string Triangle::stringAboutObject()
{
    std::ostringstream out;
    out << "length a: " << a << "\nlength b: " << b << "\nlength c: " << c << "\narea: " << area() << "\nperimeter: " << perimeter() << "\n";
    string str = out.str();
    return str;
}

string Tetragon::stringAboutObject()
{
    string str = Triangle::stringAboutObject();
    std::ostringstream out; out << str;
    out << "length d: "<< d << "\nlength e: " << e << "\nlength f: " << f;
    str = out.str();
    return str;
}