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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()));
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jww
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    `static const byte[]&` is not a type. You can't have a reference to "an array of an unknown number of things". The dimension is part of an array type. As an aside, hacking your design to crap just so you can write one compile-time assertion seems like a total false economy. Switch to a normal assert or something, then use a vector like everyone else. – Lightness Races in Orbit Sep 27 '15 at 15:18
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    array references are a little ugly, but it can be done: `static const byte (& GetNullVector())[64] { ... }`. You can use a typedef to make it less horrible :) – melak47 Sep 27 '15 at 15:19
  • Thanks Lightness Races in Orbit. `static const byte[64]&` produces the same error. – jww Sep 27 '15 at 15:20
  • Unfortunately, you cannot work with arrays this way. The following link might be useful: http://stackoverflow.com/questions/4523497/typedef-fixed-length-array – Andrey Nasonov Sep 27 '15 at 15:22
  • @melak47 - that has got to be the most convoluted declaration I have seen (but I lack acumen of many seasoned C++ folks, like Matt, you or LRO). The C++ committee should be embarrassed for producing that nearly unreadable crap. But it worked, so you should answer.... – jww Sep 27 '15 at 15:24
  • PS, does your C++03 compiler come with TR1? Then you could use `std::tr1::array` and avoid this altogether. – melak47 Sep 27 '15 at 15:34
  • Oh man... This project supports compilers going back to about the year 2000 (literally). That's GCC 2.95 and Visual Studio 6 territory. `std::tr1::array` probably won't work with the portability we need (but I often wish I had that `array`)... – jww Sep 27 '15 at 15:41

1 Answers1

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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;
}
melak47
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  • And thanks for the `typedef`. I did not think to use one because the concept seemed so simple to me.... – jww Sep 27 '15 at 15:47