Lista od celi broevi - preoptovaruvanje operatori

#include<iostream>
#include<cstring>
#include<cmath>

using namespace std;

class List{
private:
    int *niza;
    int broj;
public:
    // default constructor
    List(){ niza=new int [0]; broj=0;}
    // constructor so argument
    List(int n){
        niza=new int [n];
        broj=n;
    }
    // copy constructor
    List(const List &l){
        broj=l.broj;
        niza= new int [broj];
        for(int i=0;i<broj;i++){
            niza[i]=l.niza[i];
        }
    }
    // operator =
    List & operator=(const List &l){
        if(this!=&l){
            delete [] niza;
            broj=l.broj;
            niza= new int [broj];
            for(int i=0;i<broj;i++){
            niza[i]=l.niza[i];
        }
        }
        return *this;
    }
    // destructor
    ~List(){ delete [] niza; }
    int sum (){
        int suma=0;
        for(int i=0;i<broj;i++){
            suma+=niza[i];
        }
        return suma;
    }
    double average () { return 1.0*sum()/broj; }
    //friend funkcii
    friend bool operator==(const List &l1, const List &l2);
    friend ostream & operator<<(ostream &out,  List &l);
    friend istream & operator>>(istream &in,  List &l);
    friend List operator +(const List &l1,const List &l2);
    friend List operator-(const List &l1,const List &l2);
    // operator ++ prefix
    List & operator++(){
        for(int i=0;i<broj;++i)
            ++niza[i];
        return *this;
    }
    // operator -- postfix
    List operator--(int){
        List tmp(*this);
        for(int i=0;i<broj;++i)
            --niza[i];
        return tmp;
    }
    // operator []
    int & operator[](int index){
        return niza[index];
    }
    List & operator+=(int ind){
        int isti=0;
        for(int i=0;i<broj;++i)
            if(ind==niza[i])
                ++isti;
        if(isti<2){
            int *tmp=new int [broj+1];
            for(int i=0;i<broj;++i)
                tmp[i]=niza[i];
            tmp[broj]=ind;
            broj++;
            delete [] niza;
            niza=tmp;
        }
        return *this;
    }
    List addList(List l){
        List nova;
        // nova.broj=broj+l.broj;
        // nova.niza=new int [nova.broj];
        for(int i=0;i<broj;++i)
            nova+=niza[i];
        for(int i=0;i<l.broj;++i)
            nova+=l.niza[i];
        return nova;
    }
};
// operator ==
bool operator==( List &l1,  List &l2){
    return l1.sum()==l2.sum();
}
// operator <<
ostream & operator<<(ostream &out,  List &l){
    if(l.broj>0){
    out<<l.broj<<": ";
    for(int i=0;i<l.broj;i++)
        out<<l.niza[i]<<' ';
    out<<"sum: "<<l.sum()<<" average: "<<l.average()<<endl;
    return out;
    }
    out<<"The list is empty";
    return out;
}
// operator >>
istream & operator>>(istream &in,  List &l){
    in>>l.broj;
    l.niza= new int [l.broj];
    for(int i=0;i<l.broj;i++)
        in>>l.niza[i];
    return in;
}
// operator +
List operator +(const List &l1,const List &l2){
    if(l1.broj==l2.broj){
        List k(l1.broj);
        for(int i=0;i<k.broj;i++)
            k.niza[i]=l1.niza[i]+l2.niza[i];
        return k;
    }
        List l;
        return l;
}
// operator -
List operator-(const List &l1,const List &l2){
    if(l1.broj==l2.broj){
        List k(l1.broj);
        for(int i=0;i<k.broj;i++){
            k.niza[i]=l1.niza[i]-l2.niza[i];
        }
        return k;
    }
    List l;
    return l;
}

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,l3;
        cin >> l1;
        cin >> l2;
        l3 = l1.addList(l2);
        //cout << l1.addList(l2);
        cout<<l3;
    }

    return 0;
}