App crashes when using QCoreApplication in library

[jamalabo]

I have a GUI Application and has a shared library which is the functionality lib, that lib is based on Qt so it uses slots & signals. when calling functions from the lib the GUI throws error: QEventLoop: Cannot be used without QApplication (after connect()) so I tried to create QCoreApplication in the lib and use it, but the GUI crashes after calling the function.

In the GUI:

    MY_LIB::Manager manager;

    connect(&content, &MY_LIB::Manager::processStarted, this, &MainWindow::processStarted);

    manager.start();

in the Lib:

MY_LIB::Manager::Manager() {
    int n = 0;
    this->app = new QCoreApplication(n, nullptr);
}


void MY_LIB::Manager::start() {
 
// work that is based on events (slots & signals)
 
}

[Pl45m4]

@jamalabo

You need to create your QCoreApplication before you create any other QObject.

If your Manager is a QObject or makes use of them and uses signals/slots, you can't create your Manager instance first and your QCoreApplication inside its constructor.. This will never work.

[kshegunov]

And also, why are you faking the arguments to the QCoreApplication?
It depends on them to do some stuff with paths and such.
Pass them along from main()!

And also (2):
Why do you create the application object in a library?

[jamalabo]

@Pl45m4 so I'm supposed to give the GUI control of QCoreApplication? or make a parent class that is not QObject in lib that is in control of QCoreApplication

[VRonin]

@jamalabo said in App crashes when using QCoreApplication in library:

this->app = new QCoreApplication(n, nullptr);

From https://doc.qt.io/qt-6/qcoreapplication.html#QCoreApplication

Warning: The data referred to by argc and argv must stay valid for the entire lifetime of the QCoreApplication object. In addition, argc must be greater than zero and argv must contain at least one valid character string.

[jamalabo]

@kshegunov I don't understand the second segment, what do you mean application object, if you mean GUI objects then no there is no GUI objects created in the library,
the library works without any help of the application

[jamalabo]

@VRonin I have tried to pass the arguments from the QUI main and it still crashed.

[jamalabo]

I have tried this:
in GUI

    MY_LIB::DownloadManager::create_application(argc, argv);

    MY_LIB::DownloadableContent* content = MY_LIB::DownloadManager::create_content(url);

    connect(content, &MY_LIB::DownloadableContent::headersLoaded, this, &MainWindow::headersLoaded);

    content->loadHeaders();

in LIB

QNetworkAccessManager MY_LIB::DownloadManager::get_network_manager() {
    return QNetworkAccessManager(get_application());
}

void MY_LIB::DownloadableContent::loadHeaders() {
    QNetworkAccessManager manager = DownloadManager::get_network_manager();
    connect(&manager, &QNetworkAccessManager::finished, this, &DownloadableContent::http_headers_loaded);
    manager.head(this->gen_request());
}

void MY_LIB::DownloadableContent::http_headers_loaded(QNetworkReply *reply) {
// process headers
    emit headersLoaded();
}

I'm new to c++ & qt so if there is any "bad practice" I'm happy to hear suggestions.

[SGaist]

Hi,

Is your GUI application a Qt application as well ?

[jamalabo]

@SGaist yes

[SGaist]

So it is likely:

• You have a static instance of your DownloadableContent
• You try to use it before creating your QApplication object

If the first one: stop doing that.
If the second, as mentioned clearly in the documentation, QxxxApplication must be instanciated first and before any other QObject.

From the looks of it, your DownloadableManager class is either not properly implemented or used but in any case, it's not there that you should create the QApplication instance.

