Deducing value type from iterator for return type of template function

Question

Just experimenting with multiple return values using this simple snippet that calculates sum and mean from a container.

template<typename Iter>
std::tuple<double, double> summean(Iter first1, Iter last1)
{
    double sum = std::accumulate(first1, last1, 0.0);
    double mean = sum / (last1-first1);

    return {sum, mean};
}

The demo uses double precision calculation just as a demo. Is there an elegant way to use the precision of the values in the container in the calculations for the return types of the tuple?

Answer 1

You can use std::iterator_traits::value_type:

typedef typename std::iterator_traits<Iter>::value_type value_type;

As for the function declaration and definition, in old-school C++03 style you could do the following:

template<typename Iter>
std::pair<typename std::iterator_traits<Iter>::value_type,
          typename std::iterator_traits<Iter>::value_type> 
summean(Iter first1, Iter last1)
{
    typedef typename std::iterator_traits<Iter>::value_type value_type;
    value_type sum = std::accumulate(first1, last1, value_type());
    value_type mean = sum / (last1-first1);

    return std::make_pair(sum, mean);
}

And since C++11 you can use decltype and trailing return type to make it less verbose:

template<typename Iter>
auto summean(Iter first1, Iter last1)->decltype(std::make_tuple(*first1, *first))
{
    using value_type = typename std::iterator_traits<Iter>::value_type;
    value_type sum = std::accumulate(first1, last1, value_type());
    value_type mean = sum / (last1-first1);

    return make_tuple(sum, mean);
}

Answer 2

You probably want to return a std::pair of double instead of std::tuple. As for your problem you can use the value_type member taken from std::iterator_traits:

template<typename Iter>
auto
summean(Iter first1, Iter last1) -> decltype(std::make_pair(*first1, double{}))
{
    using value_type = typename std::iterator_traits<Iter>::value_type;
    double sum = std::accumulate(first1, last1, value_type{});
    double mean = sum / (last1-first1);

    return {sum, mean};
}