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

Sai Kamat

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
@
#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
@
#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
@
#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
@
#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
@
#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&#40;&#41;;

}
@

SGaist

Hi,

First thing, GUI manipulation cannot be done outside the main thread AKA the GUI thread. You should emit a signal from your thread to your Dialog and show the QMessageBox there.

There error message comes from the fact that you are trying to give a QObject as the parent of the QMessageBox which can only have a QWidget as a parent.

I would also recommend that you take a look at the QThread latest documentation to see the various possible implementation. Using a worker object with a timer would be cleaner in your case.

Hope it helps

Sai Kamat

Hi SGaist,

I choose to take your 1st 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?

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

mythread.h
@
//....
signals:
void alertSignal(QString message);
//....
@

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

mythread.cpp
@
//....
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
@
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?

treenn

Better create and start your thread in the dialog's constructor Dialog::Dialog(...).

create the thead's object
call connect(...)
start the thread

So it should be something like this:
@Dialog::Dialog(QWidget *parent) :
QDialog(parent),
ui(new Ui::Dialog)
{
ui->setupUi(this);
mThread1=new MyThread(this); //add the field "MyThread *mThread1" to your dialogs definition.
mThread1->setObjectName("mThread1");
connect( mThread1, SIGNAL(alertSignal(QString)), this, SLOT(alertSlot(QString)), Qt::QueuedConnection);
mThread1->start();
}@

Sai Kamat

Thank u @treenn, that sorted it out.