Undefined reference to vtable (Inheritance)

Question

I have problem with inheriting from abstract class.

Abstract class - header file:

    #ifndef CLIENT_H
    #define CLIENT_H
    #include "country.h"
    #include "currency.h"
    #include "item.h"
    #include "order.h"
    #include <string>
    #include <vector>

    using namespace std;

    class Order;

    class Client {
        string first_name;
        string last_name;
        int account_balance;
        Country country;
        Currency currency;
        vector <Order> orders;

    public:
        Client (string first_name, string last_name, Country country, Currency currency);
        void buy_item (Item item, unsigned quantity, unsigned order_ID);
        void add_order (unsigned ID);
        virtual void pay (unsigned order_ID) = 0; //
    };

    #endif // CLIENT_H

Abstract class - .cpp file:

   #include "client.h"

   Client::Client (string first_name, string last_name, Country country, Currency currency)
       : first_name(first_name), last_name(last_name), country(country), currency(currency)
   {
   account_balance = 0;
   }

Inheriting class - header file:

    #ifndef ENGLISHCLIENT_H
    #define ENGLISHCLIENT_H
    #include "client.h"
    #include <string>

    using namespace std;

    class EnglishClient : public Client {
    public:
        EnglishClient (string first_name, string last_name);
        void pay (unsigned order_ID);
    };

    #endif // ENGLISHCLIENT_H

Inheriting class - .cpp file:

    #include "englishclient.h"

    EnglishClient::EnglishClient (string first_name, string last_name)
        : Client(first_name, last_name, GB, GBP)
    {
    }

And finally the error:

GB and GBP are enum variables:

enum Country {GB, PL};

enum Currency {GBP, PLN};

Answer 1

您忘记了方法Client::buy_item ， Client::add_order和EnglishClient::pay

Answer 2

You never defined EnglishClient::pay , Client::buy_item or Client::add_order . For client, either define them or set them as abstract: void buy_item (Item item, unsigned quantity, unsigned order_ID)=0; (same for add_order)` and define them in subclass.