15. IPv4 data type

The problem requires writing a class to represent an IPv4 address. This is a 32-bit value, usually represented in decimal dotted format, such as 168.192.0.100; each part of it is an 8-bit value, ranging from 0 to 255. For easy representation and handling, we can use four unsigned char to store the address value. Such a value could be constructed either from four unsigned char or from an unsigned long. In order to be able to read a value directly from the console (or any other input stream) and be able to write the value to the console (or any other output stream), we have to overload operator>> and operator<<. The following listing shows a minimal implementation that can meet the requested functionality:

class ipv4
{
   std::array<unsigned char, 4> data;
public:
   constexpr ipv4() : data{ {0} } {}
   constexpr ipv4(unsigned char const a, unsigned char const b, 
                  unsigned char const c, unsigned char const d):
      data{{a,b,c,d}} {}
   explicit constexpr ipv4(unsigned long a) :
      data{ { static_cast<unsigned char>((a >> 24) & 0xFF), 
              static_cast<unsigned char>((a >> 16) & 0xFF),
              static_cast<unsigned char>((a >> 8) & 0xFF),
              static_cast<unsigned char>(a & 0xFF) } } {}
   ipv4(ipv4 const & other) noexcept : data(other.data) {}
   ipv4& operator=(ipv4 const & other) noexcept 
   {
      data = other.data;
      return *this;
   }

   std::string to_string() const
   {
      std::stringstream sstr;
      sstr << *this;
      return sstr.str();
   }

   constexpr unsigned long to_ulong() const noexcept
   {
      return (static_cast<unsigned long>(data[0]) << 24) |
             (static_cast<unsigned long>(data[1]) << 16) |
             (static_cast<unsigned long>(data[2]) << 8) |
              static_cast<unsigned long>(data[3]);
   }

   friend std::ostream& operator<<(std::ostream& os, const ipv4& a)
   {
      os << static_cast<int>(a.data[0]) << '.' 
         << static_cast<int>(a.data[1]) << '.'
         << static_cast<int>(a.data[2]) << '.'
         << static_cast<int>(a.data[3]);
      return os;
   }

   friend std::istream& operator>>(std::istream& is, ipv4& a)
   {
      char d1, d2, d3;
      int b1, b2, b3, b4;
      is >> b1 >> d1 >> b2 >> d2 >> b3 >> d3 >> b4;
      if (d1 == '.' && d2 == '.' && d3 == '.')
         a = ipv4(b1, b2, b3, b4);
      else
         is.setstate(std::ios_base::failbit);
      return is;
   }
};

The ipv4 class can be used as follows:

int main()
{
   ipv4 address(168, 192, 0, 1);
   std::cout << address << std::endl;

   ipv4 ip;
   std::cout << ip << std::endl;
   std::cin >> ip;
   if(!std::cin.fail())
      std::cout << ip << std::endl;
}