Array count that does not odr-use?

Question

There are several good reasons to prefer

#include <cstdlib>

template<typename T, std::size_t N>
constexpr std::size_t ARRAY_COUNT_FUNC(T (&arr)[N]) { return N; }

rather than

#define ARRAY_COUNT_MACRO(arr) (sizeof(arr)/sizeof(*arr))

One important difference is that when a pointer (not an array) is passed to ARRAY_COUNT_MACRO , it silently returns an unhelpful answer, but passing the same argument to ARRAY_COUNT_FUNC will cause a compiler error pointing out the mistake.

But the macro does have one advantage: its argument is unevaluated.

#include <utility>
struct S {
    int member_array[5];
};

// OK:
std::size_t count1 = ARRAY_COUNT_MACRO(std::declval<S&>().member_array);

// ERROR: std::declval is odr-used!
std::size_t count2 = ARRAY_COUNT_FUNC(std::declval<S&>().member_array);

Is there another approach with the benefits of both together? I. e., something that causes a compile error if the argument is not an array, and does not odr-use its argument.

Answer 1

Shamelessly ripped off from the Chromium project, as described here .

#include <utility>
#include <cstdlib>

template<typename T, std::size_t N>
constexpr std::size_t ARRAY_COUNT_FUNC(T (&arr)[N]) { return N; }

#define ARRAY_COUNT_MACRO(arr) (sizeof(arr)/sizeof(*arr))

// Template for typesafey goodness.
template <typename T, size_t N>
char (&ArraySizeHelper(T (&array)[N]))[N];
// sizeof to avoid actually calling the function.
#define arraysize(array) (sizeof(ArraySizeHelper(array)))

struct S {
    int member_array[5];
};

int main()
{

    // OK:
    std::size_t count1 = ARRAY_COUNT_MACRO(std::declval<S&>().member_array);

    // ERROR: std::declval is odr-used!
    //std::size_t count2 = ARRAY_COUNT_FUNC(std::declval<S&>().member_array);

    // OK:
    std::size_t count2 = arraysize(std::declval<S&>().member_array);

    // ERROR:
    // int * p;
    // std::size_t count3 = arraysize(p);
}

Answer 2

... And then I remembered <type_traits> has a std::is_array template. Another solution:

#include <type_traits>

template<typename T>
constexpr auto ArrayCountImpl(std::nullptr_t)
    -> typename std::enable_if<std::is_array<typename
                                   std::remove_reference<T>::type>::value,
                               std::size_t>::type
{ return std::extent<typename std::remove_reference<T>::type>::value; }

template<typename T>
std::size_t ArrayCountImpl(...)
{ static_assert(!std::is_same<T,T>::value,
                "Argument type is not an array"); }

#define ARRAY_COUNT_MACRO_2(arr) (ArrayCountImpl<decltype(arr)>(nullptr))