menubar problem, help me!!

I created a small application and subclass QMenubar, this class has a layout with some buttons, textedit,... In mainwindow class, I used setMenubar(..) to replace the system menubar by one.

Here is code:

// MainWindow is main class and called in main.cpp. This class contains toolbar, statusbar

Code:

void MainWindow::createMenus()
{
//....
 
                setMenuBar(new MenuBar(this))
}

and here is constructor of MenuBar class:

Code:

MenuBar::MenuBar(QWidget* parent):QMenuBar(parent)
{
 
    gridLayout = new QGridLayout(this);
    gridLayout->setObjectName(QString::fromUtf8("gridLayout"));
 
    hboxLayout = new QHBoxLayout();
    hboxLayout->setObjectName(QString::fromUtf8("hboxLayout"));
 
    toolButton = new QToolButton(this);
    toolButton->setObjectName(QString::fromUtf8("toolButton"));
 
    hboxLayout->addWidget(toolButton);
 
    pushButton_2 = new QPushButton(this);
    pushButton_2->setObjectName(QString::fromUtf8("pushButton_2"));
 
    hboxLayout->addWidget(pushButton_2);
 
    pushButton_3 = new QPushButton(this);
    pushButton_3->setObjectName(QString::fromUtf8("pushButton_3"));
 
    hboxLayout->addWidget(pushButton_3);
 
    spacerItem = new QSpacerItem(40, 20, QSizePolicy::Expanding, QSizePolicy::Minimum);
 
    hboxLayout->addItem(spacerItem);
 
    lineEdit = new QLineEdit(this);
    lineEdit->setObjectName(QString::fromUtf8("lineEdit"));
 
    hboxLayout->addWidget(lineEdit);
 
 
   setFixedHeight(50);
 
    gridLayout->addLayout(hboxLayout, 0, 0, 1, 1);
   this->setLayout(gridLayout);
   this->show();
 
}

The problem is that: the toolbar of mainwindow is always above the subclass Menubar, hide the area of Menubar. I want the geometry of the toolbar is below the subclass Menubar.
Any help? thanks!