[kshegunov]

Look, all of this is going the same way, so I'll spill it:

Just structure your application, as one'd normally do (like shown in a Qt example) and the problem is going to go away on its own. You don't appear to really need to wrap around the application object - QCoreApplication instance, to answer your previous question - and you really don't appear to need anything exotic here. So, you'd be doing yourself a service to stick to the beaten path.

int main(int argc, char ** argv)
{
     QCoreApplication app(argc, argv);
     // ...
     // Stuff that the library provides/needs/executes
     // You can pass the application object to it here, or get it inside the library directly with qApp, if you need to.
     return QCoreApplication::exec();
}

[jamalabo]

@kshegunov I don't understand where should I put the QCoreApplication? if the GUI is running on QApplication

In GUI

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    MainWindow w;
    w.set_arguments(argc, argv);
    w.show();
    return a.exec();
}

question: should I use the same QxxxApplication for the GUI and the LIB? and if so I get the error QEventLoop: Cannot be used without QApplication and If I try to pass the app/MainWindow as the parent it's throw the error QObject: Cannot create children for a parent that is in a different thread

[kshegunov]

No, there's only one QApplication object, the one in main()! Also I really see no reason for you getting the thread errors, are you using threads?

[jamalabo]

@kshegunov no but I'm calling the code on a slot in MainWindow (on button click), (I don't know if this runs on different thread than MainWindow)

[kshegunov]

@jamalabo said in App crashes when using QCoreApplication in library:

I don't know if this runs on different thread than MainWindow

It doesn't, then I imagine you have static QObjects that are initialized before they're allowed to, a.k.a. "the QApplication object must always be the first one created and the last destroyed". So, as said, create your application object, then pass control to your library to do whatever it does, and when that's done continue on to start the event loop with exec().

Your code looks more or less okay, assuming you've removed all QApplication creation from the MainWindow and/or related classes.

[jamalabo]

@kshegunov it seems like it's working but not properly

void MY_LIB::DownloadableContent::loadHeaders() {
    printf("load-headers\n");
    QNetworkAccessManager manager;
    printf("manager\n");
    connect(&manager, &QNetworkAccessManager::finished, this, &DownloadableContent::http_headers_loaded);
    manager.head(this->gen_request());
}

void MY_LIB::DownloadableContent::http_headers_loaded(QNetworkReply *reply) {
    printf("headers-loaded");
    emit headersLoaded();
}

after running this code I only get the output

load-headers
manager

it seems like slots are not called and it's an event loop issue.
should I pass the QParent (app) param to QNetworkAcessManager?

[JKSH]

@jamalabo said in App crashes when using QCoreApplication in library:

I don't understand where should I put the QCoreApplication? if the GUI is running on QApplication

• QApplication inherits QCoreApplication.
• You can only have one instance of Q___Application in your program.

So, you cannot create both a QApplication and a QCoreApplication in the same program.

question: should I use the same QxxxApplication for the GUI and the LIB?

Yes. Definitely. They must share the same QxxxApplication instance. (And since your code uses widgets, then you must use QApplication)

[kshegunov]

@jamalabo said in App crashes when using QCoreApplication in library:

it seems like slots are not called and it's an event loop issue.

Well, that doesn't surprise me, your QNetworkAccessManager object dies as you leave the function. So either you make it a member to your DownloadableContent class, or create it on the heap (parented properly). And if you decide on the latter, please don't do it every time you pass through that method, just the first time.

should I pass the QParent (app) param to QNetworkAcessManager?

Not necessarily, although usually you give parents to QObject instances. Since DownloadableContent already derives from QObject, what's wrong with giving the QNetworkAcessManager object a parent of the object which "owns"/"contains" it (a.k.a. this in this case)?

[jamalabo]

@kshegunov
now it crashes.

QNetworkAccessManager *NETWORK_COMBINER_LIB::DownloadableContent::get_manager() {
    return (!this->manager ? (this->manager = new QNetworkAccessManager()) : this->manager);
}

void NETWORK_COMBINER_LIB::DownloadableContent::loadHeaders(QObject *parent) {
    printf("load-headers\n");
    QNetworkAccessManager *pManager = this->get_manager();
    printf("pManager\n");
    connect(pManager, &QNetworkAccessManager::finished, this, &DownloadableContent::http_headers_loaded);
    pManager->head(this->gen_request());
}