Span utility

[chfast]

Implements some parts of std::span<> from C++20.

Taken from #359 and extended for needs in #403.

[codecov]

Codecov Report

Merging #404 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           master     #404    +/-   ##
========================================
  Coverage   99.16%   99.17%            
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  Files          43       45     +2     
  Lines       13043    13165   +122     
========================================
+ Hits        12934    13056   +122     
  Misses        109      109            

[gumb0]

if T is already const, is const needed here or how is it allowed at all?

Suggested change

	constexpr span(const T* begin, std::size_t size) noexcept : m_begin{begin}, m_size{size} {}
	constexpr span(T* begin, std::size_t size) noexcept : m_begin{begin}, m_size{size} {}

[gumb0]

Here I mean it looks at least inconsistent with the m_begin definition above, it should be either both T* or both const T*.
But not sure if it would somehow break anything?

[chfast]

Oh yes, fixed.

[axic]

Is it possible to fall back to native const if compiling on C++20 or it is not worth the hassle (it is not any faster/better than this below)?

[chfast]

I'm not sure I understand the question.

This is compatible with C++20 and you can just start using std::span<const uint64_t>. The C++20 also supports the std::span<uint64_t>.

I disabled the usage of the later, because we don't need it. And also because of the implicit conversions involved. E.g. conversion from const std::vector<uint64_t>& to span<uint64_t> should be disallowed. I prefer not to handle this in the implementation as long as we don't need it.

If by native const you mean const span<uint64_t> it is not useful as an argument type (similar to const int argument type).
I think about span as a reference type similar to pointers:

• span<int> is as int*,
• span<const int> is as const int*,
• const span<int> is as int* const,
• const span<const int> is as const int* const.

[axic]

This is compatible with C++20 and you can just start using std::span. The C++20 also supports the std::span<uint64_t>.

The question was can this header be done in a way such that any code using it will either use the native C++20 implementation or this fallback, depending on compiler settings?

[chfast]

Yes, this can be done. I can do it here after #405.

[axic]

Assuming some things will be squashed.