C++ Variable Template. This is UB?

Question

I created a simple C++14 variable template that calculates factorials (just for learning). Then I want print the first 12 factorials.

template <int n> const int fact = n * fact<n - 1>;
template <> const int fact<0> = 1;

If I replace fact<12> with fact<i> in following snippet, I get an error, because i is not constant.

int main()
{   
    for(int i = 0; i < 12; i++){
        std::cout << fact<12> << std::endl;
    }
}

But when I change it to this, I get the expected result.

int main()
{   
    for(int i = 0; i < 12; i++){
        std::cout << *(&fact<12> - i) << std::endl;
    }
}

Is this Undefined Behavior? It's working as expected on GCC 8.3. Live example here

Answer 1

It is UB. Your pointer arithmetic "works by chance" (with UB anything can happen).

You might do, instead, for example:

template <std::size_t ... Is>
void print_fact(std::index_sequence<Is...>)
{
    for (int res : {fact<Is>...}) {
        std::cout << res << std::endl;
    }
}

int main()
{
    print_fact(std::make_index_sequence<12>());
}