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### 2861. `basic_string` should require that `charT` match `traits::char_type`

Section: 23.4.3.2 [string.require] Status: C++17 Submitter: United States Opened: 2017-02-02 Last modified: 2020-09-06

Priority: Not Prioritized

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Discussion:

Addresses US 145

There is no requirement that `traits::char_type` is `charT`, although there is a requirement that `allocator::value_type` is `charT`. This means that it might be difficult to honour both methods returning reference (such as `operator[]`) and `charT&` (like `front`/`back`) when traits has a surprising `char_type`. It seems that the allocator should not rebind in such cases, making the reference-returning signatures the problematic ones.

Suggested resolution: Add a requirement that `is_same_v<typename traits::char_type, charT>` is true, and simplify so that value_type is just an alias for `charT`.

[2017-02-02 Marshall adds]

In [string.require]/3, there's already a note that the types shall be the same. In [string.view.template]/1, it says "In every specialization `basic_string_view<charT, traits, Allocator>`, the type `traits` shall satisfy the character traits requirements (21.2), and the type `traits::char_type` shall name the same type as `charT`".

[Kona 2017-02-28]

Accepted as Immediate to resolve NB comment.

Proposed resolution:

Changes are based off of N4618

1. Modify [basic.string] as indicated (in the synopsis):

   >     class basic_string {
   >     public:
   >     // types:
   >       using traits_type      = traits;
   >       using value_type       = charTtypename traits::char_type;
   >       using allocator_type   = Allocator

2. Change [string.require]/3 as indicated:

   -3- In every specialization `basic_string<charT, traits, Allocator>`, the type `allocator_traits<Allocator>::value_type` shall name the same type as `charT`. Every object of type `basic_string<charT, traits, Allocator>` shall use an object of type `Allocator` to allocate and free storage for the contained `charT` objects as needed. The `Allocator` object used shall be obtained as described in 23.2.1. [ Note: In every specialization `basic_string<charT, traits, Allocator>`, the type `traits` shall satisfy the character traits requirements (21.2), and the type `traits::char_type` shall name be the same type as `charT`; see 21.2. — end note ]