Adding a virtual function causes strange result...

[SPlatten]

I have added a virtual function to a base class:

virtual QString strGetData();

In the prototype of the base class:

QString clsXMLinterface::strGetData() {
    return "";
}

I have a number of classes derived from this class, one of my classes, prototype:

    class clsQtLineEdit : public QLineEdit, public clsXMLinterface {
    Q_OBJECT

Under the public modifier:

    QString strGetData();

The implementation:

QString clsQtLineEdit::strGetData() {
    return text();
}

With this code as soon as I build and execute within Qt Creator the execution stops in qlist.h on line 115:

    inline int size() const noexcept { return int(d->end - d->begin); }   // q6sizetype

I am using, Qt Creator 4.15.0-beta2 (4.14.83), Based on Qt 5.15.2 (Clang 11.0 (Apple), 64 bit), Built on Mar 24 2021 04:43:31

The executable is using Qt 5.15.2, this behaviour doesn't happed if I remove the virtual keyword.

Can anyone help with why this is happening?

[SPlatten]

@jsulm , fixed with:

clsXMLinterface* pobjInterface(dynamic_cast<clsXMLinterface*>(pobjButton));

[J.Hilk]

@SPlatten are you calling strGetData() in one of your constructors?

[jsulm]

@SPlatten Stops with what error/exception?
Try to add override key-word to strGetData() in your subclass.
And please provide more context:

• Which exact strGetData() is called when application stops?
• What are you doing with the returned value?

[SPlatten]

@J-Hilk, no.

[SPlatten]

@jsulm , this is where its called:

QString clsScriptHelper::getValue(const QString& crstrID) {
    QString strValue;
    if ( crstrID.isEmpty() != true ) {
        clsXMLnode* pobjNode(clsXMLnode::spobjGetNodeById(crstrID));
        if ( pobjNode != nullptr ) {
            clsXMLinterface* pobjXMLif(reinterpret_cast<clsXMLinterface*>(pobjNode));
            if ( pobjXMLif != nullptr ) {
                strValue = pobjXMLif->strGetData();
            }
        }

    }
    return strValue;
}

The prototype for the above:

Q_INVOKABLE QString getValue(const QString& crstrID);

There is presently only one call to this, the only overridden instance is defined in the class clsQtLineEdit.

Execution stops as I stated on the line I specified in the original post, how can I see if an error or exception has occurred because it isn't obvious.

[jsulm]

@SPlatten said in Adding a virtual function causes strange result...:

how can I see if an error or exception has occurred because it isn't obvious

You should see it in debugger.
Please post the stack trace.

[SPlatten]

@jsulm :

                                                                                                                                                                                                                                                                                                  
                                                                                                                                                                                                                                                                                                  
