C++ unresolved external symbol

Question

I have a class setup and from that class I am using inheritance.

In file a.h

class a
{
public:
    virtual void print();
};

In file b.h:

#include "a.h"
#include <iostream>
class b: public a
{
public:
    void print();
};

And in b.cpp

#include "a.h"
#include "b.h"
void b::print(){};

In the main file I am including both of these files:

#include "a.h"
#include "b.h"

Yet I get an unresolved symbol for the virtual function print. The file a.obj is listed as the file generating the error What am I doing wrong? If I move b.cpp into b.h below the class definition it works fine.

Answer 1

You have an implementation for b::print but not for a::print. What would happen if instantiated an object of class a and called print() on it? i.e.

a o;
o.print();

b::print overrides a::print but you still need to have an implementation of a::print (unless you make it pure virtual).

to make print pure virtual in a, define it like this:

virtual void print() = 0;

When a class has pure virtual functions, you cannot instantiate objects of that class. You must derive from that class and provide an implementation of any pure virtual functions before you have a class that can actually be instantiated.