gistfile1.m

-(std::string *) objcDataToCppString:(NSData*)objcData {
  return new std::string([objcData bytes], [objcData length]);
}

-(NSData*) cppStringToObjcData:(const std::string)cppString {
  return [NSData dataWithBytes:cppString.c_str() length:cppString.size()];
}

Warning: This is an updated Gist. @lrz helped me fix the bugs :)

I also recommend to return a pointer (or fill the string by reference) to avoid copying cost.

-(std::string *) objcDataToCppString:(NSData*)objcData {
  return new std::string([objcData bytes], [objcData length]);
}