What does "except that a default constructed array is not empty" mean?

Question

In N3337, I'm reading §23.3.2.1/3 it states:

An array satisfies all of the requirements of a container and of a reversible container (23.2), except that a default constructed array object is not empty and that swap does not have constant complexity.

In §23.2.1, Table 96 Container Requirements, it shows a default constructed object X u; where the post condition is u.empty(). Presumably then, the following:

std::array<int, 0> a;

should result with a.empty() outputting 1, which it does. So what does "empty" mean here?

Answer 1

You are looking at a border case - is a zero-sized array empty or not? Ignoring that specific question, the standard quote says that a general std::array<T,N> arr, on default construction, does not satisfy the post-condition arr.empty(). In fact, an std::array<T, N> is empty() only when N==0. And this definition is natural as well - std::array<T,N>::size() is a constexpr with value N and empty() should be synonymous with size() == 0.