Multithreading in Qt with C++

stevereine

I'm trying to create a separate thread to periodically poll another device via TCP/IP. Right now just trying to create and start a separate thread that prints "hello world" to a textbox in my main GUI. I've read dozens of links on how to do this, and tried to rearrange my code in a number of different ways, still no success.

Here's my thread class;

class WorkerThread : public QThread
{
Q_OBJECT
public:

explicit WorkerThread();
//explicit WorkerThread(QObject *parent);
~WorkerThread();

void run() override
{
  //if (pw!=0)
  //pw->setText1("starting new thread");
}

signals:
void settingtext()
{};
//void resultReady(const QString &s)
void resultReady()
{};
};

and here is the class derived from QObject that I use to set this up;

class WorkerThread : public QThread
{
Q_OBJECT
public:

explicit WorkerThread();
//explicit WorkerThread(QObject *parent);
~WorkerThread();

void run() override
{
  //if (pw!=0)
  //pw->setText1("starting new thread");
}

signals:
void settingtext()
{};
//void resultReady(const QString &s)
void resultReady()
{};
};

Included with the rest of my code, this compiles and runs fine. However, when I try to create an instance of myobject in my main code, it gives me three linker errors;

myobject *newobject = new myobject();

main.obj:-1: error: LNK2001: unresolved external symbol "public: virtual struct QMetaObject const * __cdecl myobject::metaObject(void)const " (?metaObject@myobject@@UEBAPEBUQMetaObject@@XZ)

main.obj:-1: error: LNK2001: unresolved external symbol "public: virtual void * __cdecl myobject::qt_metacast(char const *)" (?qt_metacast@myobject@@UEAAPEAXPEBD@Z)

main.obj:-1: error: LNK2001: unresolved external symbol "public: virtual int __cdecl myobject::qt_metacall(enum QMetaObject::Call,int,void * *)" (?qt_metacall@myobject@@UEAAHW4Call@QMetaObject@@HPEAPEAX@Z)

Does anybody have any hints on this? thanks.....Steve

JKSH

@stevereine said in Multithreading in Qt with C++:

I'm trying to create a separate thread to periodically poll another device via TCP/IP. Right now just trying to create and start a separate thread that prints "hello world" to a textbox in my main GUI.

You don't need a thread to periodically poll a device via TCP/IP. Just use a QTimer.

Also, you are not allowed to touch any GUI classes from a secondary thread. GUI class methods can only be called from the GUI thread (which is the thread that creates the QApplication/QGuiApplication object).

@stevereine said in Multithreading in Qt with C++:

it gives me three linker errors;
...
main.obj:-1: error: LNK2001: unresolved external symbol "public: virtual struct QMetaObject const * __cdecl myobject::metaObject(void)const " (?metaObject@myobject@@UEBAPEBUQMetaObject@@XZ)

Try running qmake. If you are using Qt Creator, the command is in the top menu bar: Build > Run qmake

If that doesn't help, post your class definition for myobject.

stevereine

My apologies, I cut and pasted my QThread class twice, where I meant to post the code for my myobject class. Here's the myobject class.

I did run qmake, the result is the same linker errors

class myobject : public QObject
{
Q_OBJECT
QThread workerThread;

public:
void startWorkInAThread()
{
//WorkerThread workerThread;
//connect(workerThread, &WorkerThread::resultReady, this, &myobject::handleResults);
//connect(workerThread, &WorkerThread::finished, workerThread, &QObject::deleteLater);
workerThread.start();
}

void handleResults()
{

};

};

JKSH

@stevereine said in Multithreading in Qt with C++:

I did run qmake, the result is the same linker errors

I forgot to mention: Make sure your class definitions are in *.h files, not in *.cpp files.

Again, I highly recommend you try to achieve your goal using a QTimer instead of threads.

stevereine

Hello JKSH,

It's working now, thanks to your suggestions.

I broke the QThread and QObject classes out into .h and .cpp files, pretty much worked after that. Also, I hit a dead end trying to periodically start the thread to poll for TCP/IP data. That's an easy thing to do in Python, not so elegant in QT C++. I went with your recommendation on using QTimer. All seems well with the app now. Thanks again, I am submitting this as solved.