Undefined reference to a superclass' virtual function

Question

Well I'm facing this problem when my subclass doesn't use a virtual function from superclass.. How to solve it? Thanks in advance for help!

this is my superclass Q header file

#ifndef Q_HPP
#define Q_HPP

#include <iostream>

using namespace std;

class Q
{
protected:
    int w;
public :
    virtual void setW ( int w );
    virtual void display() = 0;
};

#endif

this is my superclass Q source file

#include "Q.hpp"

void Q::setW ( int w )
{
    this-> w = w;
}

this is my subclass R header file

#ifndef R_HPP
#define R_HPP

#include "Q.hpp"

class R : public Q
{
public:
    void display() override;
};

#endif 

this is my subclass R source file

#include "Q.hpp"
#include "R.hpp"

void R::display()
{
    cout << w << endl;
}

error during R.cpp compilation: undefined reference to `Q::setW(int)'