Implemet an dinamic array ( I have access to positions that can't exist)

For some reason my implementation allows 0 size array access to 0 position and 5 size array access 50 position. While both positions must get me an segmentation fault error for my perspective. I think that maybe it can be caused by allocate memory wrongly but by my side it looks right ,someone can tell me why I obtain that result.

My code for array:

class vector{
 private:
  int length;
  int cap;
  T* arr;
 public:

  vector(int n){
   arr = new T[n];
   length = n;
   cap = n;
  }

  vector():vector(0){};

  vector(vector<T> & other){
   arr = new T[other.size()];
   length = other.size();
   cap = other.capacity();
  }
  ~vector(){
   delete[] arr;
  }

  T& operator[](int ind) {
   return arr[ind];
  }

  const T& operator[](int ind) const{
   return arr[ind];
    }
};

My test:

 int main(int argc, char* argv[])
 {
  vector<int> v;
  v[0] = 5;
  vector<int> v2(5);

  std::cout<<v2[50]<<" "<<v[0]<<std::endl;
 }

Output:

  0 0