Issue 3146: Excessive unwrapping in std::ref/cref (original) (raw)

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3146. Excessive unwrapping in std::ref/cref

Section: 22.10.6.7 [refwrap.helpers] Status: C++23 Submitter: Agustín K-ballo Bergé Opened: 2018-07-10 Last modified: 2023-11-22

Priority: 3

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Discussion:

The overloads of std::ref/cref that take a reference_wrapper as argument are defined as calling std::ref/cref recursively, whereas the return type is defined as unwrapping just one level. Calling these functions with arguments of multiple level of wrapping leads to ill-formed programs:

int i = 0; std::reference_wrapper ri(i); std::reference_wrapper<std::reference_wrapper> rri(ri); std::ref(rri); // error within 'std::ref'

[Note: these overloads were added by issue resolution 10.29 for TR1, which can be found atN1688, at Redmond 2004]

[2018-08-20 Priority set to 3 after reflector discussion]

[2021-05-22 Tim syncs wording to the current working draft]

[2021-08-20; LWG telecon]

Set status to Tentatively Ready after telecon review.

[2021-10-14 Approved at October 2021 virtual plenary. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4885.

1. Change 22.10.6.7 [refwrap.helpers] as indicated:

   template constexpr reference_wrapper ref(reference_wrapper t) noexcept;

   -3- Returns: ~~ref(t.get())~~t.

   […]

   template constexpr reference_wrapper cref(reference_wrapper t) noexcept;

   -5- Returns: ~~cref(t.get())~~t.