Iterative variadic templates

reverseForEach.cc

#define METHOD 1

// I admit this is not an easy function to understand, but you will love it
// in a few seconds.
void reverseForEach(...) { }


#if METHOD == 1
// This method use a separate data structure as an accumulator, and call a
// method to mutate the content of the accumulator.  If you don't see any
// use of it, replace "Adder" by "std::vector", and "add" by "push_back".

struct Adder {
    int result;

    Adder() : result(0) {}
    int add(int i) {
        result += i;
        return 0;
    }
};

template <typename... Elems>
int add_args(Elems... elems)
{
    Adder a;
    reverseForEach(a.add(elems)...);
    return a.result;
}

#elif METHOD == 2
// This one is the nicest version, but apparently some versions of GCC don't
// like having the lambda inside. Thus you might have to fallback on the
// method 3. (I have not verified with the latest versions of gcc yet)

template <typename... Elems>
int add_args(Elems... elems)
{
    int result;
    reverseForEach(([&](int i) -> int {
        result += i;
        return 0;
    })(elems)...);
    return result;
}

#elif METHOD == 3

template <typename... Elems>
int add_args(Elems... elems)
{
    int result;
    auto step = [&](int i) -> int {
        result += i;
        return 0;
    };
    reverseForEach(step(elems)...);
    return result;
}

#endif


int main()
{
    return add_args(1, 2, 3, 4);
}