Is it possible to have pure virtual slots?

[SPlatten]

@SGaist, can you please explain what you mean? I'm not sure I understand.

Found this:
https://rvarago.medium.com/default-implementation-for-pure-virtual-functions-in-c-3c525cc820af

Will give it a try tomorrow.

[SPlatten]

@SGaist All the pure virtual functions have overloaded methods in the derived class or the derived class would be abstract. Presently these are very simple functions and just contain a qDebug() << "function name" entry.

[SPlatten]

@SGaist , updated....if I comment out the 3 connects which:

//    connect(this, &QTcpSocket::connected, this, &clsModHelper::connected);
//    connect(this, &QTcpSocket::disconnected, this, &clsModHelper::disconnected);
//    connect(this, &QIODevice::readyRead, this, &clsModHelper::dataIn);

It doesn't crash and the connection is made, waitForConnected doesn't crash and the application continues to where I expect.

To clarify, in the base class, the initial prototype for these slots:

    public slots:
        virtual void connected() = 0;
        virtual void dataIn() = 0;
        virtual void disconnected() = 0;

Then in my derived class:

    public slots:
        void connected() override;
        void dataIn() override;
        void disconnected() override;

The implementation, just to prove if works:

void clsModFileIO::connected() {
    //Start the module thread
    start();
}
void clsModFileIO::dataIn() {
    qdbg() << "dataIn";
}
void clsModFileIO::disconnected() {
    emit terminateModule();
}

If I then uncomment those connections and rebuild it does exactly the same its very repeatable.

[Bonnie]

I used to have pure virtual slots. It's quite a long time ago so I was using the old syntax to connect. Maybe you can try if that will work.
P.S. I think the "override" mark is irrelevant, you can add that. The SO post is so old that c++11 was not published at that time.

[SPlatten]

@Bonnie , thank you, the only reason I put in the override keyword is to get rid of the annoying yellow text appended at the end of each line suggesting it.

[J.Hilk]

@SPlatten is clsModHelper the base class of clsModFileIO?

[SPlatten]

@J-Hilk Yes

[SPlatten]

@Bonnie , just replaced connections with:

    connect(this, SIGNAL(connected), this, SLOT(connected));
    connect(this, SIGNAL(disconnected), this, SLOT(disconnected));
    connect(this, SIGNAL(readyRead), this, SLOT(dataIn));

It worked, no more crashing, thank you.

[J.Hilk]

@SPlatten ok, than this
connect(this, &QTcpSocket::connected, this, &clsModHelper::connected);

is explicitly invoking the pure virtual base function, thats bound to crash

connect(this, &QTcpSocket::connected, this, & clsModFileIO::connected);

should work fine

[SPlatten]

@J-Hilk , thank you, but isn't this the point of using a pure virtual slot, I want the actual derived slot to be worked out by the compiler so I can have any number of derived classes based the same pure virtual slot and then the actual slot gets called in its place?

[SPlatten]

@Bonnie , not so fast, I thought it worked because it didn't crash, but now I see in the Application Output:

2020-11-06 09:11:49.460550+0000 mdFileIO[4406:86031] QObject::connect: Parentheses expected, signal clsModHelper::connected in ../mdFileIO/clsModHelper.cpp:67
2020-11-06 09:11:49.460576+0000 mdFileIO[4406:86031] QObject::connect: Parentheses expected, signal clsModHelper::disconnected in ../mdFileIO/clsModHelper.cpp:68
2020-11-06 09:11:49.460583+0000 mdFileIO[4406:86031] QObject::connect: Parentheses expected, signal clsModHelper::readyRead in ../mdFileIO/clsModHelper.cpp:69

[Bonnie]

@SPlatten
The old syntax is not like that :)
As the output says you need parentheses like

    connect(this, SIGNAL(connected()), this, SLOT(connected()));
    connect(this, SIGNAL(disconnected()), this, SLOT(disconnected()));
    connect(this, SIGNAL(readyRead()), this, SLOT(dataIn()));

[SPlatten]

@Bonnie, its different, but still not right, now it stops in moc_clsModHelper.cpp on a line which reads:

case 4: _t->connected(); break;

The Application Output shows:

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

[J.Hilk]

@SPlatten sure, but you can always call the base implementation by implicitly invoking it and in many cases you want to do that.

take for example:

class BaseClass {
public:
    virtual void functionA() { qDebug () << "A"; }
};

class DerivedClass : public BaseClass{

public:
    void functionA() override {qDebug() << "B";}

};

class DerivedClass2: public BaseClass {
public:
    void functionA() override {
        BaseClass::functionA();
        qDebug() << "B";
    }
};

int main (int argc, char *argv[])
{

    DerivedClass c1;
    DerivedClass2 c2;

    c1.functionA();
    qDebug() << "----";
    c2.functionA();

}

this outputs:

B
----
A
B

where the 2nd class implicitly invokes the base implementation.

[Bonnie]

@SPlatten
Hmmm...can you post the full header file of clsModFileIO class?

[SPlatten]

@J-Hilk Its a pure virtual slot for the purpose of defining the structure and interface required to be implemented by the derived classes, thats why they have no implementation.

[SPlatten]

@Bonnie , sure:

These are very much a work in progress:

clsModHelper.h

    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 connected() = 0;
        virtual void dataIn() = 0;
        virtual void disconnected() = 0;
        void errorOccurred(QAbstractSocket::SocketError socketError);
        void exitModule();
        void killThreads();

    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();
    };

clsModFileIO.h:

    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; }

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

Some additional information:

Qt Creator 4.13.2
Based on Qt 5.15.1 (Clang 11.0 (Apple), 64 bit)

Built on Oct 1 2020 01:16:45

From revision 2ee1af2032

Using Qt 5.15.0

[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?