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4251. Move assignment for indirect unnecessarily requires copy construction

Section: 20.4.1.5 [indirect.assign], 20.4.2.5 [polymorphic.assign] Status: WP Submitter: Jonathan Wakely Opened: 2025-05-01 Last modified: 2025-11-11

Priority: 1

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Discussion:

The move assignment operator for indirect says:

Mandates: is_copy_constructible_t<T> is true.

constructs a new owned object with the owned object of other as the argument as an rvalue

It seems like we should require is_move_constructible_v instead, and only if the allocator traits mean we need to construct an object. (Technically move-constructible might not be correct, because the allocator's construct member might use a different constructor).

Additionally, the noexcept-specifier for the move assignment doesn't match the effects. The noexcept-specifier says it can't throw if POCMA is true, but nothing in the effects says that ownership can be transferred in that case; we only do a non-throwing transfer when the allocators are equal. I think we should transfer ownership when POCMA is true, which would make the noexcept-specifier correct.

[2025-06-12; Reflector poll]

Set priority to 1 after reflector poll.

Similar change needed for std::polymorphic.

Previous resolution [SUPERSEDED]:

This wording is relative to N5008.

1. Modify 20.4.1.5 [indirect.assign] as indicated:

   
   constexpr indirect& operator=(indirect&& other)
     noexcept(allocator_traits<Allocator>::propagate_on_container_move_assignment::value ||
              allocator_traits<Allocator>::is_always_equal::value);
   

   -5- Mandates: If allocator_traits<Allocator>::propagate_on_container_move_assignment::value is false and allocator_traits<Allocator>::is_always_equal::value is false, is_copymove_constructible_t<T> is true.

   -6- Effects: If addressof(other) == this is true, there are no effects. Otherwise:

   1. (6.1) — The allocator needs updating if allocator_traits<Allocator>::propagate_on_container_move_assignment::value is true.
   2. (6.2) — If other is valueless, *this becomes valueless and the owned object in *this, if any, is destroyed using allocator_traits<Allocator>::destroy and then the storage is deallocated.
   3. (6.3) — Otherwise, if the allocator needs updating or if alloc == other.alloc is true, swaps the owned objects in *this and other; the owned object in other, if any, is then destroyed using allocator_traits<Allocator>::destroy and then the storage is deallocated *this takes ownership of the owned object of other.
   4. (6.4) — Otherwise, constructs a new owned object with the owned object of other as the argument as an rvalue, using either the allocator in *this or the allocator in other if the allocator needs updating.
   5. (6.5) — The previously owned object in *this, if any, is destroyed using allocator_traits<Allocator>::destroy and then the storage is deallocated.
   6. (6.6) — If the allocator needs updating, the allocator in *this is replaced with a copy of the allocator in other.

   -7- Postcondition: other is valueless.

[2025-11-03; Tomasz provides wording.]

[Kona 2025-11-03; approved by LWG. Status changed: New → Immediate.]

[Kona 2025-11-08; Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N5014.

1. Modify 20.4.1.5 [indirect.assign] as indicated:

   
   constexpr indirect& operator=(indirect&& other)
     noexcept(allocator_traits<Allocator>::propagate_on_container_move_assignment::value ||
              allocator_traits<Allocator>::is_always_equal::value);
   

   -5- Mandates: If allocator_traits<Allocator>::propagate_on_container_move_assignment::value is false and allocator_traits<Allocator>::is_always_equal::value is false, is_copymove_constructible_t<T> is true.

   -6- Effects: If addressof(other) == this is true, there are no effects. Otherwise:

   1. (6.1) — The allocator needs updating if allocator_traits<Allocator>::propagate_on_container_move_assignment::value is true.
   2. (6.2) — If other is valueless, *this becomes valueless and the owned object in *this, if any, is destroyed using allocator_traits<Allocator>::destroy and then the storage is deallocated.
   3. (6.3) — Otherwise, if the allocator needs updating or if alloc == other.alloc is true, swaps the owned objects in *this and other; the owned object in other, if any, is then destroyed using allocator_traits<Allocator>::destroy and then the storage is deallocated *this takes ownership of the owned object of other.
   4. (6.4) — Otherwise, constructs a new owned object with the owned object of other as the argument as an rvalue, using either the allocator in *this or the allocator in other if the allocator needs updating.
   5. (6.5) — The previously owned object in *this, if any, is destroyed using allocator_traits<Allocator>::destroy and then the storage is deallocated.
   6. (6.6) — If the allocator needs updating, the allocator in *this is replaced with a copy of the allocator in other.

   -7- Postcondition: other is valueless.

2. Modify 20.4.2.5 [polymorphic.assign] as indicated:

   
   constexpr polymorphic& operator=(polymorphic&& other)
     noexcept(allocator_traits<Allocator>::propagate_on_container_move_assignment::value ||
              allocator_traits<Allocator>::is_always_equal::value);
   

   -5- Mandates: If allocator_traits<Allocator>::propagate_on_container_move_assignment::value is false and allocator_traits<Allocator>::is_always_equal::value is false, T is complete type.

   -6- Effects: If addressof(other) == this is true, there are no effects. Otherwise:

   1. (6.1) — The allocator needs updating if allocator_traits<Allocator>::propagate_on_container_move_assignment::value is true.
   2. (6.?) — If other is valueless, *this becomes valueless.
   3. (6.2) — Otherwise, if the allocator needs updating orIf alloc == other.alloc is true, swaps the owned objects in *this and other; the owned object in other, if any, is then destroyed using allocator_traits<Allocator>::destroy and then the storage is deallocated *this takes ownership of the owned object of other.
   4. (6.3) — Otherwise, if alloc != other.alloc is true; if other is not valueless, a new owned object is constructed in *this using allocator_traits::construct with the owned object from Otherwise, constructs a new owned object with the owned object of other as the argument as an rvalue, using either the allocator in *this or the allocator in other if the allocator needs updating.
   5. (6.4) — The previously owned object in *this, if any, is destroyed using allocator_traits<Allocator>::destroy and then the storage is deallocated.
   6. (6.5) — If the allocator needs updating, the allocator in *this is replaced with a copy of the allocator in other.

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