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

Answer 1

Make sure to delete any "*.gch" files if none of the other responses help you.

Answer 2

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()
{
}

---

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;

---

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 3

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;
};