How to get factorial<a>....factorial<b> at runtime?

Question

I want to fill a look up table with integers computed at compile time:

#include <iostream>
#include <type_traits>

template <int x> using number = std::integral_constant<int,x>;    
template <int n> struct factorial : number<n * factorial<n-1>::value> {};
template <> struct factorial<0> : number<1> {};

int get_factorial(int x) {
    if (x < 1) return -1;
    if (x > 5) return -1;
    static constexpr int lookup_table[] = { 
        factorial<1>::value,
        factorial<2>::value,
        factorial<3>::value,
        factorial<4>::value,
        factorial<5>::value
    };
    return lookup_table[x-1];
}

int main() {        
    int x;
    std::cin >> x;
    std::cout << get_factorial(x) << "\n";
}

This is fine for small number of elements, but what can I do when the look up table contains a large number of elements? How to populate the array without writing each element explicitly?

The factorial is only for the example. In a more realistic scenario I would like to store ~1000 elements in the lookup table.

Answer 1

With C++14 you can use std::integer_sequence:

template <int... S>
constexpr std::array<int, sizeof...(S)> get_lookup_table_impl(std::integer_sequence<int, S...>)
{
    return { factorial<S>::value... };
}

template <int S>
constexpr auto get_lookup_table()
{
    return get_lookup_table_impl(std::make_integer_sequence<int, S>{});
}

See a fully working example here.

The trick is that std::make_integer_sequence<int, S>{} will create an instance of std::integer_sequence<int, S...>. So, the helper function get_lookup_table_impl is able to deduce its parameter pack. Then, factorial<S>::value... unpacks it and passes each value of S to factorial. Covered with curly braces, this can be used to initialize any kind of std container. I used std::array, but you can use whatever you want.