1  QListData::size() const                                                                                                                                qlist.h                                                                                                             115  0x10000799f    
2  QList<std::pair<QString, QString>>::length() const                                                                                                     qlist.h                                                                                                             360  0x100043265    
3  clsXMLnode::strGetAttribute(QString)                                                                                                                   clsMainWnd.cpp                                                                                                      4281 0x100028276    
4  clsQtLayout::addButton(QAbstractButton *, QString const&)                                                                                              clsQtLayout.cpp                                                                                                     149  0x10008ad47    
5  clsXMLnode::setWidget(QWidget *)                                                                                                                       clsMainWnd.cpp                                                                                                      3860 0x10002b026    
6  clsQtRadioButton::clsQtRadioButton(clsXMLnode *, QString *, QWidget *)                                                                                 clsQtRadionButton.cpp                                                                                               40   0x1000a6a8f    
7  clsQtRadioButton::clsQtRadioButton(clsXMLnode *, QString *, QWidget *)                                                                                 clsQtRadionButton.cpp                                                                                               38   0x1000a993d    
8  clsCNT::pCreate(clsXMLnode *, QString const&, QString&, QStringList&, QWidget *)                                                                       clsCNT.cpp                                                                                                          141  0x100005f59    
9  clsXMLnode::clsXMLnode(QStringList&, clsXMLnode *)                                                                                                     clsMainWnd.cpp                                                                                                      766  0x1000232c9    
10 clsXMLnode::clsXMLnode(QStringList&, clsXMLnode *)                                                                                                     clsMainWnd.cpp                                                                                                      545  0x10002ed35    
11 clsScriptHelper::addNodesToGUI(clsXMLnode *, QLayout *, QJsonArray&, int *, int *)                                                                     clsScriptHelper.cpp                                                                                                 317  0x1000bf08b    
12 clsScriptHelper::addNodesToGUI(clsXMLnode *, QLayout *, QJsonArray&, int *, int *)                                                                     clsScriptHelper.cpp                                                                                                 419  0x1000bfed7    
13 clsScriptHelper::addNodesToGUI(clsXMLnode *, QLayout *, QJsonArray&, int *, int *)                                                                     clsScriptHelper.cpp                                                                                                 419  0x1000bfed7    
14 clsScriptHelper::addGUIelement(QJsonObject const&)                                                                                                     clsScriptHelper.cpp                                                                                                 232  0x1000bd8c2    
15 clsScriptHelper::qt_static_metacall(QObject *, QMetaObject::Call, int, void * *)                                                                       moc_clsScriptHelper.cpp                                                                                             216  0x1000dae2a    
16 clsScriptHelper::qt_metacall(QMetaObject::Call, int, void * *)                                                                                         moc_clsScriptHelper.cpp                                                                                             303  0x1000db8dc    
17 CallMethod(QQmlObjectOrGadget const&, int, int, int, int *, QV4::ExecutionEngine *, QV4::CallData *, QMetaObject::Call)                                qv4qobjectwrapper.cpp                                                                                               1301 0x1012bf559    
18 CallPrecise(QQmlObjectOrGadget const&, QQmlPropertyData const&, QV4::ExecutionEngine *, QV4::CallData *, QMetaObject::Call)                            qv4qobjectwrapper.cpp                                                                                               1569 0x1012bb753    
19 QV4::QObjectMethod::callInternal(QV4::Value const *, QV4::Value const *, int) const                                                                    qv4qobjectwrapper.cpp                                                                                               2131 0x1012bafda    
20 QV4::FunctionObject::call(QV4::Value const *, QV4::Value const *, int) const                                                                           qv4functionobject_p.h                                                                                               202  0x1012e97bc    
21 QV4::Runtime::CallProperty::call(QV4::ExecutionEngine *, QV4::Value const&, int, QV4::Value *, int)                                                    qv4runtime.cpp                                                                                                      1448 0x1012e9799    
22 QV4::Moth::VME::interpret(QV4::CppStackFrame *, QV4::ExecutionEngine *, const char *)                                                                  qv4vme_moth.cpp                                                                                                     727  0x1012d56b0    
23 QV4::Moth::VME::exec(QV4::CppStackFrame *, QV4::ExecutionEngine *)                                                                                     qv4vme_moth.cpp                                                                                                     463  0x1012d4542    
24 QV4::ArrowFunction::virtualCall(QV4::FunctionObject const *, QV4::Value const *, QV4::Value const *, int)                                              qv4functionobject.cpp                                                                                               528  0x10127d8cf    
25 QV4::FunctionObject::call(QV4::Value const *, QV4::Value const *, int) const                                                                           qv4functionobject_p.h                                                                                               202  0x1012e9578    
26 QV4::Runtime::CallName::call(QV4::ExecutionEngine *, int, QV4::Value *, int)                                                                           qv4runtime.cpp                                                                                                      1406 0x1012e9562    
27 QV4::Moth::VME::interpret(QV4::CppStackFrame *, QV4::ExecutionEngine *, const char *)                                                                  qv4vme_moth.cpp                                                                                                     766  0x1012d5c13    
28 QV4::Moth::VME::exec(QV4::CppStackFrame *, QV4::ExecutionEngine *)                                                                                     qv4vme_moth.cpp                                                                                                     463  0x1012d4542    
29 QV4::Function::call(QV4::Value const *, QV4::Value const *, int, QV4::ExecutionContext const *)                                                        qv4function.cpp                                                                                                     69   0x10127a985    
30 QV4::Script::run(QV4::Value const *)                                                                                                                   qv4script.cpp                                                                                                       162  0x1012af6ca    
31 QJSEngine::evaluate(QString const&, QString const&, int)                                                                                               qjsengine.cpp                                                                                                       545  0x101237d86    
