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basic_string_view
should allow explicit conversion when only traits varySection: 23.3.3.2 [string.view.cons] Status: C++23 Submitter: Casey Carter Opened: 2023-01-10 Last modified: 2023-11-22
Priority: Not Prioritized
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Discussion:
basic_string_view
has a constructor that converts appropriate contiguous ranges to basic_string_view
.
This constructor accepts an argument by forwarding reference (R&&
), and has several constraints
including that specified in 23.3.3.2 [string.view.cons]/12.6:
if the qualified-id
remove_reference_t<R>::traits_type
is valid and denotes a type,is_same_v<remove_reference_t<R>::traits_type, traits>
istrue
.
This constraint prevents conversions from basic_string_view<C, T1>
and basic_string<C, T1, A>
to basic_string_view<C, T2>
. Preventing such seemingly semantic-affecting conversions from happening
implicitly was a good idea, but since the constructor was changed to be explicit it no longer seems necessary to
forbid these conversions. If a user wants to convert a basic_string_view<C, T2>
to
basic_string_view<C, T1>
with static_cast<basic_string_view<C, T1>>(meow)
instead of by writing out basic_string_view<C, T1>{meow.data(), meow.size()}
that seems fine to me.
Indeed, if we think conversions like this are so terribly dangerous we probably shouldn't be performing them ourselves
in 22.14.8.1 [format.arg]/9 and 22.14.8.1 [format.arg]/10.
[2023-02-01; Reflector poll]
Set status to Tentatively Ready after six votes in favour during reflector poll.
[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4917.
Modify 23.3.3.2 [string.view.cons] as indicated:
template<class R> constexpr explicit basic_string_view(R&& r);-11- Let
-12- Constraints:d
be an lvalue of typeremove_cvref_t<R>
.
(12.1) —
remove_cvref_t<R>
is not the same type asbasic_string_view
,(12.2) —
R
modelsranges::contiguous_range
andranges::sized_range
,(12.3) —
is_same_v<ranges::range_value_t<R>, charT>
istrue
,(12.4) —
is_convertible_v<R, const charT*>
isfalse
, and(12.5) —
d.operator ::std::basic_string_view<charT, traits>()
is not a valid expression, and.
(12.6) — if the qualified-idremove_reference_t<R>::traits_type
is valid and denotes a type,is_same_v<remove_reference_t<R>::traits_type, traits>
istrue
.