'QMessageBox::critical' : none of the 4 overloads could convert all the argument type

I want to display an error message whenever my independent thread encounters the word "alert1" in a specific .txt file. But I get the above error inside the monitorForAlerts() inside mythread.cpp file. The line expectedly executes if I were to place it inside dialog.cpp. So I guess this is due to non-inheritance of this object. Can you please advise me how to solve this error for the given code?

Here is the code:
dialog.h

Code:

---

#ifndef DIALOG_H
#define DIALOG_H
 
#include <QDialog>
#include <QtCore>
#include "mythread.h"
namespace Ui {
class Dialog;
}
 
class Dialog : public QDialog
{
    Q_OBJECT
 
public:
    explicit Dialog(QWidget *parent = 0);
    ~Dialog();
 
public slots:
 
private:
    Ui::Dialog *ui;
 
private slots:
    void on_pushButton_clicked();
    void on_pushButton_2_clicked();
};
 
#endif // DIALOG_H

---

mythread.h

Code:

---

#ifndef MYTHREAD_H
#define MYTHREAD_H
 
#include <QThread>
#include <QtCore>
#include <QDebug>
#include <QFile>
#include <Windows.h>
#include <QMessageBox>
#include <QTimer>
#define ALERTS_MESSAGE_STORAGE_PATH "E:\\QT1\\simpleGUIThread2\\simpleGUIThread2\\usbAlert.txt"
#define TIMER_VALUE                      500
class MyThread : public QThread
{
    Q_OBJECT
public:
    explicit MyThread(QObject *parent = 0);
    void run();
    QString name;
    void monitorForAlerts();
    int exec();
 
public slots:
 
signals:
    void testSignal(QString message);
 
public slots:
 
};
 
#endif // MYTHREAD_H

---

dialog.cpp

Code:

---

#include "dialog.h"
#include "ui_dialog.h"
 
Dialog::Dialog(QWidget *parent) :
    QDialog(parent),
    ui(new Ui::Dialog)
{
    ui->setupUi(this);
}
 
Dialog::~Dialog()
{
    delete ui;
}
 
void Dialog::on_pushButton_clicked()
{
    ui->label->show();
}
 
void Dialog::on_pushButton_2_clicked()
{
    ui->label->hide();
}

---

mythread.cpp

Code:

---

#include "mythread.h"
#include "dialog.h"
MyThread::MyThread(QObject *parent) :
    QThread(parent)
{
}
 
void MyThread::run()
{
    exec();
}
 
int MyThread::exec()
{
    while(1)
    {
        monitorForAlerts();
        emit(testSignal("hello world!!"));
        sleep(1);
    }
}
 
void MyThread::monitorForAlerts()
{
    QString response = ALERTS_MESSAGE_STORAGE_PATH;
    QFile resp(response);
    resp.open(QIODevice::WriteOnly);
    resp.close();
    QFile resp1(response);
    char buf[121];
    char buf1[] = "alert1";
    char buf2[] = "alert2";
 
    resp1.open(QIODevice::ReadOnly);
    while(resp1.size() == 0)
    {
        Sleep(3000);
    }
    qint64 lineLength = resp1.readLine(buf, sizeof(buf));
    resp1.close();
    if(strcmp(buf,buf1) == 0)
    {
        QFile::remove(ALERTS_MESSAGE_STORAGE_PATH);
        qDebug()<<"warning 1!!";
        QMessageBox::critical(this,tr("ERROR"),tr("Large change in illumination.\nPlease re-capture reference image.\n"));
    }
    if(strcmp(buf,buf2) == 0)
    {
        QFile::remove(ALERTS_MESSAGE_STORAGE_PATH);
        qDebug()<<"warning 2!!";
        QMessageBox::critical(this,tr("ERROR"),tr("The camera position has been moved or an object is obscuring its view.\nPlease check the device.\n"));
    }
}

---

main.cpp

Code:

---

#include "dialog.h"
#include <QApplication>
#include "mythread.h"
int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
 
    MyThread mThread1;
    mThread1.name = "mThread1";
    mThread1.start();
 
    Dialog w;
    w.show();
 
    return a.exec();
}

---

You cannot access UI classes from a secondary thread. Only the main thread, the one running QApplication::exec() is allowed to do that.

One option that you have is to let the thread emit a signal that you connect to your dialog class and let the dialog class show the message box.

Cheers,
_

Hi anda_skoa,
I took your advice. I have created a signal that the thread will emit and connect it to a slot for QDialog. Please let me know if my understanding is correct, because I do not know where to implement the connect(), since the signal is declared in mythread.h and the slot in dialog.h. The connection type argument for connect is Qt::QueuedConnection, so that gui elements from another thread different than main-thread are NOT created. Is this statement correct? and where do I place this?

Code:

---

connect( mThread, SIGNAL(alertSignal(QString)), this, SLOT(alertSlot(QString)), Qt::QueuedConnection);

---

mythread.h

Code:

---

//....
signals:
void alertSignal(QString message);
//....

---

dialog.h

Code:

---

//....
public slots:
void alertSlot(QString message);
//....

---

mythread.cpp

Code:

---

//....
    if(strcmp(buf,buf1) == 0)
    {
        QFile::remove(ALERTS_MESSAGE_STORAGE_PATH);
        qDebug()<<"warning 1!!";
        emit(alertSignal("alert1"));
    }
    else if(strcmp(buf,buf2) == 0)
    {
        QFile::remove(ALERTS_MESSAGE_STORAGE_PATH);
        qDebug()<<"warning 2!!";
        emit(alertSignal("alert2"));
    }

---

dialog.cpp

Code:

---

//...
void Dialog::alertSlot(QString message)
{
    if(strcmp(message, "alert1"))
        QMessageBox::critical(this,tr("ERROR"),tr("Large change in illumination.\nPlease re-capture reference image.\n"));
    else if(strcmp(message, "alert2"))
        QMessageBox::critical(this,tr("ERROR"),tr("The camera position has been moved or an object is obscuring its view.\nPlease check the device.\n"));
}

---

Now if this were correct, how do i implement the connect() and in which file?

This looks reasonable.

You can do the connect in main()

Code:

---

QObject::connect( &mThread, SIGNAL(alertSignal(QString)), &w, SLOT(alertSlot(QString)), Qt::QueuedConnection);

---

Cheers,
_

And the last parameter is not needed because its default value is Qt::AutoConnection which means that for objects from different threads it is Qt::QueuedConnection.

Quote:

---

Originally Posted by Lesiok

And the last parameter is not needed because its default value is Qt::AutoConnection which means that for objects from different threads it is Qt::QueuedConnection.

---

True, however in this case both objects belong to the same thread. It will still work with auto connection because another check is which thread is executing the emit and that is a different thread then the owner of the reciever object.

Doesn't hurt to explicitly specify Qt::QueuedConnection though.

Cheers,
_

GUI objects in threads aside, the original error message:

Quote:

---

QMessageBox::critical' : none of the 4 overloads could convert all the argument

---

is caused by:

Code:

---

QMessageBox::critical(this,tr("ERROR"),tr("Large change in illumination.\nPlease re-capture reference image.\n"));

---

because the first argument, this, is not a pointer to a QWidget and cannot be converted to one.