How do you validate that a string is a valid IP address in C++?

[mihailim@home ~]$ ping 0xc0a8010a
PING 0xc0a8010a (192.168.1.10) 56(84) bytes of data.
^C
--- 0xc0a8010a ping statistics ---
3 packets transmitted, 0 received, 100% packet loss, time 2479ms

bool Config::validateIpAddress(const string ipAddress)
{
    struct sockaddr_in sa;
    int result = inet_pton(AF_INET, ipAddress.c_str(), &(sa.sin_addr));
    return result != 0;
}

std::string ipAddress = "127.0.0.1";
boost::system::error_code ec;
boost::asio::ip::address::from_string( ipAddress, ec );
if ( ec )
    std::cerr << ec.message( ) << std::endl;

bool IsIPAddress(std::string & ipaddr)
    {

    StringTokenizer quads(ipaddr,".");

    if (quads.countTokens() != 4) return false;

    for (int i=0; i < 4; i++)
      {
      std::string quad = quads.nextToken();
      for (int j=0; j < quad.length(); j++
         if (!isdigit(quad[j])) return false;

      int quad = atoi(quads.GetTokenAt(i));
      if (quad < 0) || (quad > 255)) return false;
      }

    return true;
    }

bool validate_ip_address(const std::string& s)
{
   static const boost::regex e("\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3}");
   return regex_match(s, e);
}

vector<string> &split(const string &s, char delim, vector<string> &elems) {
    stringstream ss(s);
    string item;
    while(getline(ss, item, delim)) {
       elems.push_back(item);
    }
    return elems;
}

vector<string> split(const string &s, char delim) {
   vector<string> elems;
   return split(s, delim, elems);
}


bool isIPAddress(string  ipaddr){

    if (ipaddr.length()){
            vector<string> _ip=split(ipaddr,'.');
            if (_ip.size()==4){
                    for (int i=0; i < 4; i++){
                            for (int j=0; j < _ip[i].length(); j++)
                                    if (!isdigit(_ip[i][j])) return false;
                            if ((atoi(_ip[i].c_str()) < 0) || (atoi(_ip[i].c_str()) > 255)) return false;
                    }
            return true;
            }
    }
    return false;
 }