How to implement a virtual function in derived class?

Question

Consider the following UML:

When I try to implement the virtual function rate() in Single.cpp, I get an error: cannot allocate an object of abstract type 'Single'
Booking* b = new Single();

The addBooking() function should make a pointer of Single of type Booking, add the pointer to the QList (BookingList derives from QList<Booking*>) and return the Booking pointer.

When I comment out the virtual function then everything works. Why is this occurring, and how can I fix it?

Below is a minimal version of my program:

#ifndef BOOKING_H
#define BOOKING_H

class Booking
{
public:
    Booking();
    double SINGLE_PPPN = 200.00;
    virtual double rate() = 0;
};

#endif // BOOKING_H

#include "booking.h"

Booking::Booking()
{
}

#ifndef BOOKINGLIST_H
#define BOOKINGLIST_H
#include <QList>
#include <iostream>
#include "booking.h"
#include "single.h"

using namespace std;

class BookingList : public QList<Booking*>
{
public:
    BookingList();
    Booking* addBooking();
    void deleteAll();
};

#endif // BOOKINGLIST_H

#include "bookinglist.h"

BookingList::BookingList()
{
}

Booking* BookingList::addBooking()
{
    Booking* b = new Single();

    this->append(b);

    cout << "Total bookings: " << this->size() << "\n\n" <<endl;

    return b;
}

void BookingList::deleteAll()
{
    for (int i = 0; i < this->count(); i++)
    {
        cout << "Deleting Item At: " << i << endl;
        delete this->at(i);
    }
}

#ifndef SINGLE_H
#define SINGLE_H
#include "booking.h"

class Single : public Booking
{
public:
    Single();
};

#endif // SINGLE_H

#include "single.h"

Single::Single()
{
}

double Single::rate()
{
    return Booking::SINGLE_PPPN;
}

#include <QCoreApplication>

int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);

    return a.exec();
}

Answer 1

You need to declare your method override in the class definition

class Single : public Booking
{
public:
    Single();
    double rate() override;  // this line
};

then you can actually implement the method in your cpp file as you did

double Single::rate()
{
    return Booking::SINGLE_PPPN;
}