c++ - undefined reference to `vtable for MyCalss'

I've created new project with QTCreator using Empty Qt Project

MocTest.pro

QT += core
QT -= gui

TARGET = MocTest

CONFIG   += console

TEMPLATE = app


SOURCES += \
    main.cpp

main.cpp

#include 
#include 

class A : public QObject
{
    Q_OBJECT

public:
    A() {}


public slots:
    void slotA() {std::cout<<"Hi!";}
};

class B : public QObject
{
    Q_OBJECT

public:

    B () {}

    void doB() {emit ss();}

signals:
    void ss();
};

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

    QCoreApplication app (argc, argv);
    A a;
    B b;
    return app.exec();
}

I got errors

D:\Test\MocTest\debug\main.o:-1: In function `ZN1AC1Ev':

D:\Test\MocTest\main.cpp:9: error: undefined reference to `vtable for A'

D:\Test\MocTest\main.cpp:20: error: undefined reference to `vtable for B'

D:\Test\MocTest\main.cpp:4: error: undefined reference to `vtable for A'

D:\Test\MocTest\main.cpp:15: error: undefined reference to `vtable for B'

collect2.exe:-1: error: error: ld returned 1 exit status