Running function in Qthread - The application will hang - c++

Question

I have some problems with threading inside QT.
#include <QCoreApplication>
#include "handler.hpp"
int main(int argc, char *argv[]) {
QCoreApplication a(argc, argv);
Handler* handler = new Handler();
handler->StartThread();
return a.exec();
}
What I expect is that with handler->StartThread() statement the function within my thread start to write debug messages and once the internal timer within handler finishes I get the nice line [Press ...] and then a return code of 0. However this is not happening. What I get is:
I WORK... ( 0x1540 )
Worker has finished. ( 0x6f4 )
I WORK... ( 0x1540 )
Worker has finished. ( 0x6f4 )
I WORK... ( 0x1540 )
Worker has finished. ( 0x6f4 )
I WORK... ( 0x1540 )
Worker has finished. ( 0x6f4 )
Thread stopped.
And of course when I stop the execution of the application, the return code is: -1073741510. Obviously not that nice of a zero.
Anyway here's the rest of the application code:
Handler.hpp
#ifndef HANDLER_HPP
#define HANDLER_HPP
#include <QObject>
#include <QThread>
#include <QDebug>
#include <QTimer>
#include "testclass.hpp"
class Handler : public QObject
{
Q_OBJECT
public:
Handler();
~Handler();
void StartThread();
public slots:
void functionFinished();
void threadTerminated();
private:
QTimer* shutdown;
QTimer* timer;
QThread* thread;
MyClass* worker;
};
#endif // HANDLER_HPP
Handler.cpp
#include "handler.hpp"
Handler::Handler() {
shutdown = new QTimer();
thread = new QThread();
timer = new QTimer();
worker = new MyClass();
worker->moveToThread(thread);
QObject::connect(thread, SIGNAL(started()), worker, SLOT(runAgain()));
QObject::connect(timer, SIGNAL(timeout()), worker, SLOT(runAgain()));
QObject::connect(worker, SIGNAL(iFinished()), this, SLOT(functionFinished()));
QObject::connect(shutdown, SIGNAL(timeout()), thread, SLOT(quit()));
QObject::connect(thread, SIGNAL(finished()), this, SLOT(threadTerminated()));
shutdown->start(20000);
}
Handler::~Handler() {
QObject::disconnect(thread, SIGNAL(started()), worker, SLOT(runAgain()));
QObject::disconnect(timer, SIGNAL(timeout()), worker, SLOT(runAgain()));
QObject::disconnect(worker, SIGNAL(iFinished()), this, SLOT(functionFinished()));
QObject::disconnect(shutdown, SIGNAL(timeout()), thread, SLOT(quit()));
QObject::disconnect(thread, SIGNAL(finished()), this, SLOT(threadTerminated()));
if (shutdown != 0) {
delete shutdown;
shutdown = 0;
}
if (timer != 0) {
delete timer;
timer = 0;
}
if (thread != 0) {
delete thread;
thread = 0;
}
if (worker != 0) {
delete worker;
worker = 0;
}
}
void Handler::functionFinished() {
qDebug() << "Worker has finished. (" << QThread::currentThreadId() << ")";
timer->start(5000);
}
void Handler::threadTerminated() {
qDebug() << "Thread stopped.";
}
void Handler::StartThread() {
thread->start();
}
MyClass (header - testclass.hpp)
#ifndef TESTCLASS_HPP
#define TESTCLASS_HPP
#include <QTimer>
#include <QObject>
class MyClass : public QObject
{
Q_OBJECT
public:
MyClass();
public slots:
void runAgain();
signals:
void iFinished();
private:
void doWork();
};
#endif // TESTCLASS_HPP
MyClass Source - testclass.cpp
#include "testclass.hpp"
#include <QThread>
#include <QDebug>
MyClass::MyClass() {
}
void MyClass::runAgain() {
doWork();
}
void MyClass::doWork() {
qDebug() << "I WORK...\t(" << QThread::currentThreadId() << ")";
emit iFinished();
}
I've previosly read that it's not a good idea to inherit the classes to be ran inside a thread directly from QThread and so I came up with this solution but it still gets fishy though it's pretty nice. I'm open to any recommendation, this is my first time with QT so better to learn now than feel sorry later.
Oh my bad, I forgot the concrete actual question. Why doesn't the execution ends with a nice exit code of 0?

Answer 1

What I expect is that with handler->StartThread() statement the function within my thread start to write debug messages and once the internal timer within handler finishes I get the nice line [Press ...] and then a return code of 0. However this is not happening.
The reason you're not getting a finished command prompt of "[Press ...]" is due to the QCoreApplication and its exec() call. As the Qt docs state: -
Enters the main event loop and waits until exit() is called
So, you've created a second thread, set it to do some work and finish, but the main thread is still running. You need to exit the main thread.
And of course when I stop the execution of the application, the return code is: -1073741510
It sounds like you're killing the main thread with something like "Ctrl+C". Calling QCoreApplication::exit() when your 2nd thread finishes and has cleaned up, should help here.