Undefined reference to vtable ptr at constructor

I have three classes names A,B,C. Class A inherits from class B and which inherits from class C. Class A doesn't have any virtual functions. Class B has virtual function and defined with empty definition. Class C has virtual function and defined with empty definition. Though virtual functions are defined not sure the reasons undefined reference to vtable ptr from class constructor A... Please help me to resolve this issue...

Virtual functions are deifned...