This page is a snapshot from the LWG issues list, see the Library Active Issues List for more information and the meaning of C++17 status.
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
View all other issues in [string.cons].
View all issues with C++17 status.
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.
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()); […]
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 bybasic_string_view<charT, traits> sv = t;
and then behaves the same as:-?- Remarks: This constructor shall not participate in overload resolution unlessbasic_string(sv.substr(pos, n), a)is_convertible_v<const T&, basic_string_view<charT, traits>>
istrue
.