How to use named group feature in Google RE2 regular expression engine

After some googling and then submitting a question in re2-dev google group, I couldn't find any useful help about using "named groups" in RE2.
for example if you want to extract protocol, url path and query string parts from a given url string, it's good idea to use named group in RE2:

sample urls :
http://localhost:8080/hello?name=Hassan

a sample but not perfect RegEx may be :
^(?P<PROTO>https?)://(?P<URL>.+)\?(?P<QUERY>.+)?$

The best way for getting PROTO, URL and QUERY values is using named group capturing feature in RE2.
this is the Code I wrote for this :

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bool FindNamedGroups(const std::string &p_regex,const std::string &p_text,std::map<std::string,std::string> *p_group_value)
 {
  p_group_value->clear();
  RE2 rx(p_regex);
  if(!rx.ok())
  {
   std::cerr << "Invalid Regular Expression :" << p_regex << std::endl;
   return false;
  }
  size_t named_grp_size =rx.NumberOfCapturingGroups();
  if(named_grp_size>10)
  {
   std::cerr << "No support for more than 10 named groups :" << named_grp_size<< std::endl;
   return false;
  }
  const std::map<std::string,int> &grp_to_idx=rx.NamedCapturingGroups();
  RE2::Arg args[10];
  std::string vars[10];
  const RE2::Arg * const p_args[10]={&args[0],&args[1],&args[2],&args[3],&args[4],&args[5],&args[6],&args[7],&args[8],&args[9]};
  int var_count=0;

  for(var_count=0;var_count<10;var_count++)
   args[var_count]=&vars[var_count];

  re2::StringPiece sp_input(p_text);
  //after running following function. matched groups value  will be stored in p_args which point to args which point to vars!
  bool found= RE2::FindAndConsumeN(&sp_input,rx,p_args,named_grp_size);
  if(!found)
  {
   return false ;
  }

  std::map<std::string,int>::const_iterator iter_grps=grp_to_idx.cbegin();
  for(;iter_grps!=grp_to_idx.cend();++iter_grps)
  {
   (*p_group_value)[iter_grps->first]=vars[iter_grps->second-1];
  }
  return true;
 
 }

//////////// USAGE ////////////////

FindNamedGroups("^(?P<PROTO>https?)://(?P<URL>.+)\\?(?P<QUERY>.+)?$","http://localhost:8080/hello?name=Hassan",&g_v);
 iter=g_v.cbegin();
 for(;iter!=g_v.cend();++iter)
  std::cout << iter->first << " = " << iter->second << std::endl;
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