Is it possible to have pure virtual slots?

[Bonnie]

@SPlatten
As I remember, the overridden "slots" in the derived class can't be declared as slots.
So try

   public:
        void connected() override;
        void dataIn() override;
        void disconnected() override;
   public slots:
        void onFailure(QJsonObject& objCmd);
        void onSuccess(QJsonObject& objCmd);

BTW, you don't have Q_OBJECT in clsModFileIO class but you can declare slots in it? Is that possible?

[SPlatten]

@Bonnie , I've changed the header to your recommendations, but I'm still ending up with:

QAbstractSocket::UnconnectedState
libc++abi.dylib: Pure virtual function called!

[J.Hilk]

@SPlatten maybe we're talking past each other, where are those connects?

[KroMignon]

@SPlatten said in Is it possible to have pure virtual slots?:

QAbstractSocket::UnconnectedState
libc++abi.dylib: Pure virtual function called!

Have you take a look at stackoverflow?

For my understanding: yes it is possible

• on base class don't forget to add Q_OBJECT macro
• on subclass, do not add Q_SLOTS or slots for those slots
• and finally, don't forget to re-run moc if you change one of those things

[Bonnie]

As I tested, it doesn't matter where you do the connection, it is also fine to use the new syntax to connect the base class's slot.
But it is important that the pure virtual slots cannot be called in the base class's constructor (that's not Qt but c++ basic).
Do you call waitForConnected in clsModHelper's constructor?
If you do, the solution is to either move waitForConnected out of the constructor (which is the better way), or connect using Qt::QueuedConnection.

[Christian Ehrlicher]

You're doing something wrong, it works perfectly (and I don't see a reason why it should not - it's plain c++)

main.h

#pragma once
#include <QObject>
struct Ifc
{
  virtual void myVirtualSlot() = 0;
};
class TestClass : public QObject, public Ifc
{
  Q_OBJECT
public:
  TestClass();
  void myVirtualSlot() override;
};

main.cpp

#include <QtCore>
#include "main.h"

TestClass::TestClass()
{
  QTimer::singleShot(100, this, &TestClass::myVirtualSlot);
}
void TestClass::myVirtualSlot()
{
  qDebug() << "Executed!";
  QCoreApplication::exit(0);
}

int main(int argc, char** argv)
{
  QCoreApplication app(argc, argv);

  TestClass c;
  return app.exec();
}

[SPlatten]

@Christian-Ehrlicher , what is your issue? You seem to be constantly on the attack. There is nothing plain old C++ about Qt.

I've been clear what I have done and what the results are. If it isn't working for me then it could be down to something else.

My development system:

macOS Catalina
Version 10.15.7
iMac (Retina 5K, 27-inch, Late 2015)
Processor 4 GHz Quad-Core Intel Core i7
Memory 16 GB 1867 MHz DDR3
Graphics AMD Radeon R9 M395X 4 GB

Qt:

Qt Creator 4.13.2
Based on Qt 5.15.1 (Clang 11.0 (Apple), 64 bit)
Built on Oct 1 2020 01:16:46
From revision 2ee1af2032

Qt Kits:

Desktop Qt 5.14.2 clang 64bit
Desktop Qt 5.15.0 clang 64bit
Qt 5.14.2 WebAssembly
Qt 5.14.2 for iOS
Qt 5.14.2 for iOS Simulator
Qt 5.15.0 WebAssembly
Qt 5.15.0 for iOS
Qt 5.15.0 for iOS Simulator

I am using Desktop Qt 5.15.0 clang 64bit

Compiler

Clang (C++, x86 64bit in /usr/bin)
Clang (C++, x86 32bit in /ust/bin)
Apple Clang (armv7)
Apple Clang (x86_64)
Apple Clang (i386)
Apple Clang (armv7k)
Apple Clang (arm64)

[SPlatten]

@KroMignon The prototypes for both the base and derived classes were posted so you should see that the base includes Q_OBJECT macro.

