How to return a POD array reference with size?

Question

I have a C++ 03 class with a header only implementation. The class uses a static null vector shared among all classes:

static const byte nullVector[64];

When I initialized outside the class, linking failed due to duplicate symbols. So I moved it into a function and made is a static local according to How to have static data members in a header-only library?

Now I am trying to return that byte array from the accessor:

static const byte[64]& GetNullVector {
    static const byte s_NullVector[64] = {
        0,0,0,0,0,0,0,0, ... 0,0,0,0,0,0,0,0
    };
    return s_NullVector;
}

While it might look odd trying to return a byte[]&, I need it because of a compile time assert:

COMPILE_ASSERT(DIGEST_SIZE <= COUNTOF(GetNullVector()));

The COUNTOF macro needs a real array, and it fails on pointers. It worked fine when the byte array was a static class member.

How do I return a reference to the byte array, complete with its size so diagnostics continue to work as expected, under C++03?

Thanks in advance.

---

Here's what the compile error looks like. Both return types of static const byte[64] and static const byte[] produce the error.

c++ -DNDEBUG -g2 -O3 -fPIC -march=native -pipe -c validat3.cpp
In file included from validat3.cpp:16:
./hkdf.h:33:19: error: expected member name or ';' after declaration specifiers
        static const byte[]& GetNullVector {
        ~~~~~~~~~~~~~~~~~^
./hkdf.h:58:49: error: use of undeclared identifier 'GetNullVector'
        COMPILE_ASSERT(DIGEST_SIZE <= COUNTOF(GetNullVector()));

Answer 1

The syntax for C arrays sort of wraps around the identifier it's attached to (e.g. int array[64]). When you bring references into it, it gets a little uglier:

int (&array_ref)[64]

and now if you want to return such a reference from a function:

int (& GetNullVector())[64] { ... }

However with some typedefs, you can avoid having to explain this ugly declaration in the next code review ;)

typedef byte null_vec_t[64];

static const null_vec_t& GetNullVector()
{
    static const null_vec_t s_NullVector = {0};
    return s_NullVector;
}