Calculate the Fibonacci number (recursive approach) in compile time (constexpr) in C++11

Question

I wrote the program Fibonacci number calculation in compile time (constexpr) problem using the template metaprogramming techniques supported in C++11. The purpose of this i

Answer 1

Maybe just use a more efficient algorithm?

constexpr pair helper(size_t n, const pair& g)
{
    return n % 2
        ? make_pair(g.second * g.second + g.first * g.first, g.second * g.second + 2 * g.first * g.second)
        : make_pair(2 * g.first * g.second - g.first * g.first, g.second * g.second + g.first * g.first);
}

constexpr pair fibonacciRecursive(size_t n)
{
    return n < 2
        ? make_pair(n, 1)
        : helper(n, fibonacciRecursive(n / 2));
}

constexpr double fibonacci(size_t n)
{
    return fibonacciRecursive(n).first;
}

My code is based on an idea described by D. Knuth in the first part of his "The Art of Computer Programming". I can't remember the exact place in this book, but I'm sure that the algorithm was described there.