[Christian Ehrlicher]

@SPlatten said in Is it possible to have pure virtual slots?:

If it isn't working for me then it could be down to something else.

No, it must be in your code. So do what we ask every time - provide a testcase which reproduce your problem.
And virtual functions and how they work is for sure c++ and not Qt.

/edit: and you should simply try out my example.

[SPlatten]

@Christian-Ehrlicher , for the 2nd time today, hear is my code:

Base class prototype, just edited to move virtual slots out of public slot definitions:

    class clsModHelper : public QTcpSocket {
    Q_OBJECT

    protected:
        bool mblnExit;
        double mdblVersion;
        FILE* mfpDbgLog;
        qint64 mint64AppPID;
        std::thread* mpHeartbeat;
        std::thread* mpThread;
        static const int mscintConnectionTimeout;
        static const int mscintHeartbeatFrequency;
        static const char* mscpszHost;
        static clsModHelper* mspThis;
        static char msszTitle[MODULE_NAME_LENGTH + 1];
        quint16 muint16Port, muint16LauncherPID, muint16SleepTime;

        static bool blnToThisModule(clsJSON& objJSON);
        void cleanup();
        void loopUntilExit();
        void start();

        virtual void onConnected() = 0;
        virtual void onDataIn() = 0;
        virtual void onDisconnected() = 0;

    public slots:
        void onErrorOccurred(QAbstractSocket::SocketError socketError);
        void onExitModule();
        void onKillThreads();

    public:
        static const char mscszAck[]
                       ,mscszCmdHB[]
                     ,mscszCommand[]
                    ,mscszCommands[]
                       ,mscszError[]
                      ,mscszModule[]
                     ,mscszMsgType[]
                         ,mscszPID[]
                      ,mscszResult[]
                    ,mscszSkipOver[]
                        ,mscszTime[]
                        ,mscszTrue[]
                        ,mscszType[]
                    ,mscszTypeChar[]
                  ,mscszTypeDouble[]
                   ,mscszTypeFloat[]
                     ,mscszTypeInt[]
                    ,mscszTypeLong[]
                   ,mscszTypeShort[]
                  ,mscszTypeString[]
                   ,mscszTypeUChar[]
                    ,mscszTypeUInt[]
                   ,mscszTypeULong[]
                  ,mscszTypeUShort[];      
        explicit clsModHelper(QObject* pParent, int intArgc, char* parystrArgv[]
                             ,const char* cpszTitle, double dblVersion);
        virtual ~clsModHelper();
        double dblGetVersion() { return mdblVersion; }
        [[noreturn]] static void exitMsg(const char* cpszMsg
                                        ,const char* cpszPrefix = "ERROR"
                                        ,int intCode = EXIT_FAILURE);
        qint64 int64GetPID() { return mint64AppPID; }
        int intExitCode() { return 0; }
        static long lngTimeInSecs();
        static clsModHelper* psGetModuleInstance() { return clsModHelper::mspThis; }
        static const char* cpszGetTitle() { return msszTitle; }
        void queueJSON(QJsonObject& objJSON);
        void sendHeartbeat();
        void setSleepTime(quint16 uint16SleepTime) { muint16SleepTime = uint16SleepTime; }
        void threadBody();
        virtual void moduleThreadBody() = 0;
        quint16 uint16GetLauncherPID() { return muint16LauncherPID; }

    signals:
        void onFailure(QJsonObject& objCmd);
        void onSuccess(QJsonObject& objCmd);
        void terminateModule();
        void threadTerminated();
    };

