Question

Say I have a program that takes in integers. How do I stop the program from falling apart if the user enters an out of range number, or a letter or something?

Answer 1

The cin‘s base class is std::basic_istream. The input stream indicates a recoverable error in case it cannot extract the requested data from the stream. In order to check for that error bit, std::basic_istream::fail() method must be used — it returns true if there was a failure or false if everything is alright. It is important to remember that if there is an error, the data is left in the stream and, of course, the error bit(s) must also be cleared using std::basic_istream::clear(). Also, a programmer must ignore incorrect data, or otherwise an attempt to read something else will fail again. For that purpose, std::basic_istream::ignore() method can be used. As for the valid range of values, it must be checked manually. Okay, enough theory, here is a simple example:

#include <limits>
#include <iostream>

int main()
{
    int n = 0;

    for (;;) {
        std::cout << "Please enter a number from 1 to 10: " << std::flush;
        std::cin >> n;

        if (std::cin.fail()) {
            std::cerr << "Sorry, I cannot read that. Please try again." << std::endl;
            std::cin.clear();
            std::cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n');
            continue;
        }

        if (n < 1 || n > 10) {
            std::cerr << "Sorry, the number is out of range." << std::endl;
            continue;
        }

        std::cout << "You have entered " << n << ". Thank you!" << std::endl;
        break;
    }
}

Hope it helps. Good Luck!