Листа од цели броеви

#include<iostream>
#include<cstring>
#include<cmath>
#include <cstdlib>
using namespace std;
class List {
    int *niza;
    int number;
    void kopi(const List &l) {
        this->number=l.number;
        this->niza=new int[number];
        for(int i=0; i<number; i++) {
            this->niza[i]=l.niza[i];
        }
    }
public:
    List() {
        this->number=0;
        this->niza=new int[number];
        for(int i=0; i<number; i++) {
            this->niza[i]=niza[i];
        }
    }
    List(const List &l) {
        kopi(l);
    }
    List &operator=(const List &l) {
        if(this!=&l) {
            delete [] niza;
            kopi(l);
        }
        return *this;
    }
    ~List() {
        delete [] niza;
    }
    int sum() const {
        int suma=0;
        for(int i=0; i<number; i++) {
            suma+=this->niza[i];
        }
        return suma;
    }
    double average() const {
        return (float)sum()/number;
    }
    bool operator==(List &l) {
        return this->sum()==l.sum();
    }
    friend ostream &operator<<(ostream &out, const List &l) {
        out<<l.number<<": ";
        for(int i=0; i<l.number; i++) {
            out<<l.niza[i]<<" ";
        }
        out<<"sum: "<<l.sum()<<" average: "<<l.average()<<endl;
        return out;
    }
    friend istream &operator>>(istream &in,List &l) {
        in>>l.number;
        int *newlist = new int[l.number];
        l.niza=newlist;
        for(int i=0; i<l.number; i++) {
            in>>l.niza[i];
        }
        return in;
    }
    friend List operator+(const List &l1,const List &l2) {
        List l3(l1);
        if(l1.number==l2.number) {
            for(int i=0; i<l1.number; i++)
                l3.niza[i]+=l2.niza[i];
            return l3;
        } else {
            cout<<"The list is empty";
            exit(-1);
        }
    }
    friend List operator-(const List &l1,const List &l2) {
        List l3(l1);
        if(l1.number==l2.number) {
            for(int i=0; i<l1.number; i++)
                l3.niza[i]-=l2.niza[i];
            return l3;
        } else {
            cout<<"The list is empty";
            exit(-1);
        }
    }

    //operator ++ prefix
    List& operator++ () {
        for(int i=0; i<number; i++) {
            niza[i]++;
        }
        return *this;
    }
    //operator -- postfix
    List operator--(int) {
        List l(*this);
        for(int i=0; i<number; i++) {
            niza[i]--;
        }
        return l;
    }
    List &operator+=(int num) {
        int counter=0;
        for(int i=0; i<number; i++) {
            if(niza[i]==num)
                counter++;
        }
        int *tmp = new int[number+1];
        if(counter<2) {
            for(int i=0; i<number; i++) {
                tmp[i]=niza[i];
            }
            delete [] niza;
            tmp[number++]=num;
            niza=tmp;
        }
        return *this;
    }
    int &operator[] (int i) {
        if(i>=0&&i<number)
            return niza[i];
        else
            exit(-1);
    }
    List addList(List &l) {
        List tmp;
        for(int i=0; i<number; i++) {
            tmp+=niza[i];
        }
        for(int i=0; i<l.number; i++) {
            tmp+=l.niza[i];
        }
        return tmp;
    }
};
int main() {

    int testCase;
    cin>>testCase;

    if (testCase==1) {
        cout<<"Testiranje na operatorite << i >>"<<endl;
        List l;
        cin >> l;
        cout<< l;
    } else if (testCase == 2) {
        cout<<"Testiranje na operatorot = za klasata List"<<endl;
        List l1;
        cin >> l1;
        List l2;
        l2 = l1;
        cout << l2;
    } else if (testCase == 3) {
        //2 test primeri tuka
        cout << "Testiranje na operatorot +"<<endl;
        List l1,l2;
        cin >> l1;
        cin >> l2;
        List l3;
        l3 = l1+l2;
        cout << l3;
    } else if (testCase == 4) {

        cout << "Testiranje na operatorot - "<<endl;
        List l1,l2;
        cin >> l1;
        cin >> l2;
        List l3;
        l3 = l1-l2;
        cout << l3;
    } else if (testCase == 5) {
        cout << "Testiranje na operator ++ vo prefix notacija"<<endl;
        List l1;
        cin >> l1;
        cout << l1;
        cout << ++l1;
    } else if (testCase == 6) {
        cout << "Testiranje na operatorot -- vo sufix notacija"<<endl;
        List l1;
        cin >> l1;
        cout << "Pocetna vrednost: " << l1;
        List l2;
        l2 = l1--;
        cout << "Se povikuva ++ vo sufiks: " << l2;
        cout << "Posle povik: " << l1;
    } else if (testCase == 7) {

        cout << "Testiranje na operator += "<<endl;
        List l1;
        cin >> l1;
        int n;
        cin >> n;
        for (int i=0; i<n; i++) {
            int broj;
            cin >> broj;
            l1 += broj;
        }
        cout << l1;
    } else if (testCase == 8) {
        cout << "Testiranje na operatorot [ ] "<< endl;
        List l1;
        cin >> l1;
        int n;
        cin >> n;
        for (int i=0; i<n; i++) {
            int idx;
            cin >> idx;
            cout << "List["<< idx <<"] = " << l1[idx] << endl;
        }
    } else {
        //Dopolnitelno baranje
        cout <<"Testiranje na addList() metodot"<<endl;
        List l1,l2;
        cin >> l1;
        cin >> l2;
        cout << l1.addList(l2);
    }

    return 0;
}