On Wed, 12 Nov 2025 at 11:38, Tomasz Kamiński <[email protected]> wrote:
>
> This implements P3913R1: Optimize for std::optional in range adaptors.
>
> Specifically, for an opt of type optional<T> that is a view:
> * views::reverse(opt), views::take(opt, n), and views::drop(opt, n) returns
> optional<T>.
> * views::as_const(opt), optional<T&> is converted into optional<const T&>.
> optional<T const> is not used in the non-reference case because, such
> type is not move assignable, and thus not a view.
>
> libstdc++-v3/ChangeLog:
>
> * include/std/optional (__is_optional_ref_v): Define.
> * include/std/ranges (_Take::operator(), _Drop::operator())
Please mention __is_optional_and_view here as well.
But I wonder if __is_viewable_optional would be a better name, for
symmetry with the std::ranges::viewable_range concept.
What do you think?
> (_Reverse::operator()): Handle optional<T> that are view.
> (_AsConst::operator()): Handle optional<T&>.
> * testsuite/20_util/optional/range.cc: New tests.
> ---
> The changes got approved for C++26 so submitting this as proper patch.
> v3 expands existing test cases, to cover references to NonMovable and
> NonAssingable types. Use TEST_ADAPTOR macro to generate test cases
> for adaptors (except ranges::as_const).
>
> Tested on x86_64-linux locally. *optional* test also tested with all
> supported standard modes. OK for trunk?
>
> libstdc++-v3/include/std/optional | 8 +-
> libstdc++-v3/include/std/ranges | 16 ++
> .../testsuite/20_util/optional/range.cc | 161 +++++++++++++++++-
> 3 files changed, 179 insertions(+), 6 deletions(-)
>
> diff --git a/libstdc++-v3/include/std/optional
> b/libstdc++-v3/include/std/optional
> index 75a9531ccd5..41c04b10720 100644
> --- a/libstdc++-v3/include/std/optional
> +++ b/libstdc++-v3/include/std/optional
> @@ -1486,6 +1486,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> template<typename _Tp>
> class optional<_Tp&>;
>
> + template<typename _Tp>
> + constexpr bool __is_optional_ref_v = false;
> +
> + template<typename _Tp>
> + constexpr bool __is_optional_ref_v<optional<_Tp&>> = true;
> +
> template<typename _Tp>
> struct __optional_ref_base
> {};
> @@ -2187,7 +2193,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> constexpr bool
> ranges::enable_borrowed_range<optional<_Tp&>> = true;
> #endif
> -
> +
> template<typename _Tp>
> inline constexpr range_format
> format_kind<optional<_Tp>> = range_format::disabled;
> diff --git a/libstdc++-v3/include/std/ranges b/libstdc++-v3/include/std/ranges
> index 158692d92a7..ae57b9a0809 100644
> --- a/libstdc++-v3/include/std/ranges
> +++ b/libstdc++-v3/include/std/ranges
> @@ -2403,6 +2403,10 @@ namespace views::__adaptor
> using _Tp = remove_cvref_t<_Range>;
> if constexpr (__detail::__is_empty_view<_Tp>)
> return _Tp();
> +#ifdef __cpp_lib_optional_range_support // >= C++26
> + else if constexpr (__is_optional_v<_Tp> && view<_Tp>)
> + return __n ? std::forward<_Range>(__r) : _Tp();
> +#endif
> else if constexpr (random_access_range<_Tp>
> && sized_range<_Tp>
> && (std::__detail::__is_span<_Tp>
> @@ -2679,6 +2683,10 @@ namespace views::__adaptor
> using _Tp = remove_cvref_t<_Range>;
> if constexpr (__detail::__is_empty_view<_Tp>)
> return _Tp();
> +#ifdef __cpp_lib_optional_range_support // >= C++26
> + else if constexpr (__is_optional_v<_Tp> && view<_Tp>)
> + return __n ? _Tp() : std::forward<_Range>(__r);
> +#endif
> else if constexpr (random_access_range<_Tp>
> && sized_range<_Tp>
> && (std::__detail::__is_span<_Tp>
> @@ -4156,6 +4164,10 @@ namespace views::__adaptor
> using _Tp = remove_cvref_t<_Range>;
