Segmentation fault overloading << operator

Question

I overloaded operator << for my dynamic array Tvector container and i want to cout areas of squares.

std::ostream & operator<<(std::ostream &os, TVector &s) {
    for(int i = 0; i < s.length(); ++i){
        os << "[" << s[i].Area() << " ";
    }
    os << "]";
    return os;
}

Function area

double Square::Area() {
    double len_a = a.dist(b);
    double len_b = b.dist(c);
    return len_a * len_b;
}

If i just

cout << a.Area();

everything works fine. But if i want to cout dynamic array, compilator throws segmentation fault error. main function fragment

    Square a;
    std::cin >> a;
    Square a1;
    std::cin >> a1;
    TVector v;
    v.push_back(a);
    v.push_back(a1);
    std::cout << v;

P.S Code for TVector

TVector::TVector()
    : _array(nullptr)
    , _size(0)
{

int TVector::length() {
    return _size;
}

Square &TVector::operator[](int idx) {
    return _array[idx];
}

std::ostream & operator<<(std::ostream &os, TVector &s) {
    for(int i = 0; i < s.length(); ++i){
        os << "[" << s[i].Area() << " ";
    }
    os << "]";
    return os;
}

std::istream& operator>>(std::istream& is, TVector& s) {
    Square a;
    is >> a;
    s.push_back(a);
    return is;
}

TVector::~TVector()
{
    clear();
}