A workaround for partial specialization of function template?

Question

Consider the following metafunction for an integral pow (it is just an example) :

class Meta
{
    template<int N, typename T> static constexpr T ipow(T x)
    {
        return (N > 0) ? (x*ipow<N-1>(x)) 
                       : ((N < 0) ? (static_cast<T>(1)/ipow<N>(x)) 
                                  : (1))
    }
};

How to write the stop condition for such a function ?

Answer 1

Anytime you ask yourself "how to simulate partial specialization for functions", you can think "overload, and let partial ordering decide what overload is more specialized".

template<int N>
using int_ = std::integral_constant<int, N>;

class Meta
{
    template<int N, typename T> static constexpr T ipow(T x)
    {
        return ipow<N, T>(x, int_<(N < 0) ? -1 : N>());
    }

    template<int N, typename T> static constexpr T ipow(T x, int_<-1>)
    {
        //                             (-N) ??
        return static_cast<T>(1) / ipow<-N>(x, int_<-N>());
    }

    template<int N, typename T> static constexpr T ipow(T x, int_<N>)
    {
        return x * ipow<N-1>(x, int_<N-1>());
    }

    template<int N, typename T> static constexpr T ipow(T x, int_<0>)
    {
        return 1;
    }
};

I think you wanted to pass -N instead of N at the comment-marked position.

Answer 2

A simple version might go like this:

template <typename T, unsigned int N> struct pow_class
{
    static constexpr T power(T n) { return n * pow_class<T, N - 1>::power(n); }
};

template <typename T> struct pow_class<T, 0>
{
    static constexpr T power(T) { return 1; }
};

template <unsigned int N, typename T> constexpr T static_power(T n)
{
    return pow_class<T, N>::power(n);
}

Usage:

auto p = static_power<5>(2);  // 32

Answer 3

Just use static members in a class template and specialize the class template. You might want to create a forwarding function template for convenience, though.