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improve iota_view's iterator's binary operator+ (LWG-3580) #2417

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Merged
Stephan T. Lavavej (StephanTLavavej) merged 3 commits into from
Dec 17, 2021
Merged

improve iota_view's iterator's binary operator+ (LWG-3580) #2417

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41f2a71
improve iota_view's iterator's binary operator+
fsb4000 Dec 14, 2021
de0c629
correct implementation
fsb4000 Dec 14, 2021
2d520d7
Fix conditional noexcepts
CaseyCarter Dec 16, 2021
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15 changes: 10 additions & 5 deletions stl/inc/ranges
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Original file line number Diff line number Diff line change
Expand Up @@ -1140,16 +1140,21 @@ namespace ranges {
// clang-format on

_NODISCARD friend constexpr _Ioterator operator+(_Ioterator _It, const difference_type _Off) noexcept(
noexcept(static_cast<_Wi>(_It._Current + _Off))) /* strengthened */ requires _Advanceable<_Wi> {
return _Ioterator{static_cast<_Wi>(_It._Current + _Off)};
is_nothrow_move_constructible_v<_Ioterator>&& noexcept(
_It += _Off)) /* strengthened */ requires _Advanceable<_Wi> {
_It += _Off;
return _It;
}
_NODISCARD friend constexpr _Ioterator operator+(const difference_type _Off, _Ioterator _It) noexcept(
noexcept(static_cast<_Wi>(_It._Current + _Off))) /* strengthened */ requires _Advanceable<_Wi> {
is_nothrow_move_constructible_v<_Wi>&& noexcept(

@StephanTLavavej Stephan T. Lavavej (StephanTLavavej) Dec 16, 2021

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Casey Carter (@CaseyCarter) I observe that this one says is_nothrow_move_constructible_v<_Wi> but the other two said is_nothrow_move_constructible_v<_Ioterator>. You mentioned that they're equivalent, and there seems to be a rationale (as this one constructs an _Ioterator prvalue), but I wanted to point this out in case it was unintentional and should be cleaned up later. No change requested here.

@CaseyCarter Casey Carter (CaseyCarter) Dec 17, 2021

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The other two functions return an lvalue denoting a variable with automatic storage duration, so they are implicitly moved, and that variable is a function argument, so they cannot benefit from NRVO or RVO. Consequently they move construct an _Ioterator.

This function returns a prvalue, so no _Ioterators are moved or copied - C++17 delayed temporary materialization kicks in. It does need to account for potential throws from the _Ioterator constructor from _Wi, however. That constructor is annotated with noexcept(is_nothrow_move_constructible_v<_Wi>. I'm not sure why I decided to "inline" that noexcept-specifier instead of using the more direct is_nothrow_constructible_v<_Ioterator, _Wi>. I really need to work at making my conditional noexcept style more consistent.

static_cast<_Wi>(_It._Current + _Off))) /* strengthened */ requires _Advanceable<_Wi> {
return _Ioterator{static_cast<_Wi>(_It._Current + _Off)};
}
_NODISCARD friend constexpr _Ioterator operator-(_Ioterator _It, const difference_type _Off) noexcept(
noexcept(static_cast<_Wi>(_It._Current - _Off))) /* strengthened */ requires _Advanceable<_Wi> {
return _Ioterator{static_cast<_Wi>(_It._Current - _Off)};
is_nothrow_move_constructible_v<_Ioterator>&& noexcept(
_It -= _Off)) /* strengthened */ requires _Advanceable<_Wi> {
_It -= _Off;
return _It;
}
_NODISCARD friend constexpr difference_type
operator-(const _Ioterator& _Left, const _Ioterator& _Right) noexcept(
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