Movatterモバイル変換

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

4045.tuple can create dangling references fromtuple-like

Section: 22.4.4.2[tuple.cnstr]Status:WPSubmitter: Jonathan WakelyOpened: 2024-01-24Last modified: 2024-04-02

Priority:Not Prioritized

View otheractive issues in [tuple.cnstr].

View all otherissues in [tuple.cnstr].

View all issues withWP status.

Discussion:

P2165R4(Compatibility between tuple, pair and tuple-like objects)added two new constructors tostd::tuple:

  template<tuple-likeUTuple>    constexpr explicit(see below ) tuple(UTuple&& u);

and the allocator-extended equivalent.Unlike the existing constructors taking a single parameter of tuple type,these new constructors are not defined as deleted if they would create adangling reference to a temporary. The existing constructors gained thatrestriction fromP2255R2(A type trait to detect reference binding to temporary)which was approved one meeting beforeP2165R4so LWG seem to have missed the inconsistency.

The proposal also added a new constructor forstd::pair:

  template<pair-like P> constexpr explicit(see below) pair(P&& p);

Thisis deleted if it would create a dangling reference,although that seems to be an almost accidental consequence of addingthe new signature after existing ones which already have theRemarks:about being deleted.

[2024-03-12; Reflector poll]

Set status to Tentatively Ready after eleven votes in favour during reflector poll.

[Tokyo 2024-03-23; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative toN4971.

1. Modify 22.4.4.2[tuple.cnstr] as indicated:

   template<tuple-like UTuple>  constexpr explicit(see below) tuple(UTuple&& u);

   -28-LetI be the pack0, 1, ..., (sizeof...(Types) - 1).

   -29-Constraints:

   1. (29.1) –different-from<UTuple, tuple>(25.5.2[range.utility.helpers]) istrue,
   2. (29.2) –remove_cvref_t<UTuple> is not a specialization ofranges::subrange,
   3. (29.3) –sizeof...(Types) equalstuple_size_v<remove_cvref_t<UTuple>>,
   4. (29.4) –(is_constructible_v<Types, decltype(get<I>(std::forward<UTuple>(u)))> && ...) istrue, and
   5. (29.5) –eithersizeof...(Types) is not 1,or (whenTypes... expands toT)is_convertible_v<UTuple, T> andis_constructible_v<T, UTuple>are bothfalse.

   -30-Effects:For alli,initializes thei^th element of*thiswithget<i>(std::forward<UTuple>(u)).

   -31-Remarks:The expression insideexplicit is equivalent to:

     !(is_convertible_v<decltype(get<I>(std::forward<UTuple>(u))), Types> && ...)

   The constructor is defined as deleted if

     (reference_constructs_from_temporary_v<Types, decltype(get<I>(std::forward<UTuple>(u)))> || ...)

   istrue.