C++ Undefined Reference to vtable and inheritance

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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.

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