how to deal this "undefined reference to "vtable f

[maxmusy]

I have compiled these codes for A1200
But it crash , and show me these errors

undefined reference to "vtable for demo" in function Demo:emo(...)
undefined reference to "vtable for demo" in function Demo:emo(...)
undefined reference to "vtable for demo" in function Demo::~Demo(...)
undefined reference to "vtable for demo" in function Demo::~Demo(...)
undefined reference to "vtable for demo" in function Demo::~Demo(...)
undefined reference to "vtable for demo" in function Demo::~Demo(...)

and in mm.rar is my source code.

I used EZX , based on "A1200 EZX.doc"