Derived class prototype:

    class clsModFileIO : public clsModHelper {
    private:
        static clsModFileIO* mspInstance;

        void cmdLineSummary();

    public:
        static const char mscszCheck[]
                         ,mscszCmdFind[]
                         ,mscszCmdRead[]
                         ,mscszCmdSave[]
                         ,mscszCmdTell[]
                         ,mscszCmdWrite[]
                         ,mscszByteOrder[]
                         ,mscszData[]
                         ,mscszFailed[]
                         ,mscszFile[]
                         ,mscszFrom[]
                         ,mscszLength[]
                         ,mscszMode[]
                         ,mscszModeBinary[]
                         ,mscszModeText[]
                         ,mscszPattern[]
                         ,mscszPosition[]
                         ,mscszStart[]
                         ,mscszSuccess[];
        explicit clsModFileIO(QObject* pParent, int intArgc, char* parystrArgv[]);
        ~clsModFileIO();
        bool doCommand(QString& strFile, QJsonObject& objRequest
                                       , QJsonObject& objCmd
                                       , QJsonObject& objResponse
                                       , quint16 uint16Idx = 0);        
        void moduleThreadBody() override;
        static clsModFileIO* pInstance() { return mspInstance; }

        virtual void onConnected() override;
        virtual void onDataIn() override;
        virtual void onDisconnected() override;

    public slots:
        void onFailure(QJsonObject& objCmd);
        void onSuccess(QJsonObject& objCmd);
    };

With the code in this configuration the Application Output shows:

2020-11-06 18:04:44.443090+0000 mdFileIO[1591:37076] QObject::connect: No such slot clsModHelper::onConnected() in ../mdFileIO/clsModHelper.cpp:67
2020-11-06 18:04:44.443120+0000 mdFileIO[1591:37076] QObject::connect: No such slot clsModHelper::onDisconnected() in ../mdFileIO/clsModHelper.cpp:68
2020-11-06 18:04:44.443130+0000 mdFileIO[1591:37076] QObject::connect: No such slot clsModHelper::onDataIn() in ../mdFileIO/clsModHelper.cpp:69

As for a test case, I launch my main application which launches the problem application as a child process, immediately when this application is launched it attempts to connect to the launching application with a socket which is successful but the slots are not connected to the signals in QTcpSocket despite the connect statements being correct as far as I know.

[Christian Ehrlicher]

I can't compile it so no - that does not help. As you can see my example works as expected so it must be somewhere in your code. If you want help you have to provide something which we can reproduce which we can't.

[Bonnie]

@SPlatten
You have some misunderstanding.
According to the SO post, the virtual functions need to be declared as slots in the base class, but not in the derived class.
So they still need to be in the "public slots" part of clsModHelper, but only "public" part of clsModFileIO.

And as I posted, the original way you use to connect (the new syntax) is fine to use.
The point is, the signals cannot be emited in the base class's constuctor.
As my testing, this is the most likely situation to cause that "Pure virtual function called!" error.
So if you put waitForConnected in clsModHelper's constructor, the connected signal will be emited in it and cause that error.

[SPlatten]

The only way I can get rid of those messages is to move the prototypes back under the public slots area and then I get the original message:

QAbstractSocket::UnconnectedState
libc++abi.dylib: Pure virtual function called!

I will try to produce a simple example and see if I can replicate the issue.

[SPlatten]

@Bonnie I'll give that a try. To clarify here is the implementation:

clsModHelper::clsModHelper(QObject* pParent, int intArgc, char* parystrArgv[]
                          ,const char* cpszTitle, double dblVersion)
            : QTcpSocket(pParent)
            , mblnExit(false)
            , mdblVersion(dblVersion)
            , mfpDbgLog(nullptr)
            , mint64AppPID(QCoreApplication::applicationPid())
            , mpHeartbeat(nullptr), mpThread(nullptr)
            , muint16Port(0), muint16LauncherPID(0), muint16SleepTime(0) {    
    if ( intArgc < CLA_LAUNCHER_PID ) {
        std::cerr << "Insufficient arguments, aborting!";
        exit(EXIT_FAILURE);
    }
    if ( mspThis == nullptr ) {
        mspThis = this;
    }
    muint16Port = (quint16)atoi(parystrArgv[CLA_PORT]);
    muint16LauncherPID = (quint16)atoi(parystrArgv[CLA_LAUNCHER_PID]);
    //Connect up the signals
    connect(this, SIGNAL(connected()), this, SLOT(onConnected()));
    connect(this, SIGNAL(disconnected()), this, SLOT(onDisconnected()));
    connect(this, SIGNAL(readyRead()), this, SLOT(onDataIn()));
    connect(this, &QAbstractSocket::errorOccurred, this, &clsModHelper::onErrorOccurred);
    //Module specific signals
    connect(this, &clsModHelper::terminateModule, this, &clsModHelper::onKillThreads);
    connect(this, &clsModHelper::threadTerminated, this, &clsModHelper::onExitModule);
    qdbg() << state();
    //Connect to the Application
    connectToHost(clsModHelper::mscpszHost, muint16Port);

