I have some Qt application with QPlainTextEdit in Tab widget. When try to make a pointer on it
QPlainTextEdit *w = (QPlainTextEdit*)ui->tabWidget->widget(0)
and call a document() method
w->document()
I get a segfault.
But if i call document directly, e.g. ui->mainEdit->document(), then everything works fine.
Can anybody explain me why it happens?
You want to do:
QPlainTextEdit *w = ui->mainEdit;
Then w->document() will return what you want. You are getting the segmentation fault because when you cast ui->tabWidget->widget(0); gives a pointer to a tab page object. When you cast this to QPlainTextEdit* are telling your program to treat a part of memory that does not represent a QPlainTextEdit as a QPlainTextEdit. This causes trouble at the time that you call w->document() because that is in the memory location that it tries to access is not what it would expect from memory which belongs to QPlainTextEdit.
i'm almost sure, that ui->tabWidget->widget(0) return container widget inside of tabWidget. Try qDebug() << ui->tabWidget->widget(0)->metaObject()->className() and see what is printed. It's probably just "QWidget" not "QPlainTextEdit". Your edit is inside of layout of this widget
You can use qobject_cast to make sure that it returns the right type.
QPlainTextEdit *w = qobject_cast<QPlainTextEdit*>(ui->tabWidget->widget(0));
if (w)
{
...
}
It'll return 0 if the type is not of QPlainTextEdit*.
As stated, widget(0) is probably not returning what you wanted - and probably contains a container or some other item, and is probably not the way you want to be accessing your widgets unless there is no other way.
Related
I have an application with many windows (QWidgets).
I didn't save a list of open windows, though, so everytime I want to close a window, I have to retrieve it.
Particularly, each of these windows is called here SubWindow.
Every SubWindow class contains a layout with a MultiEditor *sEditors, which has a menu with an action that closes the current window.
Every SubWindow is created within the MainWindow.
I have two plans.
1) destroying the SubWindow from within itself, by adding in the SubWindow constructor
connect(sEditors, SIGNAL(closeWindow()),
this, closeWindow()));
or
2) destroying the SubWindow from within the MainWindow class, by adding in the SubWindow constructor
connect(sEditors, SIGNAL(closeWindow()),
main, SLOT(closeWindow(this)));
About 1), I don't understand how I can close and destroy a QWidget from within itself (delete this; didn't seem to work, but I can try again).
About 2) my SLOT(closeWindow(this)) doesn't seem to be triggered, so I am wondering if I can pass "this" as an argument.
Ad 1) You can use QObject::deleteLater(). This will destroy the object in the next event loop cycle, and is specifically create for situations like this
Ad 2) You cannot pass actual arguments as parameters in signal-slot connections.
You can however find out who has emitted the signal by using the sender() function in the slot. In your case, that would be the sEditors object.
Other options:
3) You can use a QSignalMapper to map signals from your editors to the Subwindows.
4) (Using Qt5 / C++11) You can use a lambda connection in your Subwindows:
connect(sEditors, SIGNAL(closeWindow()), [this] () {this->closeWindow();});
Can I pass this to a Qt slot?
A slot is a non-static method, so it already has access to this. The this you refer to is the third argument to QObject::connect. In Qt 4 syntax, you're free to omit the third argument - it defaults to this. In Qt 5 syntax, you must be explicit about it, though.
I don't understand how I can close and destroy a QWidget from within itself
To delete any QObject from within itself, use QObject::deleteLater(). Recall that a QWidget is-a QObject in terms of LSP.
my SLOT(closeWindow(this)) doesn't seem to be triggered
There's no such slot (give us a link to its documentation: you can't), and your slot signature is also invalid because the only things within the parentheses in the slot signature can be types, and this is not a type: SLOT(slotName(TYPE_LIST_HERE)), e.g. SLOT(mySlot(int,QString)).
To close a widget, use its close() slot:
connect(sEditors, SIGNAL(closeWindow()), this, SLOT(close());
Yet, by using Qt 4 connect syntax, you're leaving coding mistakes to be detected at runtime - and then if you don't pay attention to the debug output at runtime, you'll miss it. It's thus much better to use the new (Qt 5) connect syntax, and let the compiler detect errors for you:
connect(sEditors, &MultiEditor::closeWindow, this, &QWidget::close);
Alas, there's no need to tightly couple the object that sends closeWindow to SubWindow - at least not within the SubWindow::SubWindow(). Instead, you can connect at the place where you create the editor.
To delete a widget when it gets closed, simply set the Qt::WA_DeleteOnClose attribute on it and let Qt do it for you. No need to explicitly call deleteLater etc.
You might factor all of it into a factory method:
template <class T> T* SubWindow::makeEditor() {
auto sub = new T{this};
sub->setAttribute(Qt::WA_DeleteOnClose);
connect(sEditor, &MultiEditor::closeWindow, sub, &QWidget::close);
return sub;
}
MainWindow::MainWindow(/*...*/) : /*...*/ {
makeEditor<EditorType1>();
makeEditor<EditorType2>();
/*...*/
}
I have read some topics here about signals and slots and its parameters but found nothing about following problem:
I am working with Qt 5.7
I simply have 2 classes, inside 1st, I want to emit signal with string message and location(or specific object) where to display it.
Now it is like this: I have 1st class where I emit signal :
emit signalWriteToTextEdit("hallo","textEdit_3");
What I want to do is somehow pass as the second argument an object like textEdit. No QString as its now.
