basic_string_view::starts_with
Effects are incorrectSection: 27.3.3.8 [string.view.ops] Status: C++20 Submitter: Marshall Clow Opened: 2017-11-29 Last modified: 2021-02-25
Priority: 0
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Discussion:
The effects of starts_with
are described as equivalent to return compare(0, npos, x) == 0
.
false
when you check to see if any sequence begins with the empty sequence.
(There are other failure cases, but that one's easy)
As a drive-by fix, we can make the Effects: for starts_with
and ends_with
clearer.
Those are the second and proposed third changes, and they are not required.
[ 2017-12-13 Moved to Tentatively Ready after 8 positive votes for P0 on c++std-lib. ]
Previous resolution: [SUPERSEDED]This wording is relative to N4713.
Change 27.3.3.8 [string.view.ops] p20 as indicated:
constexpr bool starts_with(basic_string_view x) const noexcept;-20- Effects: Equivalent to:
return size() >= x.size() && compare(0,
nposx.size(), x) == 0;Change 27.3.3.8 [string.view.ops] p21 as indicated:
constexpr bool starts_with(charT x) const noexcept;-21- Effects: Equivalent to:
return !empty() && traits::eq(front(), x)
starts_with(basic_string_view(&x, 1));Change 27.3.3.8 [string.view.ops] p24 as indicated:
constexpr bool ends_with(charT x) const noexcept;-24- Effects: Equivalent to:
return !empty() && traits::eq(back(), x)
ends_with(basic_string_view(&x, 1));
[2018-01-23, Reopening due to a comment of Billy Robert O'Neal III requesting a change of the proposed wording]
The currently suggested wording has:
Effects: Equivalent to:
return size() >= x.size() && compare(0, x.size(), x) == 0;
but compare()
already does the size() >= x.size()
check.
Effects: Equivalent to:
return substr(0, x.size()) == x;
[ 2018-10-29 Moved to Tentatively Ready after 5 positive votes for P0 on c++std-lib. ]
Proposed resolution:
This wording is relative to N4713.
Change 27.3.3.8 [string.view.ops] p20 as indicated:
constexpr bool starts_with(basic_string_view x) const noexcept;-20- Effects: Equivalent to:
return substr(0, x.size()) == x
compare(0, npos, x) == 0;
Change 27.3.3.8 [string.view.ops] p21 as indicated:
constexpr bool starts_with(charT x) const noexcept;-21- Effects: Equivalent to:
return !empty() && traits::eq(front(), x)
starts_with(basic_string_view(&x, 1));
Change 27.3.3.8 [string.view.ops] p24 as indicated:
constexpr bool ends_with(charT x) const noexcept;-24- Effects: Equivalent to:
return !empty() && traits::eq(back(), x)
ends_with(basic_string_view(&x, 1));