    if ( waitForConnected(clsModHelper::mscintConnectionTimeout) ) {
    //Set-up title
        strcpy(msszTitle, cpszTitle);
    //Install custom message handler
        clsDebugService::serviceDebugQueue(msszTitle, true, true);
    //Build up title for output to the console
        char szTitle[MODULE_NAME_LENGTH + 24];
        sprintf(szTitle, "%s Version: %.2lf", cpszTitle, mdblVersion);
        qdbg() << szTitle;
    }
}

So now the base class prototype:

    class clsModHelper : public QTcpSocket {
    Q_OBJECT

    protected:
        bool mblnExit;
        double mdblVersion;
        FILE* mfpDbgLog;
        qint64 mint64AppPID;
        std::thread* mpHeartbeat;
        std::thread* mpThread;
        static const int mscintConnectionTimeout;
        static const int mscintHeartbeatFrequency;
        static const char* mscpszHost;
        static clsModHelper* mspThis;
        static char msszTitle[MODULE_NAME_LENGTH + 1];
        quint16 muint16Port, muint16LauncherPID, muint16SleepTime;

        static bool blnToThisModule(clsJSON& objJSON);
        void cleanup();
        void loopUntilExit();
        void start();

    public slots:
        virtual void onConnected() = 0;
        virtual void onDataIn() = 0;
        virtual void onDisconnected() = 0;
        void onErrorOccurred(QAbstractSocket::SocketError socketError);
        void onExitModule();
        void onKillThreads();

    public:
        static const char mscszAck[]
                       ,mscszCmdHB[]
                     ,mscszCommand[]
                    ,mscszCommands[]
                       ,mscszError[]
                      ,mscszModule[]
                     ,mscszMsgType[]
                         ,mscszPID[]
                      ,mscszResult[]
                    ,mscszSkipOver[]
                        ,mscszTime[]
                        ,mscszTrue[]
                        ,mscszType[]
                    ,mscszTypeChar[]
                  ,mscszTypeDouble[]
                   ,mscszTypeFloat[]
                     ,mscszTypeInt[]
                    ,mscszTypeLong[]
                   ,mscszTypeShort[]
                  ,mscszTypeString[]
                   ,mscszTypeUChar[]
                    ,mscszTypeUInt[]
                   ,mscszTypeULong[]
                  ,mscszTypeUShort[];      
        explicit clsModHelper(QObject* pParent, int intArgc, char* parystrArgv[]
                             ,const char* cpszTitle, double dblVersion);
        virtual ~clsModHelper();
        double dblGetVersion() { return mdblVersion; }
        [[noreturn]] static void exitMsg(const char* cpszMsg
                                        ,const char* cpszPrefix = "ERROR"
                                        ,int intCode = EXIT_FAILURE);
        qint64 int64GetPID() { return mint64AppPID; }
        int intExitCode() { return 0; }
        static long lngTimeInSecs();
        static clsModHelper* psGetModuleInstance() { return clsModHelper::mspThis; }
        static const char* cpszGetTitle() { return msszTitle; }
        void queueJSON(QJsonObject& objJSON);
        void sendHeartbeat();
        void setSleepTime(quint16 uint16SleepTime) { muint16SleepTime = uint16SleepTime; }
        void threadBody();
        virtual void moduleThreadBody() = 0;
        quint16 uint16GetLauncherPID() { return muint16LauncherPID; }

    signals:
        void onFailure(QJsonObject& objCmd);
        void onSuccess(QJsonObject& objCmd);
        void terminateModule();
        void threadTerminated();
    };

