Variadic templates with constants - c++

Question

I'm doing some experimenting with metaprogramming and variadic templates, and I'm having trouble with some confusing behaviour. I've stripped it down to a minimum working example, but essentially I want to keep track of how many recursive calls I'm making. I want to do this by making the first template parameter an integer, while the second template parameter is a variadic list. In simplest form it looks like this:
template<typename... List>
struct initial_call{
static const int val = next_call<0, List...>::val;
};
template<int D, typename... List>
struct next_call {
static const int val = D;
};
So ignoring the fact that this code is pointless, it doesn't compile on VS2013, claiming "unexpected type 'List", on the line inside the definition of initial_call. Without the integer in front, it works fine. So is there a way to combine variadic templates with integer template parameters?

Answer 1

You might want something like this (counting the number of types):
#include <iostream>
// More than one type: Increment size and consume.
template<size_t N, typename T, typename... List>
struct calculate_list_size {
static const size_t value = calculate_list_size<N + 1, List...>::value;
};
// Last type: Increment size and terminate.
template<size_t N, typename T>
struct calculate_list_size<N, T> {
static const size_t value = N + 1;
};
// Forward to calculate_list_size.
template<typename... List>
struct list_size {
static const size_t value = calculate_list_size<0, List...>::value;
};
// Empty list
template<>
struct list_size<> {
static const size_t value = 0;
};
int main()
{
std::cout << list_size<char, short, int>::value << '\n';
std::cout << list_size<>::value << '\n';
}