change it and/or distribute copies of it under certain conditions.
Type "show copying" to see the conditions.
There is absolutely no warranty for GDB.  Type "show warranty" for details.
This GDB was configured as "i686-pc-mingw32"...
(gdb) run
Starting program: D:\Applications\Qt\4.3.1\Workspace\Listeur/debug/Listeur.exe
 
warning: COUNT: 1
 
warning: SIZE: 1
 
warning: EMPTY: 0
 
warning: item_name : before acces
 
warning: item_name : after acces
 
warning: tree : before clear
 
warning: COUNT: 0
 
warning: SIZE: 0
 
warning: EMPTY: 1
 
warning: tree : before clear
 
warning: tree : after clear
 
warning: tree : after clear
 
 
Program exited normally.
(gdb)

D:\Applications\Qt\4.3.1\Workspace\Listeur>gdb debug/Listeur
GNU gdb 6.3
Copyright 2004 Free Software Foundation, Inc.
GDB is free software, covered by the GNU General Public License, and you are
welcome to change it and/or distribute copies of it under certain conditions.
Type "show copying" to see the conditions.
There is absolutely no warranty for GDB.  Type "show warranty" for details.
This GDB was configured as "i686-pc-mingw32"...
(gdb) run
Starting program: D:\Applications\Qt\4.3.1\Workspace\Listeur/debug/Listeur.exe

warning: COUNT: 1

warning: SIZE: 1

warning: EMPTY: 0

warning: item_name : before acces

warning: item_name : after acces

warning: tree : before clear

warning: COUNT: 0

warning: SIZE: 0

warning: EMPTY: 1

warning: tree : before clear

warning: tree : after clear

warning: tree : after clear


Program exited normally.
(gdb)

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Like it has been recommanded me on Qtfr forums, I should use the accessors "value()" or "take()" instead of "at()" in order to prevent segmentation fault errors . This would have help us so much...

[wysota]

Like it has been recommanded me on Qtfr forums, I should use the accessors "value()" or "take()" instead of "at()" in order to prevent segmentation fault errors . This would have help us so much...

I don't think it would help. Your problem was that you were dereferencing a pointer without checking it for null first.