20 Memory management library [mem]

20.4 Types for composite class design [mem.composite.types]

20.4.2 Class template polymorphic [polymorphic]

20.4.2.5 Assignment [polymorphic.asgn]

constexpr polymorphic& operator=(const polymorphic& other);

Mandates: T is a complete type.

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

Otherwise:

(2.1)
The allocator needs updating if allocator_traits<Allocator>::propagate_on_container_copy_assignment::value is true.
(2.2)
If other is not valueless, a new owned object is constructed in *this using allocator_traits<Allocator>::construct with the owned object from other as the argument, using either the allocator in *this or the allocator in other if the allocator needs updating.
(2.3)
The previously owned object in *this, if any, is destroyed using allocator_traits<Allocator>::
destroy and then the storage is deallocated.
(2.4)
If the allocator needs updating, the allocator in *this is replaced with a copy of the allocator in other.

Returns: A reference to *this.

Remarks: If any exception is thrown, there are no effects on *this.

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

Mandates: If allocator_traits<Allocator>::is_always_equal::value> is false, T is a complete type.

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

Otherwise:

(6.1)
The allocator needs updating if allocator_traits<Allocator>::propagate_on_container_move_assignment::value is true.
(6.2)
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.
(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<Allocator>::construct with the owned object from other as the argument as an rvalue, using either the allocator in *this or the allocator in other if the allocator needs updating.
(6.4)
The previously owned object in *this, if any, is destroyed using allocator_traits<Allocator>::
destroy and then the storage is deallocated.
(6.5)
If the allocator needs updating, the allocator in *this is replaced with a copy of the allocator in other.

Returns: A reference to *this.

Remarks: If any exception is thrown, there are no effects on *this or other.