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### 2476. `scoped_allocator_adaptor` is not assignable

Section: 20.5.1 [allocator.adaptor.syn] Status: C++17 Submitter: Jonathan Wakely Opened: 2015-03-02 Last modified: 2017-07-30

Priority: 0

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Discussion:

The class definition in 20.5.1 [allocator.adaptor.syn] declares a move constructor, which means that the copy assignment operator is defined as deleted, and no move assignment operator is declared.

This means a `scoped_allocator_adaptor` is not assignable, and a container using `scoped_allocator_adaptor<A...>` may not be `CopyAssignable` or `MoveAssignable` (depending on the `propagate_on_container_xxxx_assignment` traits of the outer and inner allocator types).

[2015-04-03 Howard comments]

If the contained allocators are not assignable, I think we need the ability of `= default` to automagically become `= delete`. My concern is that `is_copy_assignable<scoped_allocator_adaptor<CustomAllocator>>::value` get the right answer for both cases:

1. `is_copy_assignable<CustomAllocator>::value` is `true`.

2. `is_copy_assignable<CustomAllocator>::value` is `false`.

If we allow the vendor to declare and provide the copy assignment operator, the chance of getting #2 correct goes to zero.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4296.

> 1. Add to the synopsis in 20.5.1 [allocator.adaptor.syn]/1 [Editorial remark: The proposed wording does not explicitly specify the semantics of the added copy/move assignment operators, based on 16.3.3.4 [functions.within.classes] p1, which says:

>    "For the sake of exposition, Clauses 18 through 30 and Annex D do not describe copy/move constructors, assignment operators, or (non-virtual) destructors with the same apparent semantics as those that can be generated by default (12.1, 12.4, 12.8)."

>    — end remark]:

>    >     […]
>    >     template <class OuterA2>
>    >       scoped_allocator_adaptor(
>    >         scoped_allocator_adaptor<OuterA2, InnerAllocs...>&& other) noexcept;

>    >     scoped_allocator_adaptor& operator=(const scoped_allocator_adaptor&);
>    >     scoped_allocator_adaptor& operator=(scoped_allocator_adaptor&&);

>    >     ~scoped_allocator_adaptor();
>    >     […]

[2015-05, Lenexa]

Move to Immediate.

Proposed resolution:

This wording is relative to N4296.

1. Add to the synopsis in 20.5.1 [allocator.adaptor.syn]/1:

   >     […]
   >     template <class OuterA2>
   >       scoped_allocator_adaptor(
   >         scoped_allocator_adaptor<OuterA2, InnerAllocs...>&& other) noexcept;

   >     scoped_allocator_adaptor& operator=(const scoped_allocator_adaptor&) = default;
   >     scoped_allocator_adaptor& operator=(scoped_allocator_adaptor&&) = default;

   >     ~scoped_allocator_adaptor();
   >     […]