undefined reference to virtual table

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Hi:
 I defined a class to do some. It's not dervied from any class.
class x
{
.....
};
But with the time gone,I need  signal_slot funtion.So I change my class 
define like this
class x:public QObject
{
 Q_OBJECT
  ......
}
[:-(] ,But here have some error.When I build whole project,
Error report said:
   x.o: In function 'x::~x(void)':
    .../x.cpp:18:undefined reference to 'x virtual table'
BTW: I has add qobject.h header file in x.h and x.cpp.

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