Why does std::span lack cbegin and cend methods?

Question

Why does std::span only have begin and end methods and not their constant iterator counterparts cbegin and cend? (standard)

What I noticed too is that the proposals for span that I could find do have a definition for cbegin and cend: P0122R7. Why was it removed?

Answer 1

As an update, these are added back by P2278R4. The LWG issue pointed out below is still a problem, x.cbegin() and std::cbegin(x) would do different things, but now x.cbegin() and std::ranges::cbegin(x) do the same thing.

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This was removed as a result of LWG3320.

The issue is that x.cbegin() should really do the same thing as std::begin(std::as_const(x)), which is what std::cbegin(x) is defined as.

But that's not the case for std::span, because it doesn't actually own its elements and, as a result, only has shallow const. Given span<int> s;, s.cbegin() would've given you an int const*^† while std::cbegin(s) gives you an int*. That's very inconsistent. Could've kept s.cbegin() while having it just return begin() (as proposed by PL 247), but that's arguably confusing, so the resolution decision was to simply remove all the const members and aliases.

In any case, std::cbegin(s) always works if you want the container itself to be immutable (which isn't an issue with span to begin with).

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^†Technically, implementation defined, not necessary int const*, but this is a useful fiction for explanatory purposes.