I have a GraphicsScene:public QGraphicsScene inherited class with a single QGraphicsView looking at it and QTimer, ticking to call function
void GraphicsScene::adv()
{
if (actor)
views().at(0)->ensureVisible(actor,200,100);
advance();
}
advance() is an overriden method which is send to all QGraphicsItem objects on scene. The point of this function - I want to make sure actor is always visible.
actor is a unit:public QGraphicsPixmapItem object on GraphicsScene.
At some point in actor method I call deleteLater().
The next timer tick I receive SEGFAULT at views().at(0)->ensureVisible(actor,200,100); line
I wonder, why if (actor) passes as true after deleteLater() and what is the correct condition should I use?
I have an object being asynchronically deleted by deleteLater()
and wonder if there is a way to prevent accessing it from other
objects?
Yes, there is a way to tell programmatically whether or not the object was already deleted by using QPointer<MyQObject> as described. But that way is somewhat slow and your application code should rather have better logic to avoid that. Like, before calling deleteLater your code removes the reference for that object from, say, views() and your code should check for the view still there.
If you call deleteLater() from inside your actor, the container GraphicsScene still has its pointer on it - the object itself doesn't reset all external pointers to it.
You have to reset this pointer - the member actor of your GraphicsScene to get your if-statement in adv() working.
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>();
/*...*/
}
Assuming I have the following snippet, is it safe to call deleteLater in qto's destructor for other QT objects it might administer?
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
MyQTObject qto;
qto.show();
return a.exec();
}
Because I've analyzed similar code like this with a leak detector and all the objects for which deleteLater was called, weren't deallocated correctly unless I replaced the call with a normal delete.
If I've understood this correctly, deleteLater only registers a deletion event in the QT message queue. Can this be the problem that qto's destructor is called at the end of main's scope whereas the QT message loop already ends with the return from a.exec? Thus the deletion event will never be processed, in fact not even pushed into a message queue since there is none?
This post is rather aged, but I would like to add the answer I would have liked to come across when I was asking this myself.
deleteLater() can be very useful in combination with asynchronous operations. It especially shines, I think, with the more recent possibility to connect signals to lambda functions.
Suppose you have some longComputation() that you want to execute asynchronously (not in the sense of multithreading, in the sense of scheduling execution in the event loop). You can do like this:
void MyClass::deferLongComputation()
{
QTimer* timer = new QTimer();
connect(timer,
&QTimer::timeout,
[this, timer](){this->longComputiation(); timer->deleteLater();});
timer->setSingleShot(true);
timer->start();
}
where deleteLater() takes care of safely disposing of the QTimer once its duty has been carried out and avoid the memory leak that one would have otherwise.
The same pattern can be used in multithreading with QFutureWatcher.
As I understand it, deleteLater is most often used when you require an object to be deleted from within the call to a slot. If delete is used in this case and the object is referenced when returning from the slot, a reference to uninitialised memory occurs.
Therefore, deleteLater requests that object to be deleted by placing a message on the event loop, which is processed at some point, on returning from the slot and it is safe to be deleted.
I expect that using deleteLater in the destructor means there's a likely chance that the object goes out of scope, calls deleteLater on its managed objects, but quits before the event loop has a chance to delete the objects, as exiting from QApplication::exec() will terminate the event loop.
The question is old, but I'll leave this for the future generation)
The reply which was marked as an answer is correct but oddly formulated.
Actually your question contains a right answer:
message loop already ends with the return from a.exec? Thus the
deletion event will never be processed, in fact not even pushed into a
message queue since there is none.
This is exactly what is happening. Everything deleteLater() does is just posting a deletion event into the outter event loop. When event gets proccessed - object gets deleted. But if there are not outter event loop and no event loop is encountered later in the execution flow - event will never get posted, thus object is never deleted.
If you call deleteLater() in the object's destructor and put an object on the stack - deleteLater() is called when the object goes out of scope. In your example "going out of scope" is happening when closing brace of main() function is encountered. However, by that time, a.exec() (which represents the main event loop of Qt App) has already returned --> no event loop any more --> deleteLater() was called, but it's nowhere to post a deletion event --> objects were supposed to be "deletedLater" never get deleted...
