QLabel text doesn't update after i changed it when QMainWindow is shown

Question

I've set a placeholder in a QLabel to "user".

After ui->setupUi(this);, QLabel gets changed via ui->label->setText("superuser");. No changes are made afterwards.

As soon as QMainWindow is initialized and shown via mw.show(); in main.cpp file , QLabel is set to the placeholder "user" again.

I tried:

• qDebug()<<ui->label->text(); before and after ui->label->setText("superuser"); and placed one right at the end of constructor of QMainWindow. Results:

  user
  superuser
  superuser
  

• Checked mainwindow.ui in nano, all is right.
• Checked connections: there are no connections that are linked to the function that changes QLabel.
• Checked if mw->show(); is triggered multiple times in main.cpp, one call.
• No external thread changes the QLabel outside from QMainWindow.

Here's a minimal reproducible example:

mainwindow.cpp:

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

using namespace std;

MainWindow::MainWindow(QWidget *parent)
    : QMainWindow(parent)
    , ui(new Ui::MainWindow)
{
    ui->setupUi(this);
    this->init();
    qDebug() << "afterInit" << ui->label->text();
}

void MainWindow::init()
{
    this->set_options("SuperUser");
}

void MainWindow::set_options(QString s)
{
    debugInt++;
    qDebug() << debugInt;
    qDebug() << ui->label->text();
    ui->label->setText(s);
    qDebug() << ui->label->text();
}

void MainWindow::on_actionLogout_triggered()
{
    this->set_options("User");
}

mainwindow.h:

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QtWidgets/QMainWindow>
#include <QDebug>

QT_BEGIN_NAMESPACE
namespace Ui { class MainWindow; }
QT_END_NAMESPACE

class MainWindow : public QMainWindow
{
    Q_OBJECT

public slots:
    void set_options(QString s);

public:
    MainWindow(QWidget *parent = nullptr);
    ~MainWindow();

private:
    void init();
    Ui::MainWindow *ui;

    int debugInt=0;

private slots:
    void on_actionLogout_triggered();
};
#endif //MAINWINDOW_H

main.cpp:

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

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);

    a.setQuitOnLastWindowClosed(true);

    MainWindow w;

    w.show();

    return a.exec();
}

ui->label is set to "Current: user" in mainwindow.ui. That's the standard set text that I call placeholder.

mainwindow.ui:

<property name="text">
           <string>Current: user</string>

My QDebug output when I start the program and then press logout:

1
"Current: user"
"Current: SuperUser"
"afterInit "Current: SuperUser"
2
"Current: user"
"Current: User"

As you can see, by default ui->label is set to "Current: user". In the init process it gets overwritten to "Current: SuperUser". But when I start the program, it is shown as "Current: user". As soon as I click the actionLogout (menubarbutton in mainwindow_ui), ui->label is set to "Current: User" and gets shown right.

Note: I created a new program with the given code. Works as it should, ui->label isn't changed in any other function in my code.

Answer 1

So I´ve found the error. I have overwritten MainWindow::changeEvent(QEvent* event) method:

mainwindow.cpp

void MainWindow::changeEvent(QEvent* event)
{
  if(0 == event) return;

  switch(event->type())
  {
  case QEvent::LanguageChange: ui->retranslateUi(this); break;

  case QEvent::LocaleChange: break;

  default: break;
  }

  QMainWindow::changeEvent(event);

  }

Fix:

I add ui->label->setText( tr("Current: ") + currentUserString ); before QMainWindow::changeEvent(event) because the QLabel gets changed when MainWindow::changeEvent(QEvent* event) is executed to the set string in .qm/.ts translation file. Thats why mainwindow showed me "Current: user" (from translation file) instead of "Current: SuperUser" (changed in init function)

new mainwindow.cpp

void MainWindow::changeEvent(QEvent* event)
{
  if(0 == event) return;

  switch(event->type())
  {
  case QEvent::LanguageChange: ui->retranslateUi(this); break;

  case QEvent::LocaleChange: break;

  default: break;
  }

  if(uiIsSetUp) //bool that get set true after `ui->setupUi(this);`
    ui->label->setText( tr("Current: ") + currentUserString );

  QMainWindow::changeEvent(event);

  }