[...]
 void printNumber(const std::string& na){
   std::shared_lock<std::shared_timed_mutex> readerLock(teleBookMutex);
   auto searchEntry = teleBook.find(na);
   if(searchEntry != teleBook.end()){
     std::cout << searchEntry->first << ": " << searchEntry->second << std::endl;
   }
   else {
     std::cout << na << " not found!" << std::endl;
   }
 }
 [...]