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Subject: Re: What is the difference between "Throws: Nothing." and noexcept(true)
From: Daniel Krügler (daniel.kruegler_at_[hidden])
Date: 2020-07-15 06:16:29


Am Mi., 15. Juli 2020 um 11:18 Uhr schrieb Peter Sommerlad (C++) via
Std-Discussion <std-discussion_at_[hidden]>:
>
> Hi,
>
> while looking at <charconv> I was wondering why the from_chars() and
> to_chars() are not noexcept. I understand that noexcept might lead to
> worse code generation because of std::terminate() might need to be called.
>
> Is that the sole reason for not making those functions noexcept(true) ?
>
> TIA & Stay Healthy & Regards
> Peter.
> --
> Peter Sommerlad

Giuseppe already pointed the reason out, but let me extend a bit on
that: Our historic agreement and policy were that functions should be
only marked as (unconditionally) noexcept, when they have a wide
contract (http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2011/n3279.pdf).
The idea for this policy was that for such a function the
implementation should decide whether it will mark that function as
noexcept or not to provide for implementation freedom how to handle
precondition violations. This was never a strict, binding rule,
because we always have allowed exceptions of that rule, e.g. if we
considered specific functions as sufficiently low-level we sometimes
declared the relevant functions as noexcept regardless, but pointed
that deviation out (at least during discussions).

We recently questioned the strict interpretation of the convention
described by N3279 a bit, because we have now years of practical
experience with noexcept, and that should make it at least a bit
easier to question some historic decisions by "narrowing-down" to a
concrete unconditional noexcept specifier. If there exists evidence
for a good candidate to specify noexcept even in the presence of
narrowing contracts, this should be handled by an LWG issue.

Thanks,

- Daniel


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