Segmentation fault in Qt Designer 4.6 with custom widget - c++

Question

I had a segmentation fault when using my new Qt Widget with Qt Designer 4.6. The problem arise when trying to preview the new widget.
when using gdb I found that the problem is in qdesigner_internal::WidgetFactory::applyStyleToTopLevel:
Program received signal SIGSEGV, Segmentation fault.
qdesigner_internal::WidgetFactory::applyStyleToTopLevel (style=0x0, widget=0x1829df0) at /var/tmp/qt-x11-src-4.6.0/tools/designer/src/lib/shared/widgetfactory.cpp:777
777 /var/tmp/qt-x11-src-4.6.0/tools/designer/src/lib/shared/widgetfactory.cpp: No such file or directory.
in /var/tmp/qt-x11-src-4.6.0/tools/designer/src/lib/shared/widgetfactory.cpp
(gdb) bt
#0 qdesigner_internal::WidgetFactory::applyStyleToTopLevel (style=0x0, widget=0x1829df0) at /var/tmp/qt-x11-src-4.6.0/tools/designer/src/lib/shared/widgetfactory.cpp:777
#1 0x00007ffff7475bed in qdesigner_internal::QDesignerFormBuilder::createPreview (fw=, styleName=..., appStyleSheet=..., deviceProfile=, scriptErrors=
0x7fffffffbee0, errorMessage=0x7fffffffc3f0) at /var/tmp/qt-x11-src-4.6.0/tools/designer/src/lib/shared/qdesigner_formbuilder.cpp:404
#2 0x00007ffff7476773 in qdesigner_internal::QDesignerFormBuilder::createPreview (fw=0x0, styleName=..., appStyleSheet=..., deviceProfile=..., errorMessage=0x0)
at /var/tmp/qt-x11-src-4.6.0/tools/designer/src/lib/shared/qdesigner_formbuilder.cpp:439
#3 0x00007ffff7532b27 in qdesigner_internal::PreviewManager::createPreview (this=0x837f20, fw=0x1879200, pc=..., deviceProfileIndex=-1, errorMessage=0x7fffffffc3f0, initialZoom=-1)
at /var/tmp/qt-x11-src-4.6.0/tools/designer/src/lib/shared/previewmanager.cpp:686
#4 0x00007ffff75343cf in qdesigner_internal::PreviewManager::showPreview (this=0x837f20, fw=0x1879200, pc=..., deviceProfileIndex=-1, errorMessage=0x7fffffffc3f0)
at /var/tmp/qt-x11-src-4.6.0/tools/designer/src/lib/shared/previewmanager.cpp:760
#5 0x00007ffff753472f in qdesigner_internal::PreviewManager::showPreview (this=0x837f20, fw=0x1879200, style=..., deviceProfileIndex=-1, errorMessage=0x7fffffffc3f0)
at /var/tmp/qt-x11-src-4.6.0/tools/designer/src/lib/shared/previewmanager.cpp:659
because a null pointer was passed there:
void WidgetFactory::applyStyleToTopLevel(QStyle *style, QWidget *widget)
{
const QPalette standardPalette = style->standardPalette();
if (widget->style() == style && widget->palette() == standardPalette)
return;
//....
}
I'm new in Qt and this is my first custom widget. does anybody have a clue for solving this.
here is my widget code
MBICInput::MBICInput(QWidget *parent) : QStackedWidget(parent){
displayPage = new QWidget();
displayPage->setObjectName(QString::fromUtf8("displayPage"));
inputLB = new QLabel(displayPage);
inputLB->setObjectName(QString::fromUtf8("inputLabel"));
inputLB->setCursor(QCursor(Qt::PointingHandCursor));
addWidget(displayPage);
EditPage = new QWidget();
EditPage->setProperty("EditInputLine", QVariant(true));
EditPage->setObjectName(QString::fromUtf8("EditPage"));
inputInput = new QLineEdit(EditPage);
inputInput->setGeometry(QRect(5, 10, 231, 25));
inputInput->setObjectName(QString::fromUtf8("input"));
addWidget(EditPage);
_animation = new QString("");
_message = new QString("Message");
_validator = new QRegExpValidator(QRegExp("[a-zA-Z]+"), this);
}
MBICInput::~MBICInput() {
}
QValidator::State MBICInput::validate(QString &text, int &pos) const{
return _validator->validate(text, pos);
}
void MBICInput::paintEvent(QPaintEvent *) {
QPainter painter(this);
painter.setRenderHint(QPainter::Antialiasing);
}
QSize MBICInput::minimumSizeHint() const{
return QSize(200, 40);
}
QSize MBICInput::sizeHint() const{
return QSize(200, 40);
}
void MBICInput::setAnimation(const QString &animation){
*_animation = animation;
update();
}
QString MBICInput::animation() const{
return *_animation;
}
void MBICInput::setMessage(const QString &message){
*_message = message;
update();
}
QString MBICInput::message() const{
return *_message;
}
void MBICInput::mousePressEvent(QMouseEvent *event){
if(currentIndex()==0){
setCurrentIndex(1);
}else{
setCurrentIndex(0);
}
update();
}

Answer 1

In the applyStyleToTopLevel function, you are using the widget pointers without verifying it's a valid pointer. So when doing widget->style with a NULL widget pointer it will crash.

Answer 2

Just for information I just compiled the same code under the Qt version 4.5.3-9 and it worked just fine with the 4.5 designer. may be it's a Qt 4.6 bug...