setModelData crashes program
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Hi,
A line in my setModelData crashes the program:
setModelData code:void WhatFixViewDelegate::setModelData(QWidget* editor, QAbstractItemModel* fixModel, const QModelIndex& index) const { QComboBox* whatComboEditor = qobject_cast<QComboBox*>(editor); QString newWhatText; **newWhatText = whatComboEditor->currentText ();//crashes** qDebug() << "newWhatText: " << whatComboEditor->currentText (); fixModel->setData (index, newWhatText, Qt::EditRole); fixModel->submit (); }
The marked line causes the following error:
What is wrong with it?
Thank you. -
Your assumption inside the delegate is that editor is ComboBox. First you need confirm that you are setting delegate and createEditor is returning the combobox only. Looks like in some cases createEditor of your delegate may not be returning the combobox. Can you check createEditor function ?
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@gabor53 said in setModelData crashes program:
QString newWhatText; **newWhatText = whatComboEditor->currentText ();//crashes**
I don't program Qt in C++. But is that
**
really correct? That's a double indirection, off an initialized emptyQString
, if that's invalid you're writing to a random area of memory, hence a SIGSEGV? Is what you posted your actual compilable source code?? -
can u paste your createEditor(..). ?
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@gabor53 said in setModelData crashes program:
How to check the content of whatComboEditor?
** just to mark the line that crashesOMG, you put a literal
**
in a legal C++ code place on the faulty line, not in a comment, where I was supposed to understand it was a comment....... That did not occur to me.So as the others have said, where you go
qobject_cast<QComboBox*>(editor)
youreditor
parameter is either NULL, does not point to aQComboBox
, or points to what was aQComboBox
but is now invalid (e.g. disposed). Can't you just look at it in a debugger? -
Hi
Just a small question.
Is this code related to
https://forum.qt.io/topic/83393/centering-qcombobox-in-table-cell
where you also used a layout to center the combobox ?
I wonder if the
QComboBox* whatComboEditor = qobject_cast<QComboBox*>(editor);
then is correct ?
but add check to it
if (!whatComboEditor) {
qDebug() << "fail to cast whatComboEditor";
return;
} -
@mrjj
It is related to the referenced post. This is part of the same delegate. I added the suggested check and I got the "fail to cast whatComboEditor" message.
Here is the whole delegate:#include "whatfixviewdelegate.h" WhatFixViewDelegate::WhatFixViewDelegate(QObject* parent) : QStyledItemDelegate(parent) { } QWidget* WhatFixViewDelegate::createEditor(QWidget* parent, const QStyleOptionViewItem& option, const QModelIndex& index) const { Q_UNUSED(option); Q_UNUSED(index); QWidget* w = new QWidget( parent ); QComboBox* whatComboEditor = new QComboBox(w); whatComboEditor->setFrame(true); whatComboEditor->setStyleSheet("background-color:rgb(255,217,229);" ); whatComboEditor->setEditable(true); whatComboEditor->setMaximumWidth (100); whatComboEditor->setMinimumWidth (100); whatComboEditor->setMaximumHeight (30); whatComboEditor->setMinimumHeight (30); whatComboEditor->setSizePolicy(QSizePolicy::Expanding, QSizePolicy::Expanding); QVBoxLayout* whatLayout = new QVBoxLayout; whatLayout->addWidget (whatComboEditor, 0, Qt::AlignCenter); whatLayout->setMargin (2); w->setLayout (whatLayout); QSqlQuery queryLoadWhat("SELECT What FROM What_Table ORDER BY What asc", db); while(queryLoadWhat.next()) { QString whatItem = queryLoadWhat.value(0).toString(); whatComboEditor->addItem(whatItem); } return w; } void WhatFixViewDelegate::setEditorData(QWidget* editor, const QModelIndex& index) const { QComboBox* whatComboEditor = qobject_cast<QComboBox*>(editor); if(whatComboEditor) { whatComboEditor->addItem(index.model()->data(index, Qt::EditRole).toString()); whatComboEditor->setCurrentText(index.model()->data(index, Qt::EditRole).toString()); } } void WhatFixViewDelegate::updateEditorGeometry(QWidget* editor, const QStyleOptionViewItem& option, const QModelIndex& index) const { Q_UNUSED(index); editor->setGeometry(option.rect); } void WhatFixViewDelegate::setModelData(QWidget* editor, QAbstractItemModel* fixModel, const QModelIndex& index) const { QComboBox* whatComboEditor = qobject_cast<QComboBox*>(editor); if (!whatComboEditor) { qDebug() << "fail to cast whatComboEditor"; return; } QString newWhatText; newWhatText = whatComboEditor->currentText ();//crashes qDebug() << "newWhatText: " << whatComboEditor->currentText (); fixModel->setData (index, newWhatText, Qt::EditRole); fixModel->submit (); }
What should be the next step? Thank you.