And the derived prototype:

    class clsModFileIO : public clsModHelper {
    private:
        static clsModFileIO* mspInstance;

        void cmdLineSummary();

    public:
        static const char mscszCheck[]
                         ,mscszCmdFind[]
                         ,mscszCmdRead[]
                         ,mscszCmdSave[]
                         ,mscszCmdTell[]
                         ,mscszCmdWrite[]
                         ,mscszByteOrder[]
                         ,mscszData[]
                         ,mscszFailed[]
                         ,mscszFile[]
                         ,mscszFrom[]
                         ,mscszLength[]
                         ,mscszMode[]
                         ,mscszModeBinary[]
                         ,mscszModeText[]
                         ,mscszPattern[]
                         ,mscszPosition[]
                         ,mscszStart[]
                         ,mscszSuccess[];
        explicit clsModFileIO(QObject* pParent, int intArgc, char* parystrArgv[]);
        ~clsModFileIO();
        bool doCommand(QString& strFile, QJsonObject& objRequest
                                       , QJsonObject& objCmd
                                       , QJsonObject& objResponse
                                       , quint16 uint16Idx = 0);        
        void moduleThreadBody() override;
        static clsModFileIO* pInstance() { return mspInstance; }

        virtual void onConnected() override;
        virtual void onDataIn() override;
        virtual void onDisconnected() override;

    public slots:
        void onFailure(QJsonObject& objCmd);
        void onSuccess(QJsonObject& objCmd);
    };

With the code this way I get:

QAbstractSocket::UnconnectedState
libc++abi.dylib: Pure virtual function called!

In the Application Output.

[Bonnie]

@SPlatten
That's exactly what I have guessed.
So as I said I think it's better to not use waitForConnected() in clsModHelper's constructor.
Calling a blocking function in a constructor is not a good design anyway.

[Christian Ehrlicher]

@Bonnie said in Is it possible to have pure virtual slots?:

Calling a blocking function in a constructor is not a good design anyway.

Neither is calling a (pure) virtual function in the ctor/dtor.

[SPlatten]

Here is the demo I just created and it works, so the problem has to be specific to the class:

    class clsBaseTest : public QObject {
    Q_OBJECT

    public slots:
        //Pure virtual slot
        virtual void onTest() = 0;

    public:
        explicit clsBaseTest() {}

    signals:
       void test();
    };

    class clsDerivedTest : public clsBaseTest {
    public:
        clsDerivedTest() {}

        virtual void onTest() override {
            qDebug() << "clsDerivedTest::onTest";
        };
    };

Code to test:

    clsDerivedTest test;
    connect(&test, SIGNAL(test()), &test, SLOT(onTest()));
    emit test.test();

In the Application Output I see exactly what I am supposed to see:

2020-11-06 18:47:48.956472+0000 mdFileIO[3144:63052] clsDerivedTest::onTest

So it must be the fact that the connections are established in the base class constructor, I will try moving these to another function.

[SPlatten]

@SPlatten , I moved the socket connection from the constructor to its own function:

void clsModHelper::connectToXMLMPAM() {
    //Connect to the Application
    connectToHost(clsModHelper::mscpszHost, muint16Port);

    if ( waitForConnected(clsModHelper::mscintConnectionTimeout) != true ) {
        throw -1;
    }
}

main loops like this:

int main(int intArgc, char* parystrArgv[]) {
//Initialise the module                
    QApplication a(intArgc, parystrArgv);
    clsModFileIO obj(&a, intArgc, parystrArgv);

    try{
        obj.connectToXMLMPAM();
    } catch( exception& e) {
        std::cout << "Could not connect to XMLMPAM!";
        exit(EXIT_FAILURE);
    }
    return obj.intExitCode();
}

Now it works, thank you to everyone who helped me to resolve this issue.