Спринт 13. Не очень чистые функции - побочные эффекты.

#include <algorithm>
#include <cassert>
#include <optional>
#include <string_view>
#include <utility>
#include <vector>

using namespace std;

template <typename InputIt, typename OutSum, typename OutSqSum, typename OutMax>
void ComputeStatistics(InputIt first,
                       InputIt last,
                       OutSum& out_sum,
                       OutSqSum& out_sq_sum,
                       OutMax& out_max) {

    using Elem = std::decay_t<decltype(*first)>;

    constexpr bool need_sum = !is_same_v<OutSum, const nullopt_t>;
    constexpr bool need_sq_sum = !is_same_v<OutSqSum, const nullopt_t>;
    constexpr bool need_max = !is_same_v<OutMax, const nullopt_t>;

    optional<Elem> sum, sq_sum, max_elem;
    size_t count = 0;

    for (auto it = first; it != last; ++it, ++count) {

        if constexpr (need_sum) {
            !sum ? sum = *it : *sum += *it;
        }

        if constexpr (need_sq_sum) {
            !sq_sum ? sq_sum = *it * *it : *sq_sum += *it * *it;
        }

        if constexpr (need_max) {
            !max_elem ? max_elem = *it : max_elem = max(*max_elem, *it);
        }

    }

    if constexpr (need_sum) {
        if (sum) {
            out_sum = *sum;
        }
    }

    if constexpr (need_sq_sum) {
        if (sq_sum) {
            out_sq_sum = *sq_sum;
        }
    }

    if constexpr (need_max) {
        if (max_elem) {
            out_max = *max_elem;
        }
    }
}

struct OnlySum {int value;};
OnlySum operator+(OnlySum l, OnlySum r) {return {l.value + r.value};}
OnlySum& operator+=(OnlySum& l, OnlySum r) {return l = l + r;}

int main() {
    vector input = {1, 2, 3, 4, 5, 6};
    int sq_sum;
    std::optional<int> max;

    ComputeStatistics(input.begin(), input.end(), nullopt, sq_sum, max);

    assert(sq_sum == 91 && max && *max == 6);

    vector<OnlySum> only_sum_vector = {{100}, {-100}, {20}};
    OnlySum sum;

    ComputeStatistics(only_sum_vector.begin(), only_sum_vector.end(), sum, nullopt, nullopt);

    assert(sum.value == 20);
}