Checking for legitimate integer input in c++

Question

so i've seen many people ask this and not many solid answers floating around the web. most just check that an integer was placed in place of a string but if a floating point number was entered then it truncates the bottom half or if integers and characters are intered it truncates the characters. i need help writing a piece of code that checks for user input and asks the user to retry if his input is not valid or a combination of valid/invalid. i think the basic idea was to make a string so it accepts anything then use sstream to manipulate and then back to int if the input was legit but i cant really manage to check the other parts. if anyones run accross this or can help me out please link me to it. i'll post my code when i get a good sense of what to do.

Answer 1

Assuming that you can't use boost::lexical_cast, you can write your own version:

#include <sstream>
#include <iostream>
#include <stdexcept>
#include <cstdlib>
template <class T1, class T2>
T1 lexical_cast(const T2& t2)
{
  std::stringstream s;
  s << t2;
  T1 t1;
  if(s >> std::noskipws >> t1 && s.eof()) {
    // it worked, return result
    return t1;
  } else {
    // It failed, do something else:
    // maybe throw an exception:
    throw std::runtime_error("bad conversion");
    // maybe return zero:
    return T1();
    // maybe do something drastic:
    exit(1);
  }
}



int main() {
  std::string smin, smax;
  int imin, imax;

  while(std::cout << "Enter min and max: " && std::cin >> smin >> smax) {
    try {
      imin = lexical_cast<int>(smin);
      imax = lexical_cast<int>(smax);
      break;
    } catch(std::runtime_error&) {
      std::cout << "Try again: ";
      continue;
    }
  }

  if(std::cin) {
    std::cout << "Thanks!\n";
  } else {
    std::cout << "Sorry. Goodbye\n";
    exit(1);
  }
}

Answer 2

You can use C++11 string conversion functions like stol

try
{
    std::string value = ...;
    long number = std::stol(value); 
}
catch (std::invalid_argument const& e)
{
    // no conversion could be performed
}

Post-comments update: Visual C++ 11 shipped with Visual Studio 2012 implements std::stol as a convenient wrapper around strtol declared in <cstdlib>. I think it's safe to assume most C++11 implementations define it in most optimal way possible, not reaching for std::stringstream machinery.

Answer 3

The C function strtol (and it's siblings) will be able to tell you if the string fed to it is completely consumed.

 std::string str;
 char *endptr;
 std::cin >> str;
 long x = std::strtol(str.c_str(), &endptr, 0); 
 if (*endptr != 0) 
    cout << "That's not a valid number...";

Answer 4

I don't know if there are any classes in standard c++ lib that encapsule primitive types like in java but here how a simple and very basic implementation would look like

class Integer {
   private:
       int value;
       void parse(string);
   public:
       Integer(string);
       int intValue();
};

Integer::Integer(string sint) { parse(sint); }
int Integer::intValue() { return value; }

void Integer::parse(string sint) {
    string::iterator its = sint.begin();
    while(its != sint.end() && (! (*its < '0' || *its > '9'))) {
        its++;
    }
    if(its != sint.end()) {
        throw sint + ": Input is not a valid integer.";
    }
    value = atoi(sint.c_str());
}