How to pass an empty span object?

Question

Is there a way to pass an empty std::span<int> to a function?

I have a function like below:

bool func( const std::vector<int>& indices )
{
    if ( !indices.empty( ) )
    {
        /* do something */
    }

    ...
}


// when calling it with an empty vector
const bool isAcceptable { func( std::vector<int>( 0 ) ) };

And I want to change it to use std::span instead of std::vector so that it can also get std::array and raw array as its argument.

Now here:

bool func( const std::span<const int> indices )
{
    if ( !indices.empty( ) )
    {
        /* do something */
    }

    ...
}


// when calling it with an empty span
const bool isAcceptable { func( std::span<int>( ) ) }; // Is this valid code?

Also does std::span properly support all contiguous containers (e.g. std::vector, std::array, etc.)?

Answer 1

std::span's default constuctor is documented as:

constexpr span() noexcept;

Constructs an empty span whose data() == nullptr and size() == 0.

Hence, passing a default constructed std::span<int>() is well-defined. Calling empty() on it is guaranteed to return true.

---

Does std::span properly support all contiguous containers (e.g. std::vector, std::array, etc.)?

Basically, std::span can be constructed from anything that models a contiguous and sized range:

template<class R>
explicit(extent != std::dynamic_extent)
constexpr span(R&& range);

Constructs a span that is a view over the range range; the resulting span has size() == std::ranges::size(range) and data() == std::ranges::data(range).

In particular, std::vector does satisfy these requirements.

For C-style arrays and std::array there are special constructors (to harness their compile-time size):

template<std::size_t N>
constexpr span(element_type (&arr)[N]) noexcept;
template<class U, std::size_t N>
constexpr span(std::array<U, N>& arr) noexcept;
template<class U, std::size_t N>
constexpr span(const std::array<U, N>& arr) noexcept;

Constructs a span that is a view over the array arr; the resulting span has size() == N and data() == std::data(arr).