c++ - why would cin.fail() pass input=100a if 'input' is an int? -

int main(void) {    int valid_input();    int ll_input=0;    int resp;    hash_table obj;    int bucket;      while(1)    {       ll_input=valid_input();       if(ll_input>10)       {         break;       }       obj.insert(ll_input);     }    obj.print_ll();      return 0; }      int valid_input()  {   int input;   printf("\n\renter value: ");   std::cin >> input;   while(std::cin.fail())   {     std::cin.clear();     std::cin.ignore(std::numeric_limits<std::streamsize>::max(),'\n');     printf("\n\rbad entry!\n\renter value: ");     std::cin>>input;   }   return input; } 

the above code, works correctly alphanumeric combinations, rejects user inputs a,10aa,1003abc, etc. however, on inputs 100a, 100b, etc, passes. reason this?

the following output in snapshot

cin >> input stops reading finds non number, "100a" interpreted 2 tokens: "100" , "a".

"100" converted int , exits while(std::cin.fail()) loop. leaves "a" sitting in stream bedevil next read of cin.

the quick way catch this, (quick in terms of writing , explaining, not fastest execute if input error-prone)

std::string token; while (true) // or countdown kick out moronic {     cin >> token;     try     {         return std::stoi(token);     }     catch (...) // don't care why std::stoi failed     {     } } 

std::stoi documentation.

faster approaches, if errors expected, use strtol or similar , skip exception handling overhead.