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

Question

I\'m writing a program which needs to read a text file and check first line of the text file for a number between 0 to 10. I have come up with a couple of solutions but ther

Answer 1

I would probably read the line with std::getline, then use a Boost lexical_cast to convert to int. It will throw an exception unless the entirety of the input string can be converted to the target type -- exactly what you want.

Of course, you'll also need to check that the converted result is in the correct range, and also throw the exception if it's out of range.

Answer 2

#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 does not exist ->> No File for reading";
    exit(1);
  }

  //To check file is empty http://stackoverflow.com/a/2390938/1903116
  if(fin.peek() == std::ifstream::traits_type::eof())
  {
    cout<<"file is empty"<<endl;
    exit(1);
  }
  string tmp;
  getline(fin,tmp);
  if (isValidNumber(tmp) == false)
  {
    cerr << "Invalid number : " + tmp << endl;
  }
  else
  {
    cout << "Valid Number : " + tmp << endl;
  }
}