Error on closing app

mzimmers

Hi all -

My app has a C++ class that I expose to QML. Here's the minimal code:

// main.cpp
#include <QGuiApplication>
#include <QQmlApplicationEngine>
#include <QQmlContext>

#include "manager.h"

int main(int argc, char *argv[])
{
    QGuiApplication app(argc, argv);
    QQmlApplicationEngine engine;
    Manager manager;
    const QUrl url(u"qrc:/restapi/main.qml"_qs);
    int rc;

    qmlRegisterType<Manager>("Manager", 1, 0, "Manager");
    engine.rootContext()->setContextProperty("manager", &manager);

    QObject::connect(&engine, &QQmlApplicationEngine::objectCreated,
                     &app, [url](QObject *obj, const QUrl &objUrl) {
        if (!obj && url == objUrl)
            QCoreApplication::exit(-1);
    }, Qt::QueuedConnection);
    engine.load(url);

    rc = app.exec();

    return rc;
}

And a bit of my class definition:

// manager.h
class Manager : public QObject
{
    Q_OBJECT
    Q_PROPERTY(QByteArray replyData MEMBER m_replyData READ replyData WRITE setReplyData NOTIFY replyDataChanged)
public:
    explicit Manager(QObject *parent = nullptr);
    QByteArray replyData() { return m_replyData; }
private:
    QByteArray m_replyData;

Referencing it in my qml:

TextArea { // to display the reply
    text: manager.replyData

Everything seems to work -- my QML scene properly reflects the contents of manager.m_replyData. BUT: when I close the application, I get this error:

TypeError: Cannot read property 'replyData' of null

This error occurs on the "text: manager.replyData" line, again, only on exit.

Any ideas what's going on here? Am I somehow improperly closing the app?

Thanks...

GrecKo

@mzimmers Your Manager is constructed after your QQmlApplicationEngine so it will be destroyed before.

Swap these 2 lines to make sure the Manager always exists while the engine lives:

QQmlApplicationEngine engine;
Manager manager;

mzimmers

@GrecKo yep, that was it (I should have figured that out myself).

Curiously, when I re-ordered the declarations in my main.cpp to this:

int main(int argc, char *argv[])
{
    Manager manager;
    QGuiApplication app(argc, argv);
    QQmlApplicationEngine engine;

I got a message on startup:

qt.core.qobject.connect: QObject::connect(QObject, Unknown): invalid nullptr parameter

Re-ordering to this eliminated the error:

int main(int argc, char *argv[])
{
    QGuiApplication app(argc, argv);
    Manager manager;
    QQmlApplicationEngine engine;

I stepped through it in the debugger, and traced it to something in the ensureInitialized() call within a QNetworkAccessManager c'tor. Not really sure what was happening -- again, the application seemed to work just fine.

Thanks for the answer.

bakerinitially

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