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Re: [std-proposals] Why some standard functions having a wide contract are not marked as conditionally noexcept?

From: blacktea hamburger <greenteahamburger_at_[hidden]>
Date: Sat, 1 Oct 2022 21:10:46 +0800
But you did not explain why conditional noexcept is not used.

On Mon, Sep 26, 2022 at 1:03 AM Arthur O'Dwyer <arthur.j.odwyer_at_[hidden]>

> On Sun, Sep 25, 2022 at 8:43 AM blacktea hamburger via Std-Proposals <
> std-proposals_at_[hidden]> wrote:
>> Is conditional noexcept only for swap function, move-constructor, or
>> move-assignment operator?
>> However, according to cppreference
>> <https://en.cppreference.com/w/cpp/language/noexcept_spec#Notes>, it is
>> useful for any function templates.
> The noexcept keyword was introduced in order to solve a problem with C++11
> std::vector (as compared to C++98 std::vector). That problem (the "vector
> pessimization") applies only to the move constructor. (Well, and the
> destructor; but C++11 decided to make destructors noexcept by default, and
> the C++11 STL decided not to care at all about types whose destructors
> *weren't* noexcept.)
> It is *traditional* to mark your move-assignment operator and your ADL
> `swap` functions also with `noexcept` (and to make it conditional if you're
> writing a template where those operations are only sometimes noexcept), but
> there is no particular *engineering reason* to do so — nothing in the
> standard library actually cares whether those operations are noexcept or
> not.
> See:
> https://stackoverflow.com/questions/66459162/where-do-standard-library-or-compilers-leverage-noexcept-move-semantics-other-t/66498981#66498981
> https://quuxplusone.github.io/blog/2022/08/26/vector-pessimization/
> The standard library sometimes marks functions as `noexcept` so that they
> don't "get in the way of" the user-programmer's attempts to determine the
> noexceptness of a complicated expression; e.g.
> struct S { int f() noexcept; };
> std::unique_ptr<S> p;
> static_assert(noexcept(p->f())); // OK
> works only because `std::unique_ptr<S>::operator->() const` is marked
> `noexcept` in the library. But this is at least 50% ad-hoc; for example,
> notice that unique_ptr's `operator*` and `operator->`
> <https://en.cppreference.com/w/cpp/memory/unique_ptr/operator*> are
> marked (conditionally) noexcept, but its `operator[]
> <https://en.cppreference.com/w/cpp/memory/unique_ptr/operator_at>` is not.
> std::unique_ptr<S[]> pa;
> static_assert(!noexcept(pa[0].f())); // except on MSVC
> Microsoft's STL applies `noexcept` to unique_ptr::operator[] anyway, as vendors
> are permitted to add noexcept wherever they like
> <https://eel.is/c++draft/conforming#res.on.exception.handling-5>.
> –Arthur

Received on 2022-10-01 13:11:14