如何在运行时获得阶乘<a>…阶乘<b>?

用C++ 14可以使用 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>{});
}

请参阅一个完全有效的示例 here .

诀窍在于 std::make_integer_sequence<int, S>{} 将创建的实例 std::integer_sequence<int, S...> get_lookup_table_impl factorial<S>::value... 解包并传递 S factorial std::array ,但你想用什么就用什么。

#include <iostream>
#include <type_traits>
#include <utility>
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> {};

template<std::size_t... Is>
int get_factorial_impl(int x,std::index_sequence<Is...>)
{
    if (x < 1) return -1;
    if (x > 5) return -1;
    static constexpr int lookup_table[] = { factorial<Is+1>::value...};
    return lookup_table[x-1];
}

int get_factorial(int x)
{
    return get_factorial_impl(x,std::make_index_sequence<5>{});
}

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