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Re: [PATCH] Add constexpr to iterator adaptors, array and range access


2016-07-16 1:09 GMT+02:00 Jonathan Wakely <jwakely@redhat.com>:
> This patch implements http://wg21.link/p0031 which adds constexpr to
> most operations on std::reverse_iterator, std::move_iterator,
> std::array, as well as std::advance, std::distance, and the
> range-access functions std::begin, std::rbegin etc.
>
> Strictly speaking, those functions should only be constexpr in C++17
> and not before, but this patch makes them constexpr whenever possible.
> That means the changes are fully implemented for C++14 (and the
> feature-test macro is defined) but only partially implemented for
> C++11, because some of the functions can't be constexpr in C++11.
>
> My thinking is that if the committee has decided that these functions
> *should* be constexpr, and changed them for C++17, then it doesn't
> serve users to make them non-constexpr in C++11 and C++14 just because
> the standard says so.
>
> How do other people feel about that?
>
> The alternative would be to define a new _GLIBCXX17_CONSTEXPR macro
> and use it in all these places, so they're only constexpr in C++17
> (and probably for -std=gnu++14 too, but not -std=c++14).
>
> How strict do we want to be about obeying the "implementors can't add
> constexpr" rule in these cases?

As much as I hate the current restrictions, I tend to suggest to
follow them strictly in __STRICT_ANSI__ mode.

> Here's the patch, but I'm not committing it yet.

Since I made a similar (unintentional) omission recently myself:
Shouldn't you touch std::__debug::array as well? What about other
functions from std::__debug?

- Daniel


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