Regarding the part "when to use deleteLater()":
Kuba Ober answered:
Generally speaking, there is a narrow set of circumstances where
deleteLater should be used. Most likely you simply shouldn't be using
it...
Don't listen to it, it is absolutely incorrect as the whole answer. What you should do and what should not you better decide after reading this article. Although, it is mainly about Qt threads, the article also tells about ascynchronous programming (and, as Emerald Weapon mentioned, it is exactly what deleteLater() was created for).
Also, smart pointers and QObject parent ownership have nothing to do with scheduling for the deletion with deleteLater(). These both techniques are actually using a simple delete operation under the hood. And as the article shows and as Emerald Weapon's answer demonstrated: delete does not solve the problems deleteLater() does. So if you need to delete object you use delete, if you need to schedule it for the deletion you use deleteLater().
BTW, if you want to use smart pointer with deleteLater() you can specify the deleter:
// Shared Pointer
QSharedPointer<MyObject> obj =
QSharedPointer<MyObject>(new MyObject, &QObject::deleteLater);
// Scoped Pointer
QScopedPointer<MyObject, QScopedPointerDeleteLater> customPointer(new MyObject);
And at last, It is an NOT an error to use deleteLater() in the destructor of QObject, for non-child objects.
You are correct that the deleteLater() command is only executed by an event loop.
From the Qt documentation for QObject:
Schedules this object for deletion.
The object will be deleted when control returns to the event
loop. If the event loop is not running when this function is
called (e.g. deleteLater() is called on an object before
QCoreApplication::exec()), the object will be deleted once the
event loop is started. If deleteLater() is called after the main event loop
has stopped, the object will not be deleted.
Since Qt 4.8, if deleteLater() is called on an object that lives in a
thread with no running event loop, the object will be destroyed when the
thread finishes.
Note that entering and leaving a new event loop (e.g., by opening a modal
dialog) will \e not perform the deferred deletion; for the object to be
deleted, the control must return to the event loop from which
deleteLater() was called.
Note: It is safe to call this function more than once; when the
first deferred deletion event is delivered, any pending events for the
object are removed from the event queue.
If you want all child QObjects to be deleted when qto is deleted, make sure they are created with qto as a the parent.
Generally speaking, there is a narrow set of circumstances where deleteLater should be used. Most likely you simply shouldn't be using it.
It is an error to use it in the destructor of QObject, for non-child objects. As you've found QObjects may well be destructed without an event loop present. There are no deleteLater calls in object destructors the qtbase Qt module, for example.
One has to be careful here: for example, ~QTcpServer() invokes close() invoking d->socketEngine->deleteLater(), but the socket engine is already a child of the server and will be deleted by ~QObject() anyway.
For all I know, MyQTObject should be doing one of the following:
using a smart pointer like QScopedPointer or std::unique_ptr,
have the objects as regular (non-pointer) members,
use raw pointers and have the objects be children of it.
Qt4.8.5
QObject::connect(button,SIGNAL(clicked()),label,SLOT(setText("dd"));
The Qt Creator tell me It's wrong . What's the problem ?
That you can't pass arguments in a connect() statement. You need a "trampoline" slot that sets the text of your label (or, in Qt 5, you might choose to use a lambda).
For instance, by using a subclass:
class MyLabel : public QLabel {
Q_OBJECT
public slots:
void setTextToFoo() { setText("foo"); }
};
// ...
connect(button,SIGNAL(clicked()),label,SLOT(setTextToFoo());
It depends what exactly you are trying to achieve, to be honest, the example code you provided is not very functional, is "dd" a particular static value you are using, or potentially some other string? Where does it come from, is it in the scope of the called, or is it sent by the caller, which is the usual practice when sending arguments to slots.
Either way, in order to make a connect statement the first requirement is for the arguments to match, clicked() has no arguments while setText() has one, so there is a mismatch. As of how to resolve that mismatch, the easiest way is to use simple wrappers, although you can use a QSignalMapper and as of Qt5, lambdas and std::bind.
