| ## Safe Numerics

### Checking of Input Values Can Be Easily Overlooked

It's way too easy to overlook the checking of parameters received from outside the current program.

    #include <stdexcept>
    #include <sstream>
    #include <iostream>

    #include <boost/safe_numerics/safe_integer.hpp>

    int main(int, const char *[]){
        // problem: checking of externally produced value can be overlooked
        std::cout << "example 6: ";
        std::cout << "checking of externally produced value can be overlooked" << std::endl;
        std::cout << "Not using safe numerics" << std::endl;

        std::istringstream is("12317289372189 1231287389217389217893");

        try{
            int x, y;
            is >> x >> y; // get integer values from the user
            std::cout << x << ' ' << y << std::endl;
            std::cout << "error NOT detected!" << std::endl;
        }
        catch(const std::exception &){
            std::cout << "error detected!" << std::endl;
        }

        // solution: assign externally retrieved values to safe equivalents
        std::cout << "Using safe numerics" << std::endl;
        {
            using namespace boost::safe_numerics;
            safe<int> x, y;
            is.seekg(0);
            try{
                is >> x >> y; // get integer values from the user
                std::cout << x << ' ' << y << std::endl;
                std::cout << "error NOT detected!" << std::endl;
            }
            catch(const std::exception & e){
                std::cout << "error detected:"  << e.what() << std::endl;
            }
        }
        return 0;
    }

    example 6: checking of externally produced value can be overlooked
    Not using safe numerics
    2147483647 0
    error NOT detected!
    Using safe numerics
    error detected:error in file input: domain error

Without safe integer, one will have to insert new code every time an integer variable is retrieved. This is a tedious and error prone procedure. Here we have used program input. But in fact this problem can occur with any externally produced input.