32 clsJSON::blnDecodeAccordingToType(QJsonObject const&, QTcpSocket *)                                                                                    clsJSON.cpp                                                                                                         377  0x10001859d    
33 clsModule::manageReceivedData(QTcpSocket *)                                                                                                            clsModule.cpp                                                                                                       230  0x100069269    
34 clsServer::onReadyRead()                                                                                                                               clsListener.cpp                                                                                                     84   0x10001b1e1    
35 QtPrivate::FunctorCall<QtPrivate::IndexesList<>, QtPrivate::List<>, void, void (clsServer:: *)()>::call(void (clsServer:: *)(), clsServer *, void * *) qobjectdefs_impl.h                                                                                                  152  0x10001d1ab    
36 void QtPrivate::FunctionPointer<void (clsServer:: *)()>::call<QtPrivate::List<>, void>(void (clsServer:: *)(), clsServer *, void * *)                  qobjectdefs_impl.h                                                                                                  185  0x10001d11d    
37 QtPrivate::QSlotObject<void (clsServer:: *)(), QtPrivate::List<>, void>::impl(int, QtPrivate::QSlotObjectBase *, QObject *, void * *, bool *)          qobjectdefs_impl.h                                                                                                  418  0x10001d072    
38 QObject::event(QEvent *)                                                                                                                               qobject.cpp                                                                                                         1314 0x10278bb9f    
39 QApplicationPrivate::notify_helper(QObject *, QEvent *)                                                                                                qapplication.cpp                                                                                                    3632 0x100b8c9ea    
40 QApplication::notify(QObject *, QEvent *)                                                                                                              qapplication.cpp                                                                                                         0x100b8de11    
41 QCoreApplication::notifyInternal2(QObject *, QEvent *)                                                                                                 qcoreapplication.cpp                                                                                                1063 0x102760a34    
42 QCoreApplication::sendEvent(QObject *, QEvent *)                                                                                                       qcoreapplication.cpp                                                                                                1458 0x102761d79    
43 QCoreApplicationPrivate::sendPostedEvents(QObject *, int, QThreadData *)                                                                               qcoreapplication.cpp                                                                                                1817 0x102761d64    
44 QCocoaEventDispatcherPrivate::processPostedEvents()                                                                                                    qcocoaeventdispatcher.mm                                                                                            904  0x102232259    
45 QCocoaEventDispatcherPrivate::postedEventsSourceCallback(void *)                                                                                       qcocoaeventdispatcher.mm                                                                                            927  0x1022329c8    
46 __CFRUNLOOP_IS_CALLING_OUT_TO_A_SOURCE0_PERFORM_FUNCTION__                                                                                             (x86_64h) /System/Library/Frameworks/CoreFoundation.framework/Versions/A/CoreFoundation                                  0x7ff8188138fd 
47 __CFRunLoopDoSource0                                                                                                                                   (x86_64h) /System/Library/Frameworks/CoreFoundation.framework/Versions/A/CoreFoundation                                  0x7ff818813865 
48 __CFRunLoopDoSources0                                                                                                                                  (x86_64h) /System/Library/Frameworks/CoreFoundation.framework/Versions/A/CoreFoundation                                  0x7ff8188135e4 
49 __CFRunLoopRun                                                                                                                                         (x86_64h) /System/Library/Frameworks/CoreFoundation.framework/Versions/A/CoreFoundation                                  0x7ff81881201b 
50 CFRunLoopRunSpecific                                                                                                                                   (x86_64h) /System/Library/Frameworks/CoreFoundation.framework/Versions/A/CoreFoundation                                  0x7ff8188115dd 
51 RunCurrentEventLoopInMode                                                                                                                              (x86_64) /System/Library/Frameworks/Carbon.framework/Versions/A/Frameworks/HIToolbox.framework/Versions/A/HIToolbox      0x7ff8214444f1 
52 ReceiveNextEventCommon                                                                                                                                 (x86_64) /System/Library/Frameworks/Carbon.framework/Versions/A/Frameworks/HIToolbox.framework/Versions/A/HIToolbox      0x7ff821444247 
53 _BlockUntilNextEventMatchingListInModeWithFilter                                                                                                       (x86_64) /System/Library/Frameworks/Carbon.framework/Versions/A/Frameworks/HIToolbox.framework/Versions/A/HIToolbox      0x7ff821443fe5 
54 _DPSNextEvent                                                                                                                                          (x86_64) /System/Library/Frameworks/AppKit.framework/Versions/C/AppKit                                                   0x7ff81b240b4c 
55 -[NSApplication(NSEvent) _nextEventMatchingEventMask:untilDate:inMode:dequeue:]                                                                        (x86_64) /System/Library/Frameworks/AppKit.framework/Versions/C/AppKit                                                   0x7ff81b23f1b8 
56 -[NSApplication run]                                                                                                                                   (x86_64) /System/Library/Frameworks/AppKit.framework/Versions/C/AppKit                                                   0x7ff81b2315a9 
57 QCocoaEventDispatcher::processEvents(QFlags<QEventLoop::ProcessEventsFlag>)                                                                            qcocoaeventdispatcher.mm                                                                                            437  0x10223162f    
58 QEventLoop::processEvents(QFlags<QEventLoop::ProcessEventsFlag>)                                                                                       qeventloop.cpp                                                                                                      139  0x10275cacf    
59 QEventLoop::exec(QFlags<QEventLoop::ProcessEventsFlag>)                                                                                                qeventloop.cpp                                                                                                      232  0x10275caab    
60 QCoreApplication::exec()                                                                                                                               qcoreapplication.cpp                                                                                                1371 0x102761042    
61 main                                                                                                                                                   XMLMPAM.cpp                                                                                                         120  0x1000d3cf4    
62 start                                                                                                                                                  (x86_64) /usr/lib/dyld                                                                                                   0x1002e94fe    

