On Tue, 15 Jul 2025, Patrick Palka wrote:
> On Tue, 15 Jul 2025, Tomasz Kaminski wrote:
>
> >
> >
> > On Tue, Jul 15, 2025 at 3:55 PM Patrick Palka <ppa...@redhat.com> wrote:
> > On Tue, 15 Jul 2025, Tomasz Kaminski wrote:
> >
> > > On Tue, Jul 15, 2025 at 5:51 AM Patrick Palka <ppa...@redhat.com>
> > wrote:
> > > Tested on x86_64-pc-linux-gnu, does this look OK for trunk
> > only
> > > (since it impacts ABI)?
> > >
> > > In theory an Iterator that meets all semantic requirements of the
> > input_iterator
> > > concept, could provide a default constructor that is unconstrained,
> > but ill-formed
> > > when invoked. This can be easily done accidentally, by having a
> > default member initializer.
> > >
> > > #include <concepts>
> > >
> > > struct NoDefault
> > > { NoDefault(int); };
> > >
> > > template<typename T>
> > > struct Iterator {
> > > T x = T();
> > > };
> > >
> > > static_assert(std::default_initializable<Iterator<NoDefault>>);
> > >
> > > Default member initializers are not in immediate context, and
> > checking "std::default_initializable"
> > > is ill-formed. clang emits error here:
> > https://godbolt.org/z/EafKn6h16
> > >
> > > You can however, do this optimization for forward_iterator. The
> > difference here is that user-defined
> > > iterators provides iterator_category/iterator_concept that maps to
> > forward_iterator_tag or stronger,
> > > so we can check default_initializable.
> >
> > Good point... But it seems this is not only an issue in join_view
> > (with
> > this patch), we already require elsewhere in <ranges> that the default
> > ctor of an input iterator is properly constrained:
> >
> > filter_view, transform_view, elements_view, stride_view,
> > enumerate_view,
> > to_input_view (and perhaps iota_view also counts)
> >
> > Could you please elaborate? I understand that for this view, they default
> > constructor
> > requires that iterator is default_initializable, but if you are never
> > calling this constructor,
> > the view will function correctly for my not-properly constrained iterator.
> >
> > However, for this case the standard does not impose default_initializable
> > requirement
> > for iterator, when we are incrementing it.
>
> I might be confused, is supporting such underconstrained iterators
> a QoI issue or a correctness issue?
>
> If it's QoI, note that before P2325R3 (approved June 2021) even C++20
> input iterators were required to satisfy default_initializable, so I
> reckon it's quite rare to see C++20 input iterator written after that
> with an underconstrained default ctor. I'm not sure supporting them is
> worth the tradeoff of pessimizing join_view for input-only iterators.
... worth the tradeoff of pessimizing join_view for properly constrained
default initializable input-only iterators rather. That's a bit of a
corner case that I guess we shouldn't care much about either, so I don't
feel strongly about using default_initializable vs forward_iterator even
if this is a QoI issue, and am happy to use forward_iterator as you
prefer :)
>
> If it's a correctness issue, I definitely agree with your changes :)
>
> >
> >
> > And so these views would break similarly if the default ctor is
> > underconstrained IIUC. I don't see why we'd want to start caring
> > about
> > such iterators in join_view, if other fundamental views already don't?
> >
> > >
> > >
> > >
> > > -- >8 --
> > >
> > > LWG 3569 adjusted join_view's iterator to handle adapting
> > > non-default-constructible (input) iterators by wrapping the
> > > corresponding data member with std::optional, and we followed
> > suit in
> > > r13-2649-g7aa80c82ecf3a3.
> > >
> > > But this wrapping is unnecessary for iterators that are
> > already
> > > default-constructible. Rather than unconditionally using
> > std::optional
> > > here, which introduces time/space overhead, this patch
> > conditionalizes
> > > our LWG 3569 changes on the iterator in question being
> > > non-default-constructible.
> > >
> > >
> > > libstdc++-v3/ChangeLog:
> > >
> > > * include/std/ranges
> > (join_view::_Iterator::_M_satisfy):
> > > Adjust to handle non-std::optional _M_inner as per
> > before LWG 3569.
> > > (join_view::_Iterator::_M_get_inner): New.
> > > (join_view::_Iterator::_M_inner): Don't wrap in
> > std::optional if
> > > the iterator is already default constructible.
> > Initialize.
> > > (join_view::_Iterator::operator*): Use _M_get_inner
> > instead
> > > of *_M_inner.
> > > (join_view::_Iterator::operator++): Likewise.
> > > (join_view::_Iterator::iter_move): Likewise.
> > > (join_view::_Iterator::iter_swap): Likewise.
