Qt - double label->setText(); crashes app

[shadow parrot]

Hello, I'm trying to create a switch using QPushButton but double on_off_label->setText(); crashes it (if I delete one it's fine)

[forest zephyr]

When your question is answered use !solved to mark the question as resolved.

Remember to ask specific questions, provide necessary details, and reduce your question to its simplest form. For tips on how to ask a good question use !howto ask.

[shadow parrot]

code:

void Switch::updateStyle() {
    if (isChecked()) {
        // ON
        setIcon(QIcon("D://images/switch_ON.png"));
        setIconSize(QSize(width(), height()));
        on_off_label->setText("ON");
    } else {
        // OFF
        setIcon(QIcon("D:/images/switch_OFF.png"));
        setIconSize(QSize(width(), height()));
        on_off_label->setText("OFF");
    }
}

!code

[forest zephyr]

How to Format Code on Discord

Markup

```cpp
int main() {}
```

Result

int main() {}

Note: Back-tick (`) not quotes (')

[mild fiber]

Hard to know with details provided. What is the nature of the "crash"? Is it possible that Switch::updateStyle is called on a deleted object or on_off_label has not been initialized properly or deleted?

Make sure on_off_label is valid before using:

if (on_off_label) {
    on_off_label->setText(isChecked() ? "ON" : "OFF");
} else {
    qDebug() << "on_off_label is null!";
}

Make sure on_off_switch is properly initialized in the constructor:

Switch::Switch(QWidget *parent) : QPushButton(parent) {
    on_off_label = new QLabel(this);
}

Check this pointer inside updateStyle():

qDebug() << "this pointer:" << this;

[shadow parrot]

by "crash" I mean the app closes itself without visible error. The Switch class is in a header file and I'm not entirely sure if it's written properly.

Hearder file:

#ifndef SWITCH_H
#define SWITCH_H

#include <QPushButton>
#include <QLabel>

class Switch : public QPushButton {
    Q_OBJECT

public:
    explicit Switch(int width = 120, int height = 60, QWidget *parent = nullptr);

private slots:
    void updateStyle();
    void mousePressEvent(QMouseEvent *event) override;
    void mouseReleaseEvent(QMouseEvent *event) override;

private:
    QLabel* on_off_label;
};

#endif // SWITCH_H

[lyric sail]

Source file?

[shadow parrot]

cpp?

[lyric sail]

yes

[shadow parrot]

Switch::Switch(const int width, const int height, QWidget *parent) : QPushButton(parent) {
    setCheckable(true);
    setFixedSize(width, height);
    updateStyle();

    setStyleSheet("QPushButton { background: none; border: none; }"
                  "QPushButton:hovered { background: none; border: none; }");

    connect(this, &QPushButton::toggled, this, &Switch::updateStyle);
    on_off_label = new QLabel("OFF", this);
    on_off_label->setStyleSheet("color: #efefef");

}

void Switch::updateStyle() {
    if (isChecked()) {
        // ON
        setIcon(QIcon("D:/images/switch_ON.png"));
        setIconSize(QSize(width(), height()));
        on_off_label->setText("ON");
    } else {
        // OFF
        setIcon(QIcon("D:/images/switch_OFF.png"));
        setIconSize(QSize(width(), height()));
        on_off_label->setText("OFF");
    }
}


void Switch::mousePressEvent(QMouseEvent *event) {
    QPushButton::mousePressEvent(event);  // doesnt work yet
}

void Switch::mouseReleaseEvent(QMouseEvent *event) {
    QPushButton::mouseReleaseEvent(event);  // doesnt work yet
}

[lyric sail]

In the constructor, you're calling updateStyle(); before the line

    on_off_label = new QLabel("OFF", this);

[shadow parrot]

setStyleSheet is is related to button

but label creates problems

[lyric sail]

?

[shadow parrot]

the problem is in on_off_label->setText("ON"); line

or on_off_label->setText("OFF");

[lyric sail]

    updateStyle();                                         // Here, the function uses `on_off_label` uninitialized

    setStyleSheet("QPushButton { background: none; border: none; }"
                  "QPushButton:hovered { background: none; border: none; }");

    connect(this, &QPushButton::toggled, this, &Switch::updateStyle);
    on_off_label = new QLabel("OFF", this);                // `on_off_label` initialized here

[shadow parrot]

oh

should it be after?

or better to delete it

[lyric sail]

The initialization should come first before the use, yes.

[shadow parrot]

but still error occurs in with setText

when I delete one of on_off_label->setText() lines program runs normally

[lyric sail]

Did you move the initialization?

[shadow parrot]

to where?

[lyric sail]

Before its use

[shadow parrot]

but i initializatize on_off_label in header file

[lyric sail]

If you're using the pointer on_off_label before initialization, that's Undefined Behaviour.

No, that's the initialization:

on_off_label = new QLabel("OFF", this);

[shadow parrot]

If I comment on_off_label->setText("OFF"); it works fine so I dont thisk that's the point

[lyric sail]

It's just the declaration in the header.

[shadow parrot]

my bad

[shadow parrot]

I'm pretty sure Switch::Switch must run before other Switch class functions

[lyric sail]

This is your constructor ^

Just put the last line before the first

[shadow parrot]

and it should crash right?

[lyric sail]

Do you understand what a pointer is?

[shadow parrot]

you said to move cpp on_off_label->setStyleSheet("color: #efefef"); before its initialization

so it should crash

am I wrong?

[lyric sail]

From MY code

[shadow parrot]

ok

THANKS

finally it worked

[lyric sail]

👌 Btw, these are not slots and should be protected: instead:

    void mousePressEvent(QMouseEvent *event) override;
    void mouseReleaseEvent(QMouseEvent *event) override;

[forest zephyr]

@shadow parrot Has your question been resolved? If so, type !solved :)

[shadow parrot]

ok thx

!solved