How to run a function in another thread

Disco

I tried to run a function in the event loop of another thread by clicking a QPushButton but failed.

The sources are as follows:

// main.cpp
#include "mainwindow.h"
#include "ui_mainwindow.h"
#include "workthread.h"

#include <QApplication>
#include <QObject>

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    MainWindow w;
    w.show();

    WorkThread t;
    QObject::connect(w.ui->pushButton, &QPushButton::clicked, &t, &WorkThread::printThreadId, Qt::QueuedConnection);
    t.start();

    return a.exec();
}

// mainwindow.h
#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>

namespace Ui {
class MainWindow;
}

class MainWindow : public QMainWindow
{
    Q_OBJECT

public:
    explicit MainWindow(QWidget *parent = 0);
    ~MainWindow();

    Ui::MainWindow *ui;
};

#endif // MAINWINDOW_H

// mainwindow.cpp
#include "mainwindow.h"
#include "ui_mainwindow.h"

#include <QDebug>

#include "workthread.h"

MainWindow::MainWindow(QWidget *parent)
: QMainWindow(parent)
, ui(new Ui::MainWindow)
{
    ui->setupUi(this);

    qDebug() << "MainWindow thread:" << QThread::currentThreadId();
}

MainWindow::~MainWindow()
{
    delete ui;
}

// workthread.h
#ifndef WORKTHREAD_H
#define WORKTHREAD_H

#include <QThread>

class MainWindow;

class WorkThread : public QThread
{
    Q_OBJECT

public:
    WorkThread();

public slots:
    void printThreadId();

protected:
    void run();
};

#endif // WORKTHREAD_H

// workthread.cpp
#include "workthread.h"

#include <QDebug>

WorkThread::WorkThread()
{}

void WorkThread::printThreadId()
{
    qDebug() << "WorkThread slot thread:" << QThread::currentThreadId();
}

void WorkThread::run()
{
    qDebug() << "WorkThread thread:" << QThread::currentThreadId();

    exec();
}

mainwindow.ui

<?xml version="1.0" encoding="UTF-8"?>
<ui version="4.0">
 <class>MainWindow</class>
 <widget class="QMainWindow" name="MainWindow">
  <property name="geometry">
   <rect>
    <x>0</x>
    <y>0</y>
    <width>400</width>
    <height>300</height>
   </rect>
  </property>
  <property name="windowTitle">
   <string>MainWindow</string>
  </property>
  <widget class="QWidget" name="centralWidget">
   <widget class="QPushButton" name="pushButton">
    <property name="geometry">
     <rect>
      <x>150</x>
      <y>110</y>
      <width>99</width>
      <height>27</height>
     </rect>
    </property>
    <property name="text">
     <string>PushButton</string>
    </property>
   </widget>
  </widget>
 </widget>
 <layoutdefault spacing="6" margin="11"/>
 <resources/>
 <connections/>
</ui>

The output of the program is like:

MainWindow thread: 0x7ff21ff9d780
WorkThread thread: 0x7ff1e7fff700
WorkThread slot thread: 0x7ff21ff9d780
WorkThread slot thread: 0x7ff21ff9d780

from which we can conclude that the method invoked by the clicked signal of pushButton is run in the main thread.

So how can I run this function in my workthread after pressing the button?

J.Hilk

@Disco thats what you get for choosing the subclassing thread approach

Anyway I leave this repo link here:

https://github.com/DeiVadder/QtThreadExample

I would almost always suggest the worker approach, but if it's a fire and forget function you want to run in parallel, QThread::create it probably the way to go.