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Hi
You should find the combobox in the widget with
QComboBox* whatComboEditor = editor->findChild< QComboBox*>();
if (whatComboEditor ) { ...use it... }
in the other functions since you now have a real widget in between.
as you sayQWidget* WhatFixViewDelegate::createEditor(QWidget* parent, const QStyleOptionViewItem& option, const QModelIndex& index) const { QWidget* w = new QWidget( parent ); .... return w; <<< that is the place holder widget you return as "editor" }
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In your case, you should call findChild on
editor
. Your QComboBox is in a layout inside a QWidget so you can't cast it like that directly.An alternative would be to create a custom widget which wraps the QComboBox and that you could use in the same manner.
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Thank you. I redid it as recommended and now it looks like this:
#include "whatfixviewdelegate.h" WhatFixViewDelegate::WhatFixViewDelegate(QObject* parent) : QStyledItemDelegate(parent) { } QWidget* WhatFixViewDelegate::createEditor(QWidget* parent, const QStyleOptionViewItem& option, const QModelIndex& index) const { Q_UNUSED(option); Q_UNUSED(index); QWidget* w = new QWidget( parent ); QComboBox* whatComboEditor = new QComboBox(w); whatComboEditor->setFrame(true); whatComboEditor->setStyleSheet("background-color:rgb(255,217,229);" ); whatComboEditor->setEditable(true); whatComboEditor->setMaximumWidth (100); whatComboEditor->setMinimumWidth (100); whatComboEditor->setMaximumHeight (30); whatComboEditor->setMinimumHeight (30); whatComboEditor->setSizePolicy(QSizePolicy::Expanding, QSizePolicy::Expanding); QVBoxLayout* whatLayout = new QVBoxLayout; whatLayout->addWidget (whatComboEditor, 0, Qt::AlignCenter); whatLayout->setMargin (2); w->setLayout (whatLayout); QSqlQuery queryLoadWhat("SELECT What FROM What_Table ORDER BY What asc", db); while(queryLoadWhat.next()) { QString whatItem = queryLoadWhat.value(0).toString(); whatComboEditor->addItem(whatItem); } return w; } void WhatFixViewDelegate::setEditorData(QWidget* editor, const QModelIndex& index) const { QComboBox* whatComboEditor = editor->findChild<QComboBox*>(); if(whatComboEditor) { whatComboEditor->addItem(index.model()->data(index, Qt::EditRole).toString()); whatComboEditor->setCurrentText(index.model()->data(index, Qt::EditRole).toString()); } } void WhatFixViewDelegate::updateEditorGeometry(QWidget* editor, const QStyleOptionViewItem& option, const QModelIndex& index) const { Q_UNUSED(index); editor->setGeometry(option.rect); } void WhatFixViewDelegate::setModelData(QWidget* editor, QAbstractItemModel* fixModel, const QModelIndex& index) const { QComboBox* whatComboEditor = editor->findChild<QComboBox*>(); if (whatComboEditor) { QString newWhatText; newWhatText = whatComboEditor->currentText ();//crashes qDebug() << "newWhatText: " << whatComboEditor->currentText (); fixModel->setData (index, newWhatText, Qt::EditRole); fixModel->submit (); } }
Now it doesn't crash, but when I add an item from the QComboBox, all the data from the line disappears like this:
When I reload the table all the data are back and the newly chosen item is also there (it was saved in the db). What can I add to prevent the data from disappearing? -
@JNBarchan said in setModelData crashes program:
And always check the return result of a
qobject_cast<>
, for safety, certainly while developing! :)In my nice (non-Qt) C# background, this kind of casting is built into the language. For
qobject_cast<>
we write:ComboBox cb = editor as CombBox;
where the
as
operator returnsnull
ifeditor
cannot be cast toComboBox
at runtime. We use this ifeditor
might not be aComboBox
.However, if
editor
must be/is intended to be aComboBox
, we write:ComboBox cb = (ComboBox) editor;
where the direct cast throws a runtime Exception if
editor
is not aComboBox
(i.e.as
would returnnull
). This saves a lot of code/accidents.It would be nice if Qt offered a "variant" of
qobject_cast<>
in C++ which checked and automatically threw an Exception if the return result is NULL/0....? -
@VRonin said in setModelData crashes program:
you just need to add
Q_ASSERT(whatComboEditor);
to have thatAh, OK. It's just that you have to remember to write that every time after each
qobject_cast<>
, it would be nice to have them combined in one statement. I suppose you would write:Q_ASSERT(whatComboEditor = qobject_cast<>)
but is
Q_ASSERT
code removed in non-debug code and/or does it evaluate its argument more than once (assuming it's a macro)?I would like a
qobject_assert_cast<>
:)EDIT: Yes, I see you cannot use
Q_ASSERT(whatComboEditor = qobject_cast<>)
in non-debug as the code gets removed. -
That's not specific to Q_ASSERT, it's the same for the standard C++ assert. Always test the result of a function and not the function itself.