c++ - Non-type variadic function templates in C++11

Question

I saw a blog post which used non-type variadic templates (currently not supported by gcc, only by clang).

template <class T, size_t... Dimensions>
struct MultiDimArray { /* ... */ };

The example in the post compiles fine but I failed to get it to work with function templates.

Can anyone help figuring out the correct syntax (if such exists)?

int max(int n) { return n; } // end condition

template <int... N> // replacing int... with typename... works
int max(int n, N... rest) // !! error: unknown type name 'N'
{
    int tmp = max(rest...);
    return n < tmp? tmp : n;
}

#include <iostream>
int main() 
{
   std::cout << max(3, 1, 4, 2, 5, 0) << std::endl;   
}

Answer 1

You are simply confusing type names and non-type names. What you want simply doesn’t work.

You can probably use variadic non-type templates in functions, but not as (non-template) arguments:

template <int N, int... Rest>
int max()
{
    int tmp = max<Rest...>();
    return N < tmp ? tmp : N;
}

std::cout << max<3, 1, 4, 2, 5, 0>() << std::endl;

… although I haven’t tested this and I’m not sure how this should work given that you need to have a partial specialisation as the base case. You could solve this by dispatching to a partially specialised struct:

template <int N, int... Rest>
struct max_t {
    static int const value = max_t<Rest...>::value > N ? max_t<Rest...>::value : N;
};

template <int N>
struct max_t<N> {
    static int const value = N;
};


template <int... NS>
int max()
{
    return max_t<NS...>::value;
}

This will work.