Range.cc

// Short snippet for a custom iterator, taken from cppreference.
// Minimally adapted, just for the sake of showing it.
//   clang++ -Wall -std=c++1z -o rangetest -O3 Range.cc

#include <iostream>
#include <iterator>

namespace range {

// A simple range [From .. To]
template<int From, int To>
class Range {
public:
    // A custom forward iterator able to return the current processed number.
    class range_iterator : std::iterator<
                             std::forward_iterator_tag, // Mark it as a forward iterator.
                             int                        // Type returned on dereferencation
                           > {
    public:
        range_iterator(int current_) : current(current_) {}

        // SFINAE overload #1: If  T >= F i.e. if we have a positive range,
        // increment the current element.
        template<int F = From, int T = To>
        typename std::enable_if<(T >= F), void>::type operator++() {
            current++;
        }

        // SFINAE overload #2: If  Tr < Fr i.e. if we have a negative range
        // like  [5 .. 1], decrement the counter.
        template<int Fr = From, int Tr = To>
        typename std::enable_if<(Tr < Fr), void>::type operator++() {
            current--;
        }

        // Comparsion operators.
        bool operator==(const range_iterator& other) { return   current == other.current;  }
        bool operator!=(const range_iterator& other) { return !(current == other.current); }

        // Dereferencing operator.
        int operator*() { return current; }

    private:
        int current; // Currently "processed" element of range.
    };

    // Begin: Start from "From".
    range_iterator begin() { return range_iterator(From); }

    // End: If we have a positive range, end one past the To value, if we have a
    //      negative range (e.g. 5..1), then end at one past the lowest element.
    range_iterator end()   { return range_iterator(To >= From ? To + 1 : To - 1); }
};


} // range


int main(void) {

    auto positiveRange = range::Range<1, 5>();

    for (auto&& current : positiveRange) {
        std::cout << "+ Range it: " << current << std::endl;
    }
    std::cout << std::endl;

    auto negativeRange = range::Range<5, 1>();

    for (auto&& current : negativeRange) {
        std::cout << "- Range it: " << current << std::endl;
    }

    return 0;
}