accumulate.c++

template<typename T>
auto accumulate(T const& xs,
                typename std::iterator_traits<decltype(begin(xs))>::value_type const& v)
    -> decltype(std::plus<typename std::iterator_traits<decltype(begin(xs))>::value_type>(*std::begin(xs), *std::begin(xs)))
{
    return std::accumulate(std::begin(xs), std::end(xs), v);
}

template<typename T, typename F>
auto accumulate(T const& xs,
                typename std::iterator_traits<decltype(begin(xs))>::value_type const& v,
                F&& f)
                -> decltype(f(*std::begin(xs), *std::begin(xs)))
{
    return std::accumulate(std::begin(xs), std::end(xs), v, std::forward<F>(f));
}

template<typename T>
auto accumulate1(T const& xs) 
    -> decltype(
                    std::declval<
                        std::plus<
                            typename std::iterator_traits<decltype(begin(xs))>::value_type
                        >
                    >()
                    (*std::begin(xs), *std::begin(xs))
                )
{
    if (xs.empty()) {
        throw std::invalid_argument("Collection empty");
    }
    return std::accumulate(std::begin(xs) + 1, std::end(xs), *std::begin(xs));
}

template<typename T, typename F>
auto accumulate1(T const& xs, F&& f) 
    -> decltype(f(*std::begin(xs), *std::begin(xs)))
{
    if (xs.empty()) {
        throw std::invalid_argument("Collection empty");
    }
    return std::accumulate(std::begin(xs) + 1, std::end(xs), *std::begin(xs), f);
}