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### 3100. Unnecessary and confusing "empty `span`" wording

Section: 24.7.3.2 [span.cons] Status: C++20 Submitter: Stephan T. Lavavej Opened: 2018-04-12 Last modified: 2021-02-25

Priority: 0

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Discussion:

The `span` constructors have wording relics that mention an "empty `span`". It's unnecessary (the behavior is fully specified by the postconditions), but I left it there because I thought it was harmless. It was later pointed out to me that this is actually confusing. Talking about an "empty `span`" implies that there's just one such thing, but `span` permits `empty()` to be `true` while `data()` can vary (being null or non-null). (This behavior is very useful; consider how `equal_range()` behaves.)

To avoid confusion, the "empty `span`" wording should simply be removed, leaving the constructor behavior unchanged. Editorially, there's also a missing paragraph number.

[ 2018-04-24 Moved to Tentatively Ready after 6 positive votes on c++std-lib. ]

[2018-06 Rapperswil: Adopted]

Proposed resolution:

This wording is relative to N4741.

> 1. Edit 24.7.3.2 [span.cons] as indicated:

>    >     constexpr span() noexcept;

>    > > -1- Effects: Constructs an empty `span`.

>    > > -2- Postconditions: `size() == 0 && data() == nullptr`.

>    > > -3- Remarks: This constructor shall not participate in overload resolution unless `Extent <= 0` is `true`.

>    >     constexpr span(pointer ptr, index_type count);

>    > > -4- Requires: `[ptr, ptr + count)` shall be a valid range. If `extent` is not equal to `dynamic_extent`, then `count` shall be equal to `extent`.

>    > > -5- Effects: Constructs a `span` that is a view over the range `[ptr, ptr + count)`. If `count` is `0` then an empty span is constructed.

>    > > -6- Postconditions: `size() == count && data() == ptr`.

>    > > -?- Throws: Nothing.

>    >     constexpr span(pointer first, pointer last);

>    > > -7- Requires: `[first, last)` shall be a valid range. If `extent` is not equal to `dynamic_extent`, then `last - first` shall be equal to `extent`.

>    > > -8- Effects: Constructs a `span` that is a view over the range `[first, last)`. If `last - first == 0` then an empty `span` is constructed.

>    > > -9- Postconditions: `size() == last - first && data() == first`.

>    > > -10- Throws: Nothing.