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### 2742. Inconsistent `string` interface taking `string_view`

Section: 23.4.3.3 [string.cons] Status: C++17 Submitter: Richard Smith Opened: 2016-07-06 Last modified: 2020-09-06

Priority: 1

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Discussion:

Generally, `basic_string` has a constructor matching each `assign` function and vice versa (except the constructor takes an allocator where `assign` does not). P0254R2 violates this by adding an `assign(basic_string_view, size_type pos, size_type n = npos)` but no corresponding constructor.

[2016-08-04 Chicago LWG]

Robert Douglas provides initial wording.

We decided against another constructor overload to avoid the semantic confusion between:

>     basic_string(char const*, size_type length, Allocator = Allocator())

and

>     template<class T, class Foo = is_convertible_v<T const&, basic_string_view<charT, traits>>
>     basic_string(T const&, size_type pos, Allocator = Allocator())

where someone might call:

>     basic_string("HelloWorld", 5, 5);

and get "World", but then call

>     basic_string("HelloWorld", 5);

and instead get "Hello". The second parameter changes between length and position.

[08-2016, Chicago]

Fri PM: Move to Tentatively Ready

Proposed resolution:

This wording is relative to N4606.

1. In 23.4.3 [basic.string] add the following constructor overload:

   >     […]
   >     basic_string(const basic_string& str, size_type pos,
   >                  const Allocator& a = Allocator());
   >     basic_string(const basic_string& str, size_type pos, size_type n,
   >                  const Allocator& a = Allocator());
   >     template<class T>
   >     basic_string(const T& t, size_type pos, size_type n, const Allocator& a = Allocator());
   >     explicit basic_string(basic_string_view<charT, traits> sv,
   >                           const Allocator& a = Allocator());
   >     […]

2. In 23.4.3.3 [string.cons] add the following ctor definition:

   >     template<class T>
   >     basic_string(const T& t, size_type pos, size_type n, const Allocator& a = Allocator());

   > > -?- Effects: Creates a variable, `sv`, as if by `basic_string_view<charT, traits> sv = t;` and then behaves the same as:

   > > >     basic_string(sv.substr(pos, n), a)

   > > -?- Remarks: This constructor shall not participate in overload resolution unless `is_convertible_v<const T&, basic_string_view<charT, traits>>` is `true`.