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Re: Make non-specialized numeric_limits<T> use ill-formed

From: Michael Hava <mfh_at_[hidden]>
Date: Tue, 14 Apr 2020 13:16:16 +0000
Thanks! I wasn’t aware of P0437.

From: John McFarlane <john_at_[hidden]>
Sent: Tuesday, April 14, 2020 11:54 AM
To: std-proposals_at_[hidden]
Cc: Michael Hava <mfh_at_[hidden]>
Subject: Re: [std-proposals] Make non-specialized numeric_limits<T> use ill-formed

On Mon, 13 Apr 2020 at 15:33, Michael Hava via Std-Proposals <std-proposals_at_[hidden]<mailto:std-proposals_at_[hidden]>> wrote:
IMHO: The definition of numeric_limits is rather unfortunate (is_specialized, min vs lowest, countless members that have no meaning for certain types, …), but I seriously doubt that we can/should try to change it after 20+ years…

If anything can be done in this area, it should most probably be a new facility for these limits that follows a more modern (read: non-monolithic) design.

See P0437R1<http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p0437r1.pdf> and P1370R1<http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p1370r1.html> for work in this direction.

Note: type_traits already exposes some of the functionality of numeric_limits (is_signed, is_integral, …).

Best regards,

Food for thought: maybe a new limits-system should include support for “custom arithmetic types” (e.g. complex,…)

It should although numeric_limits does allow custom specialization.


From: Std-Proposals <std-proposals-bounces_at_[hidden]<mailto:std-proposals-bounces_at_[hidden]>> On Behalf Of Thomas Mercier via Std-Proposals
Sent: Friday, April 10, 2020 8:27 PM
To: std-proposals_at_[hidden]<mailto:std-proposals_at_[hidden]>
Cc: Thomas Mercier <thomas.mercier.jr_at_[hidden]<mailto:thomas.mercier.jr_at_[hidden]>>
Subject: [std-proposals] Make non-specialized numeric_limits<T> use ill-formed


I encountered some surprising behavior from the std::numeric_limits<T> class template when experimenting with std::byte. The integer representation of the maximum std::byte value is 0 according to std::numeric_limits<std::byte>::max(). That is because there is no specialization of std::numeric_limits<T> for std::byte, because std::byte is not an arithmetic type. Ok, fine. But the fact that the program compiles, and produces an unexpected value is worrisome!

The standard specifies that "The default numeric_­limits<T> template shall have all members, but with 0 or false values." (https://eel.is/c++draft/numeric.limits#3)

I would prefer to see this say something like, "a program which uses the default numeric_limits<T> template is ill-formed", so that an error is produced at compile time rather than a value initialized result (https://github.com/gcc-mirror/gcc/blob/master/libstdc%2B%2B-v3/include/std/limits#L321 and https://github.com/llvm/llvm-project/blob/master/libcxx/include/limits#L150).

Does anyone know why the current wording specifies "0 or false values", or what any objections to my suggested change might be?


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Received on 2020-04-14 08:19:13