sizeof in variadic template c++

Question

I need to know how many items in parameter pack of a variadic templete.

my code:

#include <iostream>

using namespace std;



template <int... Entries>
struct StaticArray
{

  int size  = sizeof... (Entries);// line A
  //int array[size] = {Entries...};// line B

};


int main()
{
   StaticArray<1,2,3,4> sa;
   cout << sa.size << endl; 

   return 0;
}

I got compilation error on line A.
if change this line to

static const unsigned short int size = sizeof...(Arguments)

It can be compiled. my first question is why I need "static const unsigned short" to get compiled. as you can see, I need a size to put in on my array. my final goal is able to print this array out in main function.

please help. thanks.. my ideal comes from this website but i dont know how to make it works http://thenewcpp.wordpress.com/2011/11/23/variadic-templates-part-1-2/

Answer 1

As per the comments, I think this is a bug in g++'s handling of the new in-class initialisation of member variables. If you change the code to

template <int... Entries>
struct StaticArray
{
    static const int size = sizeof...(Entries); // works fine
};

then it works correctly, because this uses the C++03 special case of initialising static const integral members in-class.

Similarly, if you use the new C++11 uniform initialisation syntax, it works correctly:

template <int... Entries>
struct StaticArray
{
    int size{sizeof...(Entries)}; // no problem
};

I'm pretty sure the assignment form is valid here, so I think g++ (4.8.2 on my system) is getting it wrong.

(Of course, the size can't change at run-time, so the correct declaration would probably be static constexpr std::size_t size anyway, avoiding this problem...)