class Diag {
public:
    enum Exceptions{OVERINDEXED, NULLPART, DIFFERENT};

    Diag(){ }
    Diag(int k): _v(k, 0) { }

    void reSize(int k) { _v.resize(k, 0); }

    int operator()(int i, int j) const;
    int& operator()(int i, int j);

    friend Diag operator+ (const Diag& a, const Diag& b);
    friend Diag operator* (const Diag& a, const Diag& b);
    friend std::istream& operator>> (std::istream& s, Diag& a);
    friend std::ostream& operator<< (std::ostream& s, const Diag& a);
private:
    std::vector<int> _v;
};

ostream& operator<<(ostream& s, const Diag& a)
{
    for(unsigned int i=0; i<a._v.size(); ++i){
        for(unsigned int j=0; j<a._v.size(); ++j)
            s << setw(5) << a(i,j);
        s << endl;
    }
    return s;
}