> > > ---
> > > libstdc++-v3/include/std/ranges | 49
> > +++++++++++++++++++++++++--------
> > > 1 file changed, 37 insertions(+), 12 deletions(-)
> > >
> > > diff --git a/libstdc++-v3/include/std/ranges
> > b/libstdc++-v3/include/std/ranges
> > > index efe62969d657..799fa7611ce2 100644
> > > --- a/libstdc++-v3/include/std/ranges
> > > +++ b/libstdc++-v3/include/std/ranges
> > > @@ -2971,7 +2971,12 @@ namespace views::__adaptor
> > > }
> > >
> > > if constexpr (_S_ref_is_glvalue)
> > > - _M_inner.reset();
> > > + {
> > > + if constexpr
> > (default_initializable<_Inner_iter>)
> > > + _M_inner = _Inner_iter();
> > > + else
> > > + _M_inner.reset();
> > > + }
> > > }
> > >
> > > static constexpr auto
> > > @@ -3011,6 +3016,24 @@ namespace views::__adaptor
> > > return *_M_parent->_M_outer;
> > > }
> > >
> > > + constexpr _Inner_iter&
> > > + _M_get_inner()
> > > + {
> > > + if constexpr (default_initializable<_Inner_iter>)
> > > + return _M_inner;
> > > + else
> > > + return *_M_inner;
> > > + }
> > > +
> > > + constexpr const _Inner_iter&
> > > + _M_get_inner() const
> > > + {
> > > + if constexpr (default_initializable<_Inner_iter>)
> > > + return _M_inner;
> > > + else
> > > + return *_M_inner;
> > > + }
> > > +
> > > constexpr
> > > _Iterator(_Parent* __parent, _Outer_iter __outer)
> > requires forward_range<_Base>
> > > : _M_outer(std::move(__outer)),
> > _M_parent(__parent)
> > > @@ -3024,7 +3047,9 @@ namespace views::__adaptor
> > > [[no_unique_address]]
> > > __detail::__maybe_present_t<forward_range<_Base>,
> > _Outer_iter> _M_outer
> > > = decltype(_M_outer)();
> > > - optional<_Inner_iter> _M_inner;
> > > + __conditional_t<default_initializable<_Inner_iter>,
> > > + _Inner_iter, optional<_Inner_iter>>
> > _M_inner
> > > + = decltype(_M_inner)();
> > > _Parent* _M_parent = nullptr;
> > >
> > > public:
> > > @@ -3048,7 +3073,7 @@ namespace views::__adaptor
> > >
> > > constexpr decltype(auto)
> > > operator*() const
> > > - { return **_M_inner; }
> > > + { return *_M_get_inner(); }
> > >
> > > // _GLIBCXX_RESOLVE_LIB_DEFECTS
> > > // 3500. join_view::iterator::operator->() is bogus
> > > @@ -3056,7 +3081,7 @@ namespace views::__adaptor
> > > operator->() const
> > > requires __detail::__has_arrow<_Inner_iter>
> > > && copyable<_Inner_iter>
> > > - { return *_M_inner; }
> > > + { return _M_get_inner(); }
> > >
> > > constexpr _Iterator&
> > > operator++()
> > > @@ -3067,7 +3092,7 @@ namespace views::__adaptor
> > > else
> > > return *_M_parent->_M_inner;
> > > }();
> > > - if (++*_M_inner == ranges::end(__inner_range))
> > > + if (++_M_get_inner() ==
> > ranges::end(__inner_range))
> > > {
> > > ++_M_get_outer();
> > > _M_satisfy();
> > > @@ -3097,9 +3122,9 @@ namespace views::__adaptor
> > > {
> > > if (_M_outer == ranges::end(_M_parent->_M_base))
> > > _M_inner =
> > ranges::end(__detail::__as_lvalue(*--_M_outer));
> > > - while (*_M_inner ==
> > ranges::begin(__detail::__as_lvalue(*_M_outer)))
> > > - *_M_inner =
> > ranges::end(__detail::__as_lvalue(*--_M_outer));
> > > - --*_M_inner;
> > > + while (_M_get_inner() ==
> > ranges::begin(__detail::__as_lvalue(*_M_outer)))
> > > + _M_get_inner() =
> > ranges::end(__detail::__as_lvalue(*--_M_outer));
> > > + --_M_get_inner();
> > > return *this;
> > > }
> > >
> > > @@ -3126,14 +3151,14 @@ namespace views::__adaptor
> > >
> > > friend constexpr decltype(auto)
> > > iter_move(const _Iterator& __i)
> > > - noexcept(noexcept(ranges::iter_move(*__i._M_inner)))
> > > - { return ranges::iter_move(*__i._M_inner); }
> > > +
> > noexcept(noexcept(ranges::iter_move(__i._M_get_inner())))
> > > + { return ranges::iter_move(__i._M_get_inner()); }
> > >
> > > friend constexpr void
> > > iter_swap(const _Iterator& __x, const _Iterator&
> > __y)
> > > -
> > noexcept(noexcept(ranges::iter_swap(*__x._M_inner, *__y._M_inner)))
> > > +
> > noexcept(noexcept(ranges::iter_swap(__x._M_get_inner(),
> > __y._M_get_inner())))
> > > requires indirectly_swappable<_Inner_iter>
> > > - { return ranges::iter_swap(*__x._M_inner,
> > *__y._M_inner); }
> > > + { return ranges::iter_swap(__x._M_get_inner(),
> > __y._M_get_inner()); }
> > >
> > > friend _Iterator<!_Const>;
> > > template<bool> friend struct _Sentinel;
> > > --
> > > 2.50.1.271.gd30e120486
> > >
> > >
> > >
> >
> >
> >
