template Metaprogramming: multiplying a bunch of template arguments

Question

I need to compute the product of a bunch of numbers at compile time passed to a templated struct. I succeeded to make an ugly solution :

template<std::size_t n1, std::size_t ...args>
struct mul_all
{
    static constexpr std::size_t value = n1 * mul_all<args...>;
};
template<>
struct mul_all<0>
{
    static constexpr std::size_t value = 1;
};

The problem is that each time I have to feed 0 to template args to my struct like so

int main()
{
    std::cout <<  mul_all<1,2,5,4,5,7,0>::value << " " 
              <<  mul_all<4,2,0>::value;
    return 0;
}

is there any workaround to get read of that last zero?

note: I am a beginner in TMP.

Answer 1

In C++17, with folding expression, you may directly do

template<std::size_t ...args>
struct mul_all
{
    static constexpr std::size_t value = (args * ...);
};

Before, you have to do the partial specialization:

template<std::size_t n1, std::size_t ...args>
struct mul_all
{
    static constexpr std::size_t value = n1 * mul_all<args...>::value;
};

template<std::size_t n>
struct mul_all<n>
{
    static constexpr std::size_t value = n;
};

Answer 2

You need to replace your specialization with:

template<std::size_t n1, std::size_t ...args>
struct mul_all
{
    static constexpr std::size_t value = n1 * mul_all<args...>::value;
};

template<std::size_t n>
struct mul_all<n>
{
    static constexpr std::size_t value = n;
};

Answer 3

One way is to specialize for empty varargs. For that you need the main template to be variadic args only:

// main template never used
template<std::size_t ...args> struct mul_all
{
};

// specialization for at least one arg
template<std::size_t n1, std::size_t ...args>
struct mul_all<n1, args...>
{
    static constexpr std::size_t value = n1 * mul_all<args...>::value;
};

// specialization for empty args
template<>
struct mul_all<>
{
    static constexpr std::size_t value = 1;
};

So now you can do:

mul_all<1, 2, 3>::value;

Answer 4

The C++17 approach make that nice and simple:

template <std::size_t... A>
constexpr std::size_t mul = (A * ... * std::size_t(1u));

int main() {
    constexpr std::size_t val = mul<1, 2, 3, 4>;
}

For existing C++ versions you'll need to partially specialize the case mul<v>:

template <std::size_t... V>  struct mul;
template <std::size_t V> struct mul {
    statuc constexpr std::size_t value = V;
};
template <std::size_t V, std::size_t... T> struct mul {
    statuc constexpr std::size_t value = V * mul<T...>::value;
};
template <std::size_t... V>
using mul_v = mul<V...>::value;

int main() {
    constexpr std::size_t v = mul_v<1, 2, 3, 4>;
}