скомпилировать отдельный файл Qt из командной строки: неопределенная ссылка на vtable

Я хотел бы скомпилировать однокомпонентное приложение QT из командной строки, чтобы быстро проверить некоторые функции. Смотрите код файла ниже.

Я собираю с:

qmake -project && qmake && make

И я получаю эту ошибку:

Togglebutton.o:Togglebutton.cpp:function ButtonDialog::ButtonDialog(QWidget*): error: undefined reference to 'vtable for ButtonDialog'
Togglebutton.o:Togglebutton.cpp:function ButtonDialog::ButtonDialog(QWidget*): error: undefined reference to 'vtable for ButtonDialog'
Togglebutton.o:Togglebutton.cpp:function ButtonDialog::~ButtonDialog(): error: undefined reference to 'vtable for ButtonDialog'
Togglebutton.o:Togglebutton.cpp:function ButtonDialog::~ButtonDialog(): error: undefined reference to 'vtable for ButtonDialog'

Я не гуру C ++, я пытался гуглить по неопределенной ссылке на vtable, но я не понимаю, почему я получаю эту ошибку также с одним файлом ..

Кто-нибудь может помочь мне понять неопределенную ошибку vtable?

/*
* Copyright (c) 2006-2007, Johan Thelin
*
* All rights reserved.
*
* Redistribution and use in source and binary forms, with or without modification,
* are permitted provided that the following conditions are met:
*
*     * Redistributions of source code must retain the above copyright notice,
*       this list of conditions and the following disclaimer.
*     * Redistributions in binary form must reproduce the above copyright notice,
*       this list of conditions and the following disclaimer in the documentation
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*       may be used to endorse or promote products derived from this software
*       without specific prior written permission.
*
* THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
* "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
* LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
* A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR
* CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
* EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
* PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
* PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
* LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
* NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
* SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
*
*/

#include <QPushButton>
#include <QHBoxLayout>
#include <QMessageBox>

#include <QApplication>
#include <QDialog>
class QPushButton;

class ButtonDialog : public QDialog
{
Q_OBJECT

public:
ButtonDialog( QWidget *parent=0 );

private slots:
void buttonClicked();
void buttonToggled();

private:
QPushButton *clickButton;
QPushButton *toggleButton;
};

ButtonDialog::ButtonDialog( QWidget *parent ) : QDialog( parent )
{
clickButton = new QPushButton( "Click me!", this );
toggleButton = new QPushButton( "Toggle me!", this );
toggleButton->setCheckable( true );

QHBoxLayout *layout = new QHBoxLayout( this );
layout->addWidget( clickButton );
layout->addWidget( toggleButton );

connect( clickButton, SIGNAL(clicked()), this, SLOT(buttonClicked()) );
connect( toggleButton, SIGNAL(clicked()), this, SLOT(buttonToggled()) );
}

void ButtonDialog::buttonClicked()
{
QMessageBox::information( this, "Clicked!", "The button was clicked!" );
}

void ButtonDialog::buttonToggled()
{
QMessageBox::information( this, "Toggled!", QString("The button is %1!").arg(toggleButton->isChecked()?"pressed":"released") );
}int main( int argc, char **argv )
{
QApplication app( argc, argv );

ButtonDialog dlg;
dlg.show();

return app.exec();
}