In a Qt console application, why is a QTimer required for the code to exit properly? - qt

Question

I've wrote this code looking at examples online of how I'm supposed to run a console program that doesn't just run and quit and one that does. Based on a Qt console application. This one here, I wanted it to quit. I've understood pretty much everthing excepth the QTimer::singleShot line. If the line is commented out, the application will run but will not quit. If it is left, the application will run and quit as expected. Can anyone explain to me why?
dostuff.h
#ifndef DOSTUFF_H
#define DOSTUFF_H
#include <QObject>
#include <iostream>
class DoStuff: public QObject
{
Q_OBJECT
public :
DoStuff(QObject *parent = 0);
public slots:
void run();
signals:
void finished();
};
#endif // DOSTUFF_H
And the implementation dostuff.cpp
#include "dostuff.h"
DoStuff::DoStuff(QObject *parent):QObject(parent)
{
}
void DoStuff::run(){
for (int i = 0; i < 10000; i++){
std::cout << "Processing " << i << std::endl;
}
emit(finished());
}
My main.cpp
#include <QCoreApplication>
#include <QTimer>
#include "dostuff.h"
int main(int argc, char *argv[])
{
QCoreApplication a(argc, argv);
DoStuff *dostuff = new DoStuff(&a);
QObject::connect(dostuff,SIGNAL(finished()),&a,SLOT(quit()));
dostuff->run();
// WHY THIS??
QTimer::singleShot(10,dostuff,SLOT(run()));
return a.exec();
}

Answer 1

QTimer is not required to exit properly; You just need to provide a way to get your application to break the event loop at some point. In GUI application, Qt does that automatically when the last window is closed.
In Console applications, you can:
Either run your application without an event loop (if you have a straight-forward simple control flow in your application).
Or (if you require an event loop to handle some events or cross thread signal/slots) you need to have some event that makes your application break the event loop and quit. This event should only be triggered when the application has finished its job.
The code sample you have in your question is really simple, and does not require an event loop to run properly. The only effect the QTimer has in your code is that it delays execution for 10 ms. Here is the same code sample without running an event loop:
#include <QtCore>
class DoStuff: public QObject
{
Q_OBJECT
public :
DoStuff(QObject *parent = 0) : QObject(parent) {}
public slots:
void run() {
for (int i = 0; i < 10000; i++){
qInfo() << "Processing " << i;
}
emit finished();
}
signals:
void finished();
};
int main(int argc, char *argv[]) {
QCoreApplication a(argc, argv);
DoStuff dostuff;
QObject::connect(&dostuff, &DoStuff::finished,
&a, &QCoreApplication::quit);
dostuff.run();
return 0; //no event loop required
}
#include "main.moc"
If you start an event loop, you may notice that the quit slot does not work when not using QTimer::singleShot. The reason for this is that quit is called before the event loop is even started (and the call has no effect at all). That's why according to the docs, it is recommended to connect to quit using a queued connection:
It's good practice to always connect signals to this slot using a QueuedConnection. If a signal connected (non-queued) to this slot is emitted before control enters the main event loop (such as before "int main" calls exec()), the slot has no effect and the application never exits. Using a queued connection ensures that the slot will not be invoked until after control enters the main event loop.
So, if you want to have an event loop in your code above, you just need to connect using a Qt::QueuedConnection:
#include <QtCore>
class DoStuff: public QObject
{
Q_OBJECT
public :
DoStuff(QObject *parent = 0) : QObject(parent) {}
public slots:
void run() {
for (int i = 0; i < 10000; i++){
qInfo() << "Processing " << i;
}
emit finished();
}
signals:
void finished();
};
int main(int argc, char *argv[]) {
QCoreApplication a(argc, argv);
DoStuff dostuff;
QObject::connect(&dostuff, &DoStuff::finished,
&a, &QCoreApplication::quit,
Qt::QueuedConnection);
// ^^^^^^^^^^^^^^^^
// use a queued connection
dostuff.run();
return a.exec(); //start an event loop
}
#include "main.moc"

Answer 2

Timer is needed to postpone execution. Since you want to have a running event loop, a.exec() has to be called, then the timer executes your code. When your code finishes running, it triggers finished signal, that is tied to QCoreApplication::quit - that's the needed exit for event loop running inside a.exec().
Btw, you have to remove: dostuff->run(); from your code.