nmea_checksum.cc

#include <sstream>
#include <iostream>
#include <numeric>
#include <string>
#include <iostream>

std::string &righttrim(std::string &s) {
    s.erase(std::find_if(s.rbegin(), s.rend(),
            std::not1(std::ptr_fun<int, int>(std::isspace))).base(), s.end());
    return s;
}

char ck(char sum, char ch)
{
   return sum ^ ch;
}

bool nmea_checksum(std::string sentence)
{
   std::string trimmed_sentence = righttrim(sentence);
   char verify = (char)std::stoul(trimmed_sentence.substr(trimmed_sentence.find("*")+1, trimmed_sentence.length()), NULL, 16);
   char check = std::accumulate(trimmed_sentence.begin()+1, trimmed_sentence.end()-3, 0, ck);
   return (verify == check);
}

int main()
{
   std::string sentence = "$GPGGA,172814.0,3723.46587704,N,12202.26957864,W,2,6,1.2,18.893,M,-25.669,M,2.0,0031*4F";

   if(nmea_checksum(sentence)) {
     std::cout << "Valid!" << std::endl;
   }

   return 0;
}