Undefined references

class Item
{
    public:
        int Price, Value;
        string Name, Description;
        virtual void Effect() =0;
};

class Medicine : public Item
{
    public:
        void Effect();
        Medicine();
        ~Medicine(){};
};

class DCD : public Item
{
    public:
        void Effect();
        DCD();
        ~DCD(){};
};

class BlueLobster : public Item
{
    public:
        void Effect();//Turn player into lobster and summon a lobster army to damage all enemies
        BlueLobster();
        ~BlueLobster(){};
};

class Inventory
{
    public:
        void AddItem();
        vector<Item*> InventoryList;
        vector<int> InventoryPointer;
        vector<int> InventoryNum;
        Inventory();
    private:
        Medicine MedicineItem;
        DCD DCDItem;
        BlueLobster BlueLobsterItem;
};

Inventory::Inventory()
{
    InventoryList.push_back(&MedicineItem);
    InventoryList.push_back(&DCDItem);
    InventoryList.push_back(&BlueLobsterItem);
}

void Inventory::AddItem()
{


    return;
}

Medicine::Medicine()
{
    Name = "Medicine";
    Description = "Heals your Daemons by 50 HP";
    Price = 100;
    Value = 75;
}

DCD::DCD()
{
    Name = "DCD";
    Description = "Daemon Capture Device/nUsed to catch Daemons";
    Price = 150;
    Value = 100;
}

BlueLobster::BlueLobster()
{
    Name = "Blue Lobster";
    Description = "Summon Netham45's Army >:D";
    Price = 500;
    Value = 50;
}