Question

So, assume I have a template structure-function fib<i>::value. I want to get nth fibonacci number in runtime. For this i create array fibs[] = { fib<0>::value, ... , fib<maxN>::value }. Unfortunatelly, for some functions maxN can be very large and I can’t fill it with hands only. So I writed some preprocessor directives to make task easier.

#define fib(x) (fib<(x)>::value)
#define fibLine_level_0(x) fib(5*(x) + 0), fib(5*(x) + 1), fib(5*(x) + 2), fib(5*(x) + 3), fib(5*(x) + 4)
#define fibLine_level_1(x) fibLine_level_0(2*(x) + 0), fibLine_level_0(2*(x) + 1)
#define fibLine_level_2(x) fibLine_level_1(2*(x) + 0), fibLine_level_1(2*(x) + 1)
#define fibLine_level_3(x) fibLine_level_2(2*(x) + 0), fibLine_level_2(2*(x) + 1)

#define cAarrSize(x) (sizeof(x) / sizeof(x[0]))

And I use it so:

int fibs[] = { fibLine_level_3(0) };

for (int i = 0; i < cAarrSize(fibs); i++)
    cout << "fib(" << i << ") = " << fibs[i] << endl;

The code that you may need:

template<int i>
struct fibPair{
    static const int fst = fibPair<i-1>::snd;
    static const int snd = fibPair<i-1>::fst + fibPair<i-1>::snd;
};

template<>
struct fibPair<0> {
    static const int fst = 0;
    static const int snd = 1;
};

template<int i>
struct fib {
    static const int value = fibPair<i>::fst;
};

But this code is really ugly. What to do to make it more beautiful?

Constraints: this code must be used in sport programming. That means – no third-party libraries and sometimes no C++11 (but it can be)

Answer 1

Fib structure can be rewritten as follows:

template <size_t i>
struct fib
{
    static const size_t value = fib<i - 1>::value + fib<i - 2>::value;
};

template <>
struct fib<0>
{
    static const size_t value = 0;
};

template <>
struct fib<1>
{
    static const size_t value = 1;
};

Compile-time array of the Fibonacci numbers can be calculated using C++11.

Edit 1 (changed the type of fib values).

Edit 2:

Compile-time generation of Fibonacci numbers array (based on this answer).

template<unsigned... args> struct ArrayHolder
{
    static const unsigned data[sizeof...(args)];
};

template<unsigned... args>
const unsigned ArrayHolder<args...>::data[sizeof...(args)] = { args... };

template<size_t N, template<size_t> class F, unsigned... args>
struct generate_array_impl
{
    typedef typename generate_array_impl<N-1, F, F<N>::value, args...>::result result;
};

template<template<size_t> class F, unsigned... args>
struct generate_array_impl<0, F, args...>
{
    typedef ArrayHolder<F<0>::value, args...> result;
};

template<size_t N, template<size_t> class F>
struct generate_array
{
    typedef typename generate_array_impl<N-1, F>::result result;
};

int main()
{
    const size_t count = 10;
    typedef generate_array<count, fib>::result fibs;

    for(size_t i = 0; i < count; ++i)
        std::cout << fibs::data[i] << std::endl;
}

All you need is to provide generate_array with the generation «function» (our fib struct).