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3146. Excessive unwrapping in std::ref/cref

Section: 20.14.6.6 [refwrap.helpers] Status: WP Submitter: Agustín K-ballo Bergé Opened: 2018-07-10 Last modified: 2021-10-17

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<int> ri(i);
std::reference_wrapper<std::reference_wrapper<int>> 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 at N1688, 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 20.14.6.6 [refwrap.helpers] as indicated:

    template<class T> constexpr reference_wrapper<T> ref(reference_wrapper<T> t) noexcept;
    

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

    […]
    template<class T> constexpr reference_wrapper<const T> cref(reference_wrapper<T> t) noexcept;
    

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