Reverse number in compile time using user-defined literal

#include <iostream>

template <char c, char... Str>
struct reverse_int
{
    static const int value = 10 * reverse_int<Str...>::value + (c - '0');
};

template <char c>
struct reverse_int<c>
{
    static const int value = c - '0';
};

template <char... Str>
constexpr int operator "" _reverse()
{
    return reverse_int<Str...>::value;
}

template <int Num, int temp = 0>
struct reverse_int2
{
    static const int value = reverse_int2<Num / 10, temp * 10 + Num % 10>::value;
};

template <int temp>
struct reverse_int2<0, temp>
{
    static const int value = temp;
};

int main(int argc, char *argv[])
{
    using namespace std;
    cout << 54321_reverse << endl
          << reverse_int2<12345>::value << endl;
}