[J.Hilk]

this minimal example works like a charm:

class clsXMLinterface {
public:
    virtual QString strGetData();
};

QString clsXMLinterface::strGetData() {
    return "";
}

class clsQtLineEdit : public QLineEdit, public clsXMLinterface{
    Q_OBJECT

public:
    clsQtLineEdit(QWidget*parent = nullptr) : QLineEdit(parent) {}

public:
    QString strGetData() override {return text();}
};

int main(int argc, char *argv[])
{
    QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);
    QApplication app(argc, argv);

    clsQtLineEdit t;
    t.show();



    return app.exec();
}
#include "main.moc"

[jsulm]

@SPlatten said in Adding a virtual function causes strange result...:

clsXMLnode::strGetAttribute(QString)

You should check what is happening there

[SPlatten]

@jsulm , I didn't use the override keyword, should I?

[J.Hilk]

@SPlatten said in Adding a virtual function causes strange result...:

reinterpret_cast

when using virtual inheritance !?!?!?

use dynamic_cast or if QObject classes object_cast

reinterpret_cast takes the raw pointer and considers it as being of the derived type. However, because of the virtual inheritance, a slight adjustment must be done to the pointer to point to the correct method dispatch table, and that's precisely what dynamic_cast will do.

just as wrong, but probably wouldn't have resulted in a crash would be the use of static_cast.

[Christian Ehrlicher]

This post is deleted!

[SPlatten]

@J-Hilk said in Adding a virtual function causes strange result...:

dynamic_cast

I've replaced reinterpret_cast with dynamic_cast, same result.

[jsulm]

@SPlatten said in Adding a virtual function causes strange result...:

I didn't use the override keyword, should I?

Yes you should. To ask the compiler to check whether you're really overriding virtual method. Can help to detect bugs at compile time.

As I suggested: check what is happening in clsXMLnode::strGetAttribute(QString).
My guess is that the strGetData() method from base class is called which returns an empty string, so the code then fails later.

[Christian Ehrlicher]

@SPlatten said in Adding a virtual function causes strange result...:

I've replaced reinterpret_cast with dynamic_cast, same result.

For sure not.

btw: dynamic_cast was proposed to your exact same problem some weeks ago already: https://forum.qt.io/topic/132420/how-to-access/4

[SPlatten]

@jsulm, I have a breakpoint on the first line of the function getValue, execution doesn't get as far as the breakpoint before the issue occurs.

[SPlatten]

@Christian-Ehrlicher That issue was resolved, this is new and dynamic_cast wasn't used to resolve the last issue, it was resolved with the suggested qobject_cast.

[J.Hilk]

@SPlatten your stack trace actually points to your clsXMLnode class.
clsXMLnode::strGetAttribute

[jsulm]

@SPlatten I'm talking about clsXMLnode::strGetAttribute(QString).
If you check the stack trace you posted you will see that it is called shortly before the application stopps...

[Christian Ehrlicher]

@SPlatten said in Adding a virtual function causes strange result...:

it was resolved with the suggested qobject_cast.

Since your class is derived from QObject you can also use qobject_cast. Please learn the basics.