## Safe Numerics

### Arithmetic Operations Can Overflow Silently

A variation of the above is when a value is incremented/decremented beyond its domain.

    //  Copyright (c) 2018 Robert Ramey
    //
    // Distributed under the Boost Software License, Version 1.0. (See
    // accompanying file LICENSE_1_0.txt or copy at
    // http://www.boost.org/LICENSE_1_0.txt)

    #include <iostream>

    #include <boost/safe_numerics/safe_integer.hpp>

    int main(int, const char *[]){
        std::cout << "example 2:";
        std::cout << "undetected overflow in data type" << std::endl;
        // problem: undetected overflow
        std::cout << "Not using safe numerics" << std::endl;
        try{
            int x = INT_MAX;
            // the following silently produces an incorrect result
            ++x;
            std::cout << x << " != " << INT_MAX << " + 1" << std::endl;
            std::cout << "error NOT detected!" << std::endl;
        }
        catch(const std::exception &){
            std::cout << "error detected!" << std::endl;
        }
        // solution: replace int with safe<int>
        std::cout << "Using safe numerics" << std::endl;
        try{
            using namespace boost::safe_numerics;
            safe<int> x = INT_MAX;
            // throws exception when result is past maximum possible
            ++x;
            assert(false); // never arrive here
        }
        catch(const std::exception & e){
            std::cout << e.what() << std::endl;
            std::cout << "error detected!" << std::endl;
        }
        return 0;
    }

    example 2:undetected overflow in data type
    Not using safe numerics
    -2147483648 != 2147483647 + 1
    error NOT detected!
    Using safe numerics
    addition result too large
    error detected!

When variables of unsigned integer type are decremented below zero, they "roll over" to the highest possible unsigned version of that integer type. This is a common problem which is generally never detected.