Re: [boost] [review][fixed_string] End of review period approaching and a plea

On Wed, Dec 4, 2019 at 1:50 PM Peter Dimov via Boost wrote:

Andrey Semashev wrote:

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On 2019-12-04 18:24, Alexander Grund via Boost wrote:

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I would hope to see close to the same semantics as std::array, which I believe allows data() to return nullptr.

I don't think so:

"There is a special case for a zero-length array (|N == 0|). In that case, array.begin() == array.end(), which is some unique value. The effect of calling front() or back() on a zero-sized array is undefined." from https://en.cppreference.com/w/cpp/container/array

IMO this excludes nullptr as that won't be unique

I don't think iterators from different instances of a container are comparable. IOW, "unique" means distinct from any possible values of valid iterators obtained from this particular container instance.

That might not apply to pointers, though.

Pointers are comparable with ==, and array usually returns reinterpret_cast(this) from data() to satisfy the uniqueness requirement.

Or used to return - I see that libstdc++ returns nullptr now, probably because reinterpret_cast can't be constexpr.

https://github.com/gcc-mirror/gcc/blob/master/libstdc%2B%2B-v3/include/std/a...

Precisely for this reason, but also the uniqueness requirement was on `begin()` and `end()` and not `.data()`: "In the case that N == 0, begin() == end() == unique value. The return value of data() is unspecified." -- Working Draft, http://eel.is/c++draft/array#zero-2 begin() and end() are already restricted library-wide to not be comparable with begin() and end() -- let alone any iterator -- from a container that does not generate them, so cross-container uniqueness has never been a thing. There is an issue open to generate better wording around this to make it more clear: https://cplusplus.github.io/LWG/lwg-active.html#2157