#include <iostream>
#include <array>
#include <numeric>
#include <cmath>
 
template <typename T, unsigned S>
struct vec {
  T v[S];
 
  T const* begin() const { return v; }
  T const* end()   const { return v+S; }
 
  auto length() const-> decltype(std::sqrt(std::declval<T>()))
  {
    auto square = [](T const& a, T const& b){return a+b*b;};
    T sum_of_squares = std::accumulate(begin(), end(), T{}, square);
    return std::sqrt(sum_of_squares);
  }
};
 
int main()
{
  vec<int, 3> v = {{2,1,2}};
  std::cout << v.length() << '\n';
 
  vec<int, 2> v2 = {{6,8}};
  std::cout << v2.length() << '\n';
}
 

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