QThread issue of one thread calling another thread

micha_eleric

main.cpp

#include "CClass2.h"

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);

    CClass1 c1; 
    CClass2 c2; 
    CClass3 c3; 

    c1.SetC2(&c2);
    c3.SetC2(&c2);

    c2.SetC1(&c1);
    c2.SetC3(&c3);

    c2.StartThreads();

    c1.show();
    return a.exec();
}

CClass1.h

#ifndef CCLASS1_H
#define CCLASS1_H

#include <QObject>
#include "MainWindow.h"
#include <QApplication>

class CClass2;

class CClass1 : public QMainWindow
{Q_OBJECT
//?? data
private:
    // data
    Ui_MainWindow *m_ui;
    CClass2 *m_c2; 

public:
    // constructor
    CClass1(QWidget *parent = 0);
    // destructor
    virtual ~CClass1();

    // get/set
    void SetC2(CClass2 *c2);

    // functions
    void Button();

};

#endif // CCLASS1_H

CClass.cpp

#include "CClass1.h"
#include "CClass2.h"

CClass1::CClass1(QWidget *parent): QMainWindow(parent)
{
    m_ui = new Ui_MainWindow();
    m_ui->setupUi(this);
    connect(m_ui->pushButton, &QPushButton::clicked, this, &CClass1::Button);
}

CClass1::~CClass1()
{

}

void CClass1::SetC2(CClass2 *c2)
{
    m_c2 = c2;
}

void CClass1::Button()
{
    m_c2->CallButton();
}

CClass2.h

#ifndef CCLASS2_H
#define CCLASS2_H

#include <QObject>
#include "CClass1.h"
#include "CClass3.h"
#include <QThread>

class CClass2 : public QThread
{
    Q_OBJECT
    CClass1 *m_c1;
    CClass3 *m_c3;
    QThread* m_threadC1;
    QThread* m_threadC3; 

public:
    // constructor
    CClass2();
    // destructor
    virtual ~CClass2();

    //get/set
    void SetC1(CClass1 *c1);
    void SetC3(CClass3 * c3);

    // function
    void StartThreads();
    void CallButton();

signals:
    void EmitButton();

};

#endif // CCLASS2_H

CClass2.cpp

#include "CClass2.h"

CClass2::CClass2() : QThread()
{
    m_threadC3 = new QThread();
}

CClass2::~CClass2()
{

}

void CClass2::SetC1(CClass1 *c1) 
{
    m_c1 = c1;
}

void CClass2::SetC3(CClass3 * c3) /
{
    m_c3 = c3;
    m_c3->moveToThread(m_threadC3);
    connect(m_c3, &CClass3::finished, m_threadC3, &QThread::quit);
    connect(m_c3, &CClass3::finished, m_c3, &CClass3::deleteLater);
    connect(m_threadC3, &QThread::finished, m_threadC3, &QThread::deleteLater);
            m_c3->start();
}

void CClass2::StartThreads()
{
    connect(this, &CClass2::EmitButton, m_c3, &CClass3::ReceiveButton);
}

void CClass2::CallButton()
{
    emit EmitButton();
}

CClass3.h

#ifndef CCLASS3_H
#define CCLASS3_H

#include <QThread>
#include <iostream>

class CClass2;

class CClass3 : public QThread
{
private:
    // data
    CClass2 *m_c2; 

public:
    // constructor
    CClass3();
    // destructor
    virtual ~CClass3();

    // get/set
    void SetC2(CClass2 *c2);

public slots:
    void ReceiveButton();

};

#endif // CCLASS3_H

CClass3.cpp

#include "CClass2.h"
#include "CClass3.h"

CClass3::CClass3() : QThread()
{
    std::cout << "CClass3\n" << std::flush;
}

CClass3::~CClass3()
{

}

void CClass3::SetC2(CClass2 *c2)
{
    m_c2 = c2;
}

void CClass3::ReceiveButton()
{
    std::cout << "button\n" << std::flush;
}

CClass3::ReceiveButton() is not called.
what have i done wrong?

Christian Ehrlicher

When you want to use signals and slots the called thread must be run an event loop as explained in the documentation - Queued signals are placed in the thread's event loop - when there is none it can't work.

micha_eleric

@Christian-Ehrlicher read the document many times, still not fully understanding how to use qthreads properly. any other sites you recommend?

ChrisW67

@micha_eleric said in QThread issue of one thread calling another thread:

void CClass2::SetC3(CClass3 * c3) /
{
    m_c3 = c3;
    m_c3->moveToThread(m_threadC3);
    connect(m_c3, &CClass3::finished, m_threadC3, &QThread::quit);
    connect(m_c3, &CClass3::finished, m_c3, &CClass3::deleteLater);
    connect(m_threadC3, &QThread::finished, m_threadC3, &QThread::deleteLater);
            m_c3->start();
}

Surely you mean to start m_threadC3? Actually this is very confused. You have m_threadC3, presumably a QThread, and CClass3 which is also a QThread. Have a closer read of the example near the top of the docs that @Christian-Ehrlicher linked to. Neither the worker nor the controller is a QThread object.

There are also these notes How to Really, Truly Use QThreads that date from Qt 4 before the Qt 5 documentation started to change to reflect much the same advice.

Christian Ehrlicher

@micha_eleric said in QThread issue of one thread calling another thread:

any other sites you recommend?

No

As I said - it's not about threads but about slots - you must run an event loop in your thread so slots can be called. You don't so no slots.

micha_eleric

@Christian-Ehrlicher thought

class CClass1 : public QMainWindow

was and event loop

not the slot you mean?

public slots:
    void ReceiveButton();

JonB

@micha_eleric
Have you read the basic explanation in https://doc.qt.io/qt-6/qthread.html#details (the link @Christian-Ehrlicher gave you)?

A QThread object manages one thread of control within the program. QThreads begin executing in run(). By default, run() starts the event loop by calling exec() and runs a Qt event loop inside the thread.

micha_eleric

@JonB yes, many times

micha_eleric

@micha_eleric said in QThread issue of one thread calling another thread:
changing

class CClass3 : public QThread

to

class CClass3 : public QObject

fixed issue, along with issues raised from change