c++ - How to read from input file (text file) and validate input as valid integer? -

i'm writing program needs read text file , check first line of text file number between 0 10. have come couple of solutions there still problem:

how read file:

const string filename= argv[1]; ifstream fin(argv[1]); if(!fin.good()){     cout<<"file not exist ->> no file reading";     exit(1); } getline(fin,tmp); if(fin.eof()){     cout<<"file empty"<<endl; } stringstream ss(tmp); 

first used atoi:

const int filenum = atoi(tmp.c_str());     if(filenum<1 || filenum>10){         cout<<"number of files incorrect"<<endl;         //exit(1);     } 

if first line character, change 0 want call exception , terminate program.

then used isdigit entry string , not work string. used each character in string still not work.

   stringstream ss(tmp);    int i;    ss>>i;    if(isdigit(tmp[0])||isdigit(tmp[1])||tmp.length()<3)    {} 

#include <iostream> #include <fstream> #include <cstdio> #include <cstdlib> using namespace std;  bool isvalidnumber (string str) {   if (str.length() > 2 || str.length() == 0)     return false;   else if (str.length() == 2 && str != "10")     return false;   else if (str.length() == 1 && (str[0] < '0' || str[0] > '9'))     return false;   return true; }  int main() {   ifstream fin(argv[1]);   if(!fin.good())   {     cout<<"file not exist ->> no file reading";     exit(1);   }    //to check file empty http://stackoverflow.com/a/2390938/1903116   if(fin.peek() == std::ifstream::traits_type::eof())   {     cout<<"file empty"<<endl;     exit(1);   }   string tmp;   getline(fin,tmp);   if (isvalidnumber(tmp) == false)   {     cerr << "invalid number : " + tmp << endl;   }   else   {     cout << "valid number : " + tmp << endl;   } }