Qt5 Tutorial QThreads - Creating Threads - 2020

In this tutorial, we will learn how to create Threads.

As we already know, this is not a recommended way of using QThread not only because we're using lower level APIs for threads but also we may have a scaling issues later on. Qt recommends using QtConcurrent framework which has higher level of functional type APIs and we can even cancel, pause, or resume the results from a thread run.

However, we still need to know this level of APIs as well. In the next section of my Qt5 tutorial (Creating QThreads using QtConcurrent), we'll transform the code in this tutorial using QtConcurrent namespace.

Starting from Qt Console Application, we need to create MyThread class.

We make the MyThread class get inherited from QThread.

Let's look at main.cpp:

#include <QCoreApplication>
#include "mythread.h"

int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);
    
    MyThread thread1("A"), thread2("B"), thread3("C");

    thread1.start();
    thread2.start();
    thread3.start();

    return a.exec();
}

In the code, we creates three instances of MyThread class with QString names ("A", "B", and "C").

The void QThread::start(Priority priority = InheritPriority) slot begins execution of the thread by calling run() which we overrides in MyThread class.

OK. Let's look at other codes: mythread.cpp and mythread.h:

// mythread.h

#ifndef MYTHREAD_H
#define MYTHREAD_H
#include <QThread>
#include <QString>

class MyThread : public QThread
{
public:
    // constructor
    // set name using initializer
    explicit MyThread(QString s);

    // overriding the QThread's run() method
    void run();
private:
    QString name;
};

#endif // MYTHREAD_H


 

// mythread.cpp

#include "mythread.h"
#include <QDebug>

MyThread::MyThread(QString s) : name(s)
{
}

// We overrides the QThread's run() method here
// run() will be called when a thread starts
// the code will be shared by all threads

void MyThread::run()
{
    for(int i = 0; i <= 100; i++)
    {
        qDebug() << this->name << " " << i;
    }
}

We can see the three threads are running simultaneously accessing the same code segment (i.e. run()):

...
"A"   87
"A"   88
"A"   89
"C"   91
"C"   92
"C"   93
"A"   90
"A"   91
"A"   92
"A"   93
"A"   94
"A"   95
"A"   96
"A"   97
"C"   94
"C"   95
"C"   96
"C"   97
"C"   98
"C"   99
"C"   100
"A"   98
"A"   99
"A"   100