Problem with QTimer : the function that would be load is not load!

[lifedj]

Hi, I have a problem in using QTimer.
I'm trying to use this piece of code, but I don't know why the function "cameraTimerTimeout()" doesn't start:

mainwindow.h

#ifndef MAINWINDOW_H
#define MAINWINDOW_H
 
#include <QMainWindow>
#include <QTimer>
#include <iostream>
 
namespace Ui {
class MainWindow;
}
 
class MainWindow : public QMainWindow
{
    Q_OBJECT
 
public:
    explicit MainWindow(QWidget *parent = 0);
    ~MainWindow();
 
private slots:
    void on_pushButton_clicked();
    void cameraTimerTimeout();
    void cattura();
 
private:
    Ui::MainWindow *ui;
    QTimer cameraTimer;
};
 
#endif // MAINWINDOW_H

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>
#include <QTimer>
#include <iostream>

namespace Ui {
class MainWindow;
}

class MainWindow : public QMainWindow
{
    Q_OBJECT
    
public:
    explicit MainWindow(QWidget *parent = 0);
    ~MainWindow();
    
private slots:
    void on_pushButton_clicked();
    void cameraTimerTimeout();
    void cattura();

private:
    Ui::MainWindow *ui;
    QTimer cameraTimer;
};

#endif // MAINWINDOW_H

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main.cpp

#include <QtGui/QApplication>
#include "mainwindow.h"
 
int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    MainWindow w;
    w.show();
 
    return a.exec();
}

#include <QtGui/QApplication>
#include "mainwindow.h"

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

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mainwindow.cpp

#include "mainwindow.h"
#include "ui_mainwindow.h"
 
MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);
}
 
MainWindow::~MainWindow()
{
    delete ui;
}
 
void MainWindow::on_pushButton_clicked()
{
    MainWindow w;
    ui->pushButton->setText("Ferma riconoscimento");
    w.cattura();
}
 
 
void MainWindow::cattura()
{
        cameraTimer.start(33); // 33 ms = 30 fps
        connect(&cameraTimer, SIGNAL(timeout()),this,SLOT(cameraTimerTimeout()));
 
       return;
 
}
 
void MainWindow::cameraTimerTimeout()
{
    std::cout << "CIAO2" << std::endl;
 
}

#include "mainwindow.h"
#include "ui_mainwindow.h"

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

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

void MainWindow::on_pushButton_clicked()
{
    MainWindow w;
    ui->pushButton->setText("Ferma riconoscimento");
    w.cattura();
}


void MainWindow::cattura()
{
        cameraTimer.start(33); // 33 ms = 30 fps
        connect(&cameraTimer, SIGNAL(timeout()),this,SLOT(cameraTimerTimeout()));
       
       return;

}

void MainWindow::cameraTimerTimeout()
{
    std::cout << "CIAO2" << std::endl;

}

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Thank you in advance!

[ChrisW67]

Line 18 of mainwindow.cpp makes no sense. You are creating another instance of the MainWindow object and calling cattura() on that (line 20) before immediately discarding the instance (line 21).

[lifedj]

I know, but it is a simplified version of the code! In the function cattura I have to do other things! ;-)

[ChrisW67]

It is a simplified and broken piece of code that doesn't work how you expect because you are throwing away the object you are expecting to do the work. The bug is in on_pushButton_clicked() and has nothing to do with cattura().

[lifedj]

If it is so I didn't understand what you said in the first answer!
Can you explain with more details what is the problem and how can I solve it?

I thought that you were saying me that I call the function "cattura" but in it I do only 2 things! Excuse me for the misunderstanding!

---

Added after 4 minutes:

I have just understood!
I create an instance of something that is already istanced! I deleted only the line 18 and now it works fine (with substition of "cattura();" with "cattura();"!

Excuse me and thank you! ;-)

[ChrisW67]

You're welcome... everybody has done something similar at one time or another.

[d_stranz]

void MainWindow::cattura()
{
        cameraTimer.start(33); // 33 ms = 30 fps
        connect(&cameraTimer, SIGNAL(timeout()),this,SLOT(cameraTimerTimeout()));
 
       return;
 
}

void MainWindow::cattura()
{
        cameraTimer.start(33); // 33 ms = 30 fps
        connect(&cameraTimer, SIGNAL(timeout()),this,SLOT(cameraTimerTimeout()));
       
       return;

}

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Since "cameraTimer" is a member variable of MainWindow, this connect() statement should be made in the MainWindow constructor, not in this slot.

What will happen in the code above is that each time you push the capture button, you will make *another* connection to the camera timeout signal. So, the first time you push the button, the slot will be called once on timeout. The second time, it will be called twice, the third time it will be called three times, and so on. By the time you get to 10 pushes, you'll be saying "CIAO" so many times you will wish it really was leaving.