menubar Hide/remove

[high_flyer]

I can't reproduce your problem.
I don't see any menu bar if I remove the menu bar in designer it doesn't get showed in my window.
Here is the generate ui_*.h file as you can see no menu bar is added.
So probably there is not correlation between what you think you do, and what you really do.
Did you remember to save your ui changes?

class Ui_MainWindow
{
public:
    QWidget *centralwidget;
    QStatusBar *statusbar;
    QToolBar *toolBar;
 
    void setupUi(QMainWindow *MainWindow)
    {
        if (MainWindow->objectName().isEmpty())
            MainWindow->setObjectName(QString::fromUtf8("MainWindow"));
        MainWindow->resize(800, 600);
        centralwidget = new QWidget(MainWindow);
        centralwidget->setObjectName(QString::fromUtf8("centralwidget"));
        MainWindow->setCentralWidget(centralwidget);
        statusbar = new QStatusBar(MainWindow);
        statusbar->setObjectName(QString::fromUtf8("statusbar"));
        MainWindow->setStatusBar(statusbar);
        toolBar = new QToolBar(MainWindow);
        toolBar->setObjectName(QString::fromUtf8("toolBar"));
        MainWindow->addToolBar(Qt::TopToolBarArea, toolBar);
 
        retranslateUi(MainWindow);
 
        QMetaObject::connectSlotsByName(MainWindow);
    } // setupUi
 
    void retranslateUi(QMainWindow *MainWindow)
    {
        MainWindow->setWindowTitle(QApplication::translate("MainWindow", "MainWindow", 0, QApplication::UnicodeUTF8));
        toolBar->setWindowTitle(QApplication::translate("MainWindow", "toolBar", 0, QApplication::UnicodeUTF8));
    } // retranslateUi
 
};

class Ui_MainWindow
{
public:
    QWidget *centralwidget;
    QStatusBar *statusbar;
    QToolBar *toolBar;

    void setupUi(QMainWindow *MainWindow)
    {
        if (MainWindow->objectName().isEmpty())
            MainWindow->setObjectName(QString::fromUtf8("MainWindow"));
        MainWindow->resize(800, 600);
        centralwidget = new QWidget(MainWindow);
        centralwidget->setObjectName(QString::fromUtf8("centralwidget"));
        MainWindow->setCentralWidget(centralwidget);
        statusbar = new QStatusBar(MainWindow);
        statusbar->setObjectName(QString::fromUtf8("statusbar"));
        MainWindow->setStatusBar(statusbar);
        toolBar = new QToolBar(MainWindow);
        toolBar->setObjectName(QString::fromUtf8("toolBar"));
        MainWindow->addToolBar(Qt::TopToolBarArea, toolBar);

        retranslateUi(MainWindow);

        QMetaObject::connectSlotsByName(MainWindow);
    } // setupUi

    void retranslateUi(QMainWindow *MainWindow)
    {
        MainWindow->setWindowTitle(QApplication::translate("MainWindow", "MainWindow", 0, QApplication::UnicodeUTF8));
        toolBar->setWindowTitle(QApplication::translate("MainWindow", "toolBar", 0, QApplication::UnicodeUTF8));
    } // retranslateUi

};

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[mstegehu]

In the end this is very poor documented and very easy to accomplish:

Create menu:

// Add help menu
    m_helpMenu = m_mainWindow->menuBar ()->addMenu (Tr ("&Help"));
 
    m_helpMenu->setObjectName ("HelpMenu");
    m_helpMenu->setWindowTitle (Tr ("Help Menu"));
 
    m_helpMenu->addAction (m_userManualAction);
    m_helpMenu->addAction (m_quickStartManualAction);
    m_helpMenu->addAction (m_aboutBoxAction);

// Add help menu
    m_helpMenu = m_mainWindow->menuBar ()->addMenu (Tr ("&Help"));

    m_helpMenu->setObjectName ("HelpMenu");
    m_helpMenu->setWindowTitle (Tr ("Help Menu"));

    m_helpMenu->addAction (m_userManualAction);
    m_helpMenu->addAction (m_quickStartManualAction);
    m_helpMenu->addAction (m_aboutBoxAction);

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Remove menu without destroy:

m_mainWindow->menuBar ()->removeAction (m_helpMenu->menuAction ());

m_mainWindow->menuBar ()->removeAction (m_helpMenu->menuAction ());

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