SebastianWilke

#include <array>
#include <cstddef>  // std::size_t

template <std::size_t N, std::size_t M = 0U>
struct dotprod {
    template <typename InputIt1, typename InputIt2, typename T>
    constexpr static T sum(InputIt1 u, InputIt2 v, T init) {
        const T uv_m{*(u + M) * T{*(v + M)}};
        return uv_m + dotprod<N, M + 1U>::sum(u, v, init);
    }

SebastianWilke

// 3.1415926535897932384626433832795028841971693993751058209749445923

#include <stdio.h>
#include <stdint.h>

double pi_series_representation (int N)
{
  double result = 0.0;
  int8_t sign = 1;