For starters, you cannot specify the actual argument instance in the connect statement, even with arguments on both sides you only need to specify the types to help resolve overloads (it is terrible with the new connection syntax in Qt5), and not any actual identifiers or literals.
In case of the more usual scenario, where the data is send to the slot by the caller, the identifier or literal is specified in the emit signal(value) statement. Since you don't have clicked(const QString &) you need a wrapper slot that you connect to clicked() and emit with the value in that wrapper slot, or subclass the button and add your own overload of clicked(QString).
In case the value is in the scope of the called, then subclassing doesn't make much sense, all you need is the wrapper slot in the scope of the called object.
If you want more, you will have to use Qt 5, whose syntax is significantly more powerful.
If the question is whats wrong, just remember the parameter number must be the same for the Signal and the Slot. Asking a collegue and according the Peppe, setText(QString) wait for One parameter and the Clicked() is empty...A custom slot is to call the setText() method indirectly.
You can look that : http://qt-project.org/doc/qt-4.8/widgets-calculator.html
It uses the QWidget, an important part of Qt interfaces beside QML.
I have a class derived from QThread: class MyClass : public QThread. In the run method I have "my own" message loop:
run() {
// exec(); // while not reached
while (_runMessageLoop && ...) {
hr = CallDispatch(.....);
if (hr== 0) QThread::msleep(100);
// QCoreApplication::processEvents(); // Does not work
}
}
Since exec() is not executed, I have no Qt event loop. This obviously causes signal / slots not to work correctly. Is there any chance to combine the Qt and my own message loop? Or do I need a frequently firing timer in order to do what I have accomplished in my infinite loop?
The right way "Qt-wise" is to use a timer and let Qt manage the event loop.
If you need to depend on external things, you can use things like QAbstractSocket to send events when data comes in over an external socket, eg.
This is not really the answer for implementing the event loop correctly, I'm fairly sure there is a way, but more of a workaround:
Start the thread normally, exec() and all, and connect the start signal to a slot (make sure it gets called in the right thread), then put your loop there, and call Qt's processEvents() in that loop. That makes sure Qt event loop gets properly set up.
I'm trying to get the information of several of a class' member variables on the receiving end of a slot/signal setup, so I'd like to pass the entire class through. Unfortunately, after the class has been passed, the member variables seem to be empty. Here's some code snippets:
This sets up the signal to pass the class
signals:
void selected(const ControlIcon *controlIcon);
this is the slot/signal connection
connect(controllerList->serialController, SIGNAL(selected(const ControlIcon*)),
infoView, SLOT(serialControllerSelected(const ControlIcon*)));
I emit the signal from within the class to be passed
emit selected(this);
Here's the code to call on the class' member data
QLabel *ASCIIStringHolder = new QLabel;
ASCIIStringHolder->setText(controlIcon->m_ASCIIString);
Nothing shows up in the label, and when I set a breakpoint, I can see that there's nothing inside m_ASCIIString.
I looked to make sure that it was being assigned some text in the first place, and that's not the problem. I also tried the signal/slot setup with and without const.
Any help would be appreciated.
Qt signal/slot mechanism needs metainformation about your custom types, to be able to send them in emitted signals.
To achieve that, register your type with qRegisterMetaType<MyDataType>("MyDataType");
Consult official QMetaType documentation for more information about this.
First, since you are using an auto connection, do both sender and receiver live in the same thread? If not, it could happen that the call is queued and when it arrives, the data in the sender was already modified. You could try to use a direct connection just to make sure this isn't the problem.
Second, just for the fun of it, did you try to access the sender by using qobject_cast<ControlIcon*>(sender()) within the slot? This is how it is usually done if the signal doesn't pass this as an argument. Like this:
QLabel *ASCIIStringHolder = new QLabel;
// this is instead of the argument to the slot:
ControlIcon *controlIcon = qobject_cast<ControlIcon*>(sender());
ASCIIStringHolder->setText(controlIcon->m_ASCIIString);
The signal can't be declared to be passing a class and then actually pass the child of that class. I changed the signal, slot, and connect() to be SerialController (the child of ControllerIcon), and everything worked fine.