> if constexpr (__detail::__is_reverse_view<_Tp>)
> return std::forward<_Range>(__r).base();
> +#ifdef __cpp_lib_optional_range_support // >= C++26
> + else if constexpr (__is_optional_v<_Tp> && view<_Tp>)
> + return std::forward<_Range>(__r);
> +#endif
> else if constexpr (__detail::__is_reversible_subrange<_Tp>)
> {
> using _Iter = decltype(ranges::begin(__r).base());
> @@ -9312,6 +9324,10 @@ namespace views::__adaptor
> return views::all(std::forward<_Range>(__r));
> else if constexpr (__detail::__is_empty_view<_Tp>)
> return views::empty<const element_type>;
> +#if __cpp_lib_optional >= 202506L && __cpp_lib_optional_range_support // >=
> C++26
> + else if constexpr (__is_optional_ref_v<_Tp>)
> + return optional<const typename _Tp::value_type&>(__r);
> +#endif
> else if constexpr (std::__detail::__is_span<_Tp>)
> return span<const element_type,
> _Tp::extent>(std::forward<_Range>(__r));
> else if constexpr (__detail::__is_constable_ref_view<_Tp>)
> diff --git a/libstdc++-v3/testsuite/20_util/optional/range.cc
> b/libstdc++-v3/testsuite/20_util/optional/range.cc
> index 981969cb614..4238c9cf28e 100644
> --- a/libstdc++-v3/testsuite/20_util/optional/range.cc
> +++ b/libstdc++-v3/testsuite/20_util/optional/range.cc
> @@ -10,6 +10,26 @@
>
> #include <testsuite_hooks.h>
>
> +struct NonMovable
> +{
> + constexpr NonMovable() {}
> + constexpr NonMovable(int) {}
> +
> + NonMovable(NonMovable&&) = delete;
> + NonMovable& operator=(NonMovable&&) = delete;
> +
> + friend bool operator==(NonMovable const&, NonMovable const&) = default;
> +};
> +
> +struct NonAssignable
> +{
> + NonAssignable() = default;
> + NonAssignable(NonAssignable&&) = default;
> + NonAssignable& operator=(NonAssignable&&) = delete;
> +
> + friend bool operator==(NonAssignable const&, NonAssignable const&) =
> default;
> +};
> +
> template<typename T>
> constexpr
> void
> @@ -24,10 +44,11 @@ test_range_concepts()
> constexpr bool is_ref_opt = std::is_reference_v<T>;
> static_assert(std::ranges::borrowed_range<O> == is_ref_opt);
>
> - // an optional<const T> is not assignable, and therefore does not satisfy
> ranges::view
> - constexpr bool is_const_opt = std::is_const_v<T>;
> - static_assert(std::ranges::view<O> == !is_const_opt);
> - static_assert(std::ranges::viewable_range<O> == !is_const_opt);
> + // for any T (including const U) such that optional<T> is not assignable,
> + // it does not satisfy ranges::view
> + constexpr bool is_opt_view = std::is_reference_v<T> || std::movable<T>;
> + static_assert(std::ranges::view<O> == is_opt_view);
> + static_assert(std::ranges::viewable_range<O> == is_opt_view);
>
> }
>
> @@ -108,7 +129,7 @@ test_non_empty(const T& value)
> ++count;
> VERIFY(count == 1);
>
> - if constexpr (!std::is_const_v<V> && !std::is_array_v<V>) {
> + if constexpr (std::is_move_assignable_v<V>) {
> for (auto& x : non_empty)
> x = V{};
> VERIFY(non_empty);
> @@ -168,6 +189,99 @@ constexpr void test_not_range()
> static_assert(!requires(std::optional<T> o) { o.end(); });
> };
>
> +template<typename T>
> +constexpr bool is_optional = false;
> +
> +template<typename T>
> +constexpr bool is_optional<std::optional<T>> = true;
> +
> +template<bool usesOptional, typename T, typename U = std::remove_cv_t<T>>
> +constexpr void test_as_const(std::type_identity_t<U> u)
> +{
> + std::optional<T> o(std::in_place, std::forward<U>(u));
> + auto cv = std::views::as_const(o);
> + static_assert(is_optional<decltype(cv)> == usesOptional);
> + static_assert(std::is_same_v<decltype(*cv.begin()), const
> std::remove_reference_t<T>&>);
> + VERIFY(!std::ranges::empty(