Inside 2nd class is the slot:
void writeToTextEdit(QString info, QString where){
where.append(info); //I would like to do something like this
}
Just dont know how to consider that second parameter "where" as accessible object for example textEdit, so I could change its content.
I am thinking also if this is possible:
Is there some method for Ui object like finding elements by name?
Is it possible to go with foreach over all elements in ui and check their names...? I tried but dont know how to go through that.
If its not clear, I will explain more
After a little digging, I came across the QObject::findChild function. This will allow a string lookup recursively through the UI, those I am unsure of performance.
Edit for more detail:
Returns the child of this object that can be cast into type T and that is called name, or 0 if there is no such object. Omitting the name argument causes all object names to be matched. The search is performed recursively.
If there is more than one child matching the search, the most direct
ancestor is returned. If there are several direct ancestors, it is
undefined which one will be returned. In that case, findChildren()
should be used.
Just use QWidget* or QObject* (if it is not always a widget) as the argument type
signals:
void writeToTextEdit(const QString &what, QWidget *where)
or if it is always a QTextEdit even more specifically
signals:
void writeToTextEdit(const QString &what, QTextEdit *where)
Though it is not clear why the code needs to emit a signal if it has access to the target object, it could simply call setText directly
I have a mdiArea. I add subwindows to this mdiArea every time an image is opened. The widget I set for each subwindow is imageFileDialog which inherits from the QDialog. Within this dialog I have a spinbox. I want to be able to set the value of this spinbox for every dialog I have in the mdiArea after they have been created. I do not know how to iterate through the dialogs. I tried to think of ways to do this.
for (int j=0; j < ui->mdiArea->subWindowList().size(); j++)
{
imageFileDialog *ifd = ui->mdiArea->subWindowList()[j]->widget();
ifd->setSpinBox(0);
}
but I have an error because I cannot cast the widget as my imageFileDialog class that inherits from QDialog.
I though I might be able to set a connection upon creation of my imageFileDialog:
imageFileDialog *ifd = new imageFileDialog();
ifd->connect(this, SIGNAL(emitImageFileValue(double)), SLOT(ifd->setSpinBox(double)));
subWindow1->setWidget(ifd);
but this is unlike any connection I've tried to make before. Everything compiles fine, but the signal emitted does not reach my slot. I'm hoping someone has tried this before and has some suggestions! Thanks in advance.
Use qobject_cast for the first problem and for the second issue you need to pass only the slot name to the SLOT() macro (without ifd->), or pass the ifd pointer as separate parameter to connect, something like:
QObject::connect(this, SIGNAL(emitImageFileValue(double)), idf, SLOT(setSpinBox(double)));
My goal is: user can choose file (only *mp3) and after clicking twice on it it should play (so the QString to file should be send to play() function)
Firstly I started to work with QTreeView, but it has signal when the file is selected.
So I decided to create QFileDialog and used it as widget built-in into MainWindow.
The only problem that I have, that after double-click it disappears. It is possible to avoid it?
Should I work with some QDialog::finished() signal or, QDialog::done()?
First, you can get a double-click signal from QTreeView; it's:
void doubleClicked( const QModelIndex & index );
Second, if you really want to use the QFileDialog that way, first override closeEvent( QCloseEvent * event). Inside, if you want to close the dialog, do event->accept();, otherwise just do event->ignore();. Connect to QFileDialog::currentChanged( const QString & path ); to get the filename the user double-clicks. One last thing--be sure to create the QFileDialog on the heap (using new), not on the stack (a local), and call show() on it instead of exec().
Remember that you can supply it with a parent (this) and you won't need to delete it later.
connect(file_dialog, SIGNAL(finished(int)), file_dialog, SLOT(open()));
This seems to work fine. The geometry stays fixed and it remembers the last path allright..
this generates a segmentation fault becuase of "QColor colorMap[9]";. If I remove colorMap the segmentation fault goes away. If I put it back. It comes back. If I do a clean all then build all, it goes away. If I increase its arraysize it comes back. On the other hand if I reduce it it doesnt come back. I tired adding this array to another project and
What could be happening. I am really curious to know. I have removed everything else in that class. This widget subclassed is used to promote a widget in a QMainWindow.
class LevelIndicator : public QWidget
{
public:
LevelIndicator(QWidget * parent);
void paintEvent(QPaintEvent * event );
float percent;
QColor colorMap[9];
int NUM_GRADS;
};
the error happens inside ui_mainwindow.h at one of these lines:
hpaFwdPwrLvl->setObjectName(QString::fromUtf8("hpaFwdPwrLvl"));
verticalLayout->addWidget(hpaFwdPwrLvl);
I know i am not providing much but I will give alink to the app. Im trying to see if anyone has a quick answer for this.
If I do a clean all then build all, it goes away.
This makes it sound as though your build system isn't recognizing a dependency and that a change to that class definition isn't triggering a rebuild of something that should be recompiled when the definition changes.
Make sure class LevelIndicator is defined in exactly one place (generally that would be a header file that gets included by whatever modules need to use a LevelIndicator object). Also make sure that any global/static instances of LevelIndicator objects are following the one definition rule.
Firstly it might not be QColor, that may simply be changing the memory layout enough that a buffer overrun somewhere else triggers a segfault - try a different size QColor ..[1] for example.
Can QColor be used as an array like this, does it have the correct default ctor?