compiler errors - C++ undefined reference to vtable on compile

Why do I keep getting an error on compile with this code?

#ifndef OPERATOR_H
#define OPERATOR_H
#include 
#include 
#include 
#include "Individual.h"

using namespace std;

class Operator

{
public:
Operator();

virtual void execute (Individual* parent);

private:

};
#endif

Then in the cpp file I have

#include 
#include 
#include 
#include "Operator.h"

using namespace std;


Operator::Operator()
{  

}

void execute(Individual* parent)
{    

}