C++ Undefined Reference to vtable and inheritance

Question

File A.h

#ifndef A_H_ #define A_H_  class A { public:     virtual ~A();     virtual void doWork(); };  #endif 

File Child.h

#ifndef CHILD_H_ #define CHILD_H_  #include "A.h"  class Child: public A { private:     int x,y; public:     Child();     ~Child();     void doWork(); }; #endif 

And Child.cpp

#include "Child.h"  Child::Child(){     x = 5; }  Child::~Child(){...}  void Child::doWork(){...}; 

The compiler says that there is a undefined reference to vtable for A. I have tried lots of different things and yet none have worked.

My objective is for class A to be an Interface, and to seperate implementation code from headers.

Answer 1

Why the error & how to resolve it?

You need to provide definitions for all virtual functions in class A. Only pure virtual functions are allowed to have no definitions.

i.e: In class A both the methods:

virtual ~A(); virtual void doWork(); 

should be defined(should have a body)

e.g.:

A.cpp

void A::doWork() { } A::~A() { } 

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Caveat:
If you want your class A to act as an interface(a.k.a Abstract class in C++) then you should make the method pure virtual.

virtual void doWork() = 0; 

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Good Read:

What does it mean that the "virtual table" is an unresolved external?
When building C++, the linker says my constructors, destructors or virtual tables are undefined.

Answer 2

My objective is for A to be an Interface, and to seperate implementation code from headers.

In that case, make the member function as pure virtual in class A.

class A {   // ...   virtual void doWork() = 0; };