public timer sometimes crashes at runtime

I'm building a timer with an interval that can adjusted by a slider. Here's a shot of my ui:

Attachment 11739

The upper lcdNumber counts up by one every second. I want to use the slider to speed it up and slow it down. Here is my header file

Code:

---

#ifndef MAINWINDOW_H
#define MAINWINDOW_H
 
#include <QMainWindow>
 
namespace Ui {
class MainWindow;
}
 
class MainWindow : public QMainWindow
{
    Q_OBJECT
 
public:
    explicit MainWindow(QWidget *parent = 0);
    ~MainWindow();
    QTimer *timer;  // here the timer is made public
 
private:
    Ui::MainWindow *ui;
 
private slots:
    void inkup();
    void on_horizontalSlider_valueChanged(int value);
};
 
#endif // MAINWINDOW_H

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And here is my application

Code:

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#include "mainwindow.h"
#include "ui_mainwindow.h"
#include "qapplication.h"
#include <QtWidgets>
 
MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);
 
    QTimer *timer = new QTimer(this);
    connect(timer, SIGNAL(timeout()), this, SLOT(inkup()));
    timer->start(1000);
}
 
void MainWindow::inkup() {
int eks = ui->lcdNumber->value();
    eks += 1;
    ui->lcdNumber->display(eks);
}
 
MainWindow::~MainWindow()
{
    delete ui;
}
 
void MainWindow::on_horizontalSlider_valueChanged(int speed)
{
    ui->lcdNumber_2->display(timer->remainingTime());
 
    ui->lcdNumber_2->display(speed);
 
    timer->setInterval(speed);  // this causes a crash at run time
}

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In the horizontal slider event handler, I can display in the second lcdNumber the timer property remainingTime(). No problem. I can display the changed slider value so I know it's okay. But when I try and set the timer interval I get a run time crash. In fact most of the properties that pop up for the timer when I type timer-> also cause a crash. What am I doing wrong?

Quote:

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Originally Posted by twoten

But when I try and set the timer interval I get a run time crash. In fact most of the properties that pop up for the timer when I type timer-> also cause a crash. What am I doing wrong?

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Because you are accessing an uninitialized pointer.
Your member variable "timer" never gets initialized, so it points to some random memory address.

Your constructor initializes a local variable with the same name, so you probably didn't want this to be a local variable.

Cheers,
_

Can you give me an example of the correct way to do this?

Initialize the timer before the initialization ui.

Quote:

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Originally Posted by twoten

Can you give me an example of the correct way to do this?

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Instead of

Code:

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QTimer *timer = new QTimer(this);

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you write

Code:

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timer = new QTimer(this);

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Cheers,
_

Fantastic! Not only did it work but I also understand why! Thank you very much!