main.cpp

struct csv_field_result {
  std::string const field;
  std::string const rest;
};

csv_field_result
parse_csv_field(std::string const &s,char const separator,char const quote) {
    if(s.empty())
      return csv_field_result{"",""};
    if(s[0] != quote) {
      std::string::size_type const commapos = s.find(separator);
      if(commapos == std::string::npos)
	return csv_field_result{s,""};
      return csv_field_result{s.substr(0,commapos),s.substr(commapos+1)};
    }
    std::string field_string;
    for(std::string::size_type i{1}; i < s.length(); ++i) {
      if(s[i] == quote) {
	if(i+1 == s.length())
	  return csv_field_result{field_string,""};
	if(s[i+1] != quote) {
	  if(s[i+1] == separator)
	    return csv_field_result{field_string,s.substr(i+2)};
	  throw std::runtime_error(std::string{"expected ‘"}+separator+"’ after quote, got ‘"+s[i+1]+"’");
	}
	++i;
      }
      field_string += s[i];
    }
    throw std::runtime_error("quote not terminated");
}