Segmentation fault(core dumped) occured after add an private member in widget class.

[Hupe]

I am building a tool with QT 5,12 to run in the arm64 board. It run well when there was only one private member. When I add more private members, it can compile successfully, but run to the segmentation fault.

QT_BEGIN_NAMESPACE
namespace Ui { class Widget; }
QT_END_NAMESPACE

class Widget : public QWidget
{
    Q_OBJECT

public:
    Widget(QWidget *parent = nullptr);
    ~Widget();
    QLCDNumber    *rebootTimeLcdNum;
    QTimer *mtimer;

private:
    Ui::Widget *ui;
    QPushButton *startButton;
    //QLCDNumber    *rebootTimeLcdNum;
    //QTimer *mtimer;
    //int a;

private slots:
    void on_startButton_clicked();
    void onTimerTimeout();
};

Widget::Widget(QWidget *parent)
    : QWidget(parent)
    , ui(new Ui::Widget)
{
    ui->setupUi(this);
    connect(startButton, SIGNAL(clicked()), this, SLOT(on_startButton_clicked()));
 } 

I use gdb to debug and find that the fault occurred when connecting the slot function on_startButton_clicked() . And the fault won't occur if I put the member to public.

Does anyone experencd the same problem?

[jsulm]

@Hupe You did not assign a valid pointer to startButton

[Hupe]

Thanks! Is there any advice on how to assign a valid pointer or some textbooks to refer to?

[JonB]

@Hupe
You don't need a textbook to refer to, you need to assign your startButton to whatever button you have which you are wanting to attach the signal/slot to!

[jsulm]

@Hupe This belongs to absolute basics of C++. Do you know how memory is allocated in C++ using "new"?

[Hupe]

I found setupUi(QWidget *Widget) in ui_widget.h has already new the button. And it works well if there is only one private member *startButton.

[JonB]

@Hupe
You need to attach the signal to the button defined in the Designer. But it sounds/looks like you are declaring some new QPushButton *startButton; in your Widget class, which is not the one from Designer? Why are you doing that?

[Hupe]

Thanks! I found the problem.