Что не так с реализацией алгоритма generate_n для контейнера вектор?

generate_n(back_inserter(sp), size, Generator());
            for ( Iter1 = sp1.begin( ) ; Iter1 != sp1.end( ) ; Iter1++ )
                cout << "Результат генерации:\t" << *Iter1 << endl;

struct Generator 
{
    Generator():val(0) {}
    
    int operator()() 
    {
        return val++;
    }
 
private:
    int val;
};

class spisok_pokupok
{
public:
    spisok_pokupok(void);
    spisok_pokupok(string pokupka1, int kolychestvo1);
    ~spisok_pokupok(void);
    bool operator == (spisok_pokupok SP1);
    void operator = (spisok_pokupok SP1);
    friend int operator = (int i, spisok_pokupok SP1);
    friend bool operator > (spisok_pokupok SP1, spisok_pokupok SP2);
    friend bool operator < (spisok_pokupok SP1, spisok_pokupok SP2);
    friend bool operator >= (spisok_pokupok SP1, spisok_pokupok SP2);
    friend bool operator <= (spisok_pokupok SP1, spisok_pokupok SP2);
    friend spisok_pokupok operator + (spisok_pokupok SP1, spisok_pokupok SP2);
    friend int operator + (int i, spisok_pokupok SP1);
    friend ostream& operator << (ostream & out, spisok_pokupok& SP1);
    friend bool kolychestvo(spisok_pokupok& k1);
    friend bool UniqueNumber(spisok_pokupok& k1);
protected:
    string pokupka1; //покупка
    int kolychestvo1; //количество
};