undefined reference to vtable error

[PaulDaviesC]

Here is a simple Qt code I have written to study signals and slots concept :-

#include<QObject>
class A : public QObject{
        Q_OBJECT
        public:
                A(){val=0;}
        public slots:
                void changeval(int n){
                        val=n;
                        emit changed(val);
                }
        signals:
                void changed(int n);
        private:
                int val;
};
int main()
{
        A l,m;
}

#include<QObject>
class A : public QObject{
        Q_OBJECT
        public:
                A(){val=0;}
        public slots:
                void changeval(int n){
                        val=n;
                        emit changed(val);
                }
        signals:
                void changed(int n);
        private:
                int val;
};
int main()
{
        A l,m;
}

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I followed the following to "make" it
1.qmake -project
2.qmake
3.make
And at the last phase, I got an error as :

communicate.o: In function `main':
communicate.cpp:(.text+0x2d): undefined reference to `vtable for A'
communicate.cpp:(.text+0x4d): undefined reference to `vtable for A'
communicate.cpp:(.text+0x68): undefined reference to `vtable for A'
communicate.cpp:(.text+0x8d): undefined reference to `vtable for A'
collect2: ld returned 1 exit status
make: *** [firstapp] Error 1

communicate.o: In function `main':
communicate.cpp:(.text+0x2d): undefined reference to `vtable for A'
communicate.cpp:(.text+0x4d): undefined reference to `vtable for A'
communicate.cpp:(.text+0x68): undefined reference to `vtable for A'
communicate.cpp:(.text+0x8d): undefined reference to `vtable for A'
collect2: ld returned 1 exit status
make: *** [firstapp] Error 1

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I don't have no idea how does this happen. Can you please help me out?