What's the purpose of std::dynamic_extent in std::span

Question

I know std::span is static. It is just view over bunch of vector/array/etc. elements.

I see constructors of span, and it seems like std::dynamic_extent is used in 4-6. But in those constructors, there is a required template parameter for the size - std::size_t N. To me this means that the size/count/len is known at compile time. So what really is std::dynamic_extent?

Is the answer something more than “you can use `std::span` with _or without_ a size known at compile time”? — Davis Herring, Aug 04 '23 at 22:31
@DavisHerring I think they're asking how `std::span` can have a dynamic extent when the size template parameter `std::size_t N` always needs to be specified at compile time. — Brian61354270, Aug 04 '23 at 22:46
yep @DavisHerring , that's exactly I was wondering. Confusing API! — code muncher, Aug 06 '23 at 17:04

Brian61354270 · Accepted Answer · 2023-08-04T22:48:56.677

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The definition of is std::dynamic_extent is

inline constexpr std::size_t dynamic_extent 
    = std::numeric_limits<std::size_t>::max();

It's a special value of a std::size_t that's used to indicate that the std::span has a dynamic extent.

There is a required template argument for the size - std::size_t N. To me this means that the size/count/len is known at compile time.

The "size" of the std::span is still specified at compile time, it's just that when the "size" takes on that special value, it's treated as a dynamic extent.

edited Aug 04 '23 at 22:48

answered Aug 04 '23 at 22:37

Brian61354270

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Ah that's one confusing API. Thx – code muncher Aug 06 '23 at 17:14

score 0 · Answer 2 · answered Aug 05 '23 at 16:43

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The constructor would set the initial size, but when span has dynamic_extent, its size might evolve

std::array<int, 4> arr{1, 2, 3, 4};
std::span<int> s{arr}; // whole array view     // size is 4
s = s.subspan(1, 2)}; // restricted array view // size is 2 now

answered Aug 05 '23 at 16:43

Jarod42

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In that case my new `s` has again new size - 2. If I had: `auto s2 = s.subspan(1,2)` then s2 would be static_extent. (I should try this!) – code muncher Aug 06 '23 at 17:03
`s.subspan<1, 2>()` would return static extent. `s.subspan(1, 2)` returns dynamic extent. – Jarod42 Aug 06 '23 at 18:29
yep! I see now. – code muncher Aug 06 '23 at 20:44

What's the purpose of std::dynamic_extent in std::span

2 Answers2