| ## Safe Numerics

### Array Index Value Can Exceed Array Limits

Using an intrinsic C++ array, it's very easy to exceed array limits. This can fail to be detected when it occurs and create bugs which are hard to find. There are several ways to address this, but one of the simplest would be to use safe_unsigned_range;

    //  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 <stdexcept>
    #include <iostream>
    #include <array>

    #include <boost/safe_numerics/safe_integer_range.hpp>

    void detected_msg(bool detected){
        std::cout << (detected ? "error detected!" : "error NOT detected! ") << std::endl;
    }

    int main(int, const char *[]){
        // problem: array index values can exceed array bounds
        std::cout << "example 5: ";
        std::cout << "array index values can exceed array bounds" << std::endl;
        std::cout << "Not using safe numerics" << std::endl;
        std::array<int, 37> i_array;

        // unsigned int i_index = 43;
        // the following corrupts memory.
        // This may or may not be detected at run time.
        // i_array[i_index] = 84; // comment this out so it can be tested!
        std::cout << "error NOT detected!" << std::endl;

        // solution: replace unsigned array index with safe_unsigned_range
        std::cout << "Using safe numerics" << std::endl;
        try{
            using namespace boost::safe_numerics;
            using i_index_t = safe_unsigned_range<0, i_array.size() - 1>;
            i_index_t i_index;
            i_index = 36; // this works fine
            i_array[i_index] = 84;
            i_index = 43; // throw exception here!
            std::cout << "error NOT detected!" << std::endl; // so we never arrive here
        }
        catch(const std::exception & e){
            std::cout <<  "error detected:" << e.what() << std::endl;
        }
        return 0;
    }

    example 5: array index values can exceed array bounds
    Not using safe numerics
    error NOT detected!
    Using safe numerics
    error detected:Value out of range for this safe type: domain error

Collections like standard arrays and vectors do array index checking in some function calls and not in others so this may not be the best example. However it does illustrate the usage of `safe_range<T>` for assigning legal ranges to variables. This will guarantee that under no circumstances will the variable contain a value outside of the specified range.