This page is a snapshot from the LWG issues list, see the Library Active Issues List for more information and the meaning of Resolved status.

2561. [fund.ts.v2] Incorrect exception specifications for 'swap' in C++ Extensions for Library Fundamentals

Section: 5.3.4 [fund.ts.v2::optional.object.swap], 3.7.8 [fund.ts.v2::propagate_const.modifiers] Status: Resolved Submitter: Daniel Krügler Opened: 2015-11-14 Last modified: 2022-07-28

Priority: 3

View all issues with Resolved status.

Discussion:

Addresses: fund.ts.v2

As pointed out in N4511, the Library fundamentals are affected by a similar problem as described in LWG 2456⁽ⁱ⁾. First, it is caused by optional's member swap (5.3.4 [fund.ts.v2::optional.object.swap]):

void swap(optional<T>& rhs) noexcept(see below);

with

The expression inside noexcept is equivalent to:
is_nothrow_move_constructible_v<T> && noexcept(swap(declval<T&>(), declval<T&>()))

Again, the unqualified lookup for swap finds the member swap instead of the result of a normal argument-depending lookup, making this ill-formed.

A second example of such a problem recently entered the arena with the addition of the propagate_const template with another member swap (3.7.8 [fund.ts.v2::propagate_const.modifiers]):

constexpr void swap(propagate_const& pt) noexcept(see below);
-2- The constant-expression in the exception-specification is noexcept(swap(t_, pt.t_)).

A working approach is presented in N4511. By adding a new trait to the standard library and referencing this by the library fundamentals (A similar approach had been applied in the file system specification where the quoted manipulator from C++14 had been referred to, albeit the file system specification is generally based on the C++11 standard), optional's member swap exception specification could be rephrased as follows:

The expression inside noexcept is equivalent to:

is_nothrow_move_constructible_v<T> && is_nothrow_swappable_v<T>noexcept(swap(declval<T&>(), declval<T&>()))

and propagate_const's member swap exception specification could be rephrased as follows:

constexpr void swap(propagate_const& pt) noexcept(see below);
-2- The constant-expression in the exception-specification is is_nothrow_swappable_v<T>noexcept(swap(t_, pt.t_)).

[2016-02-20, Ville comments]

Feedback from an implementation:

libstdc++ already applies the proposed resolution for propagate_const, but not for optional.

[2016-02-20, Daniel comments]

A recent paper update has been provided: P0185R0.

[2016-03, Jacksonville]

Add a link to 2456⁽ⁱ⁾

[2016-11-08, Issaquah]

Not adopted during NB comment resolution

[2020-03-30; Daniel comments]

This has strong overlap with LWG 3413⁽ⁱ⁾, which describes a sub-set of the problem here. Rebasing of the library fundamentals on C++20 has removed the mentioned problem for optionals free swap, so there are now no longer any further free swap function templates with conditionally noexcept specifications except for propagate_const (but now handled by LWG 3413⁽ⁱ⁾).

[2022-07-28 Resolved by P0966R1 and LWG 3413⁽ⁱ⁾. Status changed: New → Resolved.]

Proposed resolution:

pFad - Phonifier reborn

Note: This service is not intended for secure transactions such as banking, social media, email, or purchasing. Use at your own risk. We assume no liability whatsoever for broken pages.

Alternative Proxies:

2561. [fund.ts.v2] Incorrect exception specifications for 'swap' in C++ Extensions for Library Fundamentals

Pfad - The Proxy pFad of © 2024 Garber Painting. All rights reserved.