On Tue, Jul 2, 2019 at 1:59 AM Bjorn Reese <breese_at_[hidden]> wrote:

> On 7/1/19 11:47 PM, Jorg Brown wrote:
> > A vector can be empty; a string can be empty; a string_view can be
> > empty; even a std::array can be empty. So why block a std::span from
> > being empty, just because its extent isn't dynamic? I think this would
> > end up blocking some compile-time programming cases.
>
> A span with dynamic extent can be empty.
>
> > Speaking of which, it's annoying that this doesn't currently work, due
> > to data()'s implemenetation within std::array:
> >
> > constexpr const std::array<int, 0> ar0{};
> > constexpr auto spanned_empty_array = std::span<const int, 0>{ar0};
> >
> > See https://godbolt.org/z/L4-9r8
>
> It does not seem to work for begin/end (or cbegin/cend) either when
> N = 0.
>

Sorry, I sent the wrong link; the inclusion of "constexpr" in that program
triggers a bug that needs to be fixed in libc++'s implementation of
std::array. (Specifically, the bug is that std::array::data() isn't always
constexpr) If you delete the constexpr on the second line, it will compile
just fine.

Here's a better longer example => https://godbolt.org/z/rki77g which is
this code, and compiles with -std=c++2a -stdlib=libc++ :

// Ref is useful for seeing (by its function signature) what the type of
something is.
template<typename T> void Ref(T&&);

constexpr const std::array<int, 0> ar0{};
auto spanned_empty_array = std::span /* <int, std::dynamic_extent> */ {ar0};
constexpr int size_of_empty_span = spanned_empty_array.size();

int get_size_of_empty_span() {
  Ref(spanned_empty_array);
  return size_of_empty_span;
}

Note that the parameters to std::span are deduced here - which is the point
I'm making: something that was supposed to have a static zero extent would
now be deduced as having a dynamic extent. If you comment out the template
parameters to span in this code, which forces the std::span to be dynamic,
then the code no longer compiles, because the size of the span is no longer
known at compile time.

-- Jorg