Constexpr basic_string_view

[andrewerf]

Checklist

• I did not find a duplicate of this feature request in the Github Issues section.

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Description

The bpstd::basic_string_view uses std::char_traits as a background. This produces a error in a constexpr context:
error: call to non-'constexpr' function 'static std::size_t std::char_traits<char>::length(const char_type*)'.
This is correspond to the standard because char_traits::length is marked constexpr since C++17 only.

P.S. Idk whether it's a bug or a feature request.

[bitwizeshift]

Hi @andrewerf, sorry for not getting back to this sooner. This is a good issue to raise, but unfortunately not one with a clear-cut solution, which is why it is the way it is currently.

As you have noticed, std::char_traits::length is only constexpr in C++17 and above. For string_view to be constexpr, you need a CharTraits argument for basic_string_view that is also constexpr. One option would be for Backport to implement a custom bpstd::char_traits that is constexpr to support this -- but then this introduces a whole new set of issues. The problem arises that std::string::char_traits will not be the same as bpstd::string_view::char_traits, since we would be defining a custom char_traits. Not only would this affect any template magic that detects T::char_traits for correctness, but this will also negatively impact std::string's explicit conversion from string_view.

In C++17, std::basic_string learned support for explicit conversion from any T type where std::is_convertible_v<const T&, std::basic_string_view<CharT, Traits>> is true. To replicate this in Backport as best as possible, I've given basic_string_view an explicit conversion operator to std::basic_string<...,Allocator>. This doesn't perform any T to std::string_view conversions -- but it's a limited/reasonable subset to produce somewhat similar behavior, and it also saves having to implement/wrap all of string in Backport.

If a new bpstd::char_traits type were added -- then this conversion would now be ill-formed because std::char_traits is not the same as bpstd::char_traits. The only way to fix this would either be for Backport to relax conversion requirements -- which can be bad/buggy/unportable to std::string_view, or Backport would need to implement basic_string and have it also be in terms of char_traits -- which is also really not so nice.

Ultimately I figured the best way to work around this is to simply require users to define a constexpr version of char_traits if this were needed at compile-time. This way, in general, string_view works as expected with normal std::string objects -- but if constexpr is needed in pre-c++17, then it's an opt-in choice.

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If you have any other suggested ways of dealing with this outside of what I mentioned above, I would be interested to discuss and possibly implement it -- but otherwise my best recommendation would just be to own a custom constexpr_char_traits that you use when constexpr support is needed.