I'm trying to implement an exit button, but I'm unable to use `connect()` method to do this. Actually my button does not emit any signal as far as I know, but I can't find the problem.
Here is part of my code that uses `connect()` method:

```
#include "MyMainWindow.h"

MyMainWindow::MyMainWindow(QWidget * parent, Qt::WindowFlags flag) :
        QMainWindow(parent, flag)
{
    this->setFixedSize(1120, 630);
    menu = new MyMenu(this);
    this->setCentralWidget(menu);
    this->show();
    // the connect implementation
    connect(menu->exit, SIGNAL(clicked()), this, SLOT(this->exit_button_clicked()));
}

MyMainWindow::~MyMainWindow()
{
}

void MyMainWindow::exit_button_clicked()
{
    this->close();
}

```

`MyMainWindow` is a friend class of `MyMenu` and `exit` is a private `QPushButton`. Now I want some help for solving this problem.