C++ Weekly - Ep 13 Fibonacci: You're Doing It Wrong

fib-template.cpp

#include <iostream>
#include <array>
#include <utility>
#include <sstream>

template<int I>
struct Fib
{
  static const int val Fib<I-1>::val + Fib<I-2>::val;
};

template<>
struct Fib<0>
{
  static const int val = 0;
};

template<>
struct Fib<1>
{
  static const int val = 1;
};


template<size_t ... I>
const int fib_impl(std::index_sequence<I...>, const int i)
{
  constexpr std::array<int, sizeof...(I)> a
   = {Fib<I>::val...};

  return a[i];
}

const int fib(const int i)
{
  return fib_impl(std::make_index_sequence<47>(), i);
}

int main(int argc, const char *argv[])
{
  // Fixed value only
  //# std::cout << Fib<46>::val << '\n';

  if (argc == 2)
  {
    std::stringstream ss(argv[1]);
    int i;
    ss >> i;
    std::cout << fib(i) << '\n';
  }
  else
  {
    std::cerr << "fib-template: Invalid number of command line arguments" << '\n';
    return 1;
  }

  // Added to avoid warning
  return 0;
}

This example, written by Jason Turner and corrected by me, computes the Fibonacci sequence up to the limit associated with a long int, at compile time and puts it into an array that's indexable AT RUN TIME.