Is there a way to enforce full initialization of std::array

Question

I am using std::array<size_t, N> (N is a fixed template-variable).

#include<array>
template<size_t N>
struct A{
   size_t function(std::array<size_t, N> arr){ return arr[N-1];} // just an example
};

int main(){
   A<5> a;
   a.function({{1,2,3,4,5}}));
}

And it works fine. The problem is that this other code is silently allowed:

   A.function({{1,2,3}}));

That is, even with missed elements the array is initialized somehow, even if it is well defined (e.g. remaining elements initialized to zero, I am not sure) this is a possible source of errors.

Is there a way to enforce the initialization of the extra elements? E.g. by generating a compiler error or a warning.

One option that I contemplated is to use initializer_list

   size_t function2(std::initializer_list<size_t> il){ assert(il.size() == N); ...}

The problem is that this generated a runtime error at best and a check in every call. I would prefer a compiler error/warning.

I am not so much bothered by the default initialization of std::array<>{} but by the incomplete initialization. (Maybe there is nothing it can be done about it, since this is inherited from the behavior of T[N] static array.)

I tried using clang 3.5 and gcc 5.

Answer 1

You can enforce this by using parameter pack, but with a bit different syntax:

#include <array>

template<size_t N>
struct A{
   template<typename... T>
   size_t function(T&&... nums)
   {
     static_assert(sizeof...(nums) == N, "Wrong number of arguments");
     std::array<size_t, N> arr = { std::forward<size_t>(nums)... };
     return arr[N-1];
   }
};

int main(){
   A<5> a;
   a.function(1,2,3,4,5); // OK
   a.function(1,2,4,5);   // Compile-time error
}

But, I think there is no good way to enforce that in compile time. I would just use assert(il.size() == N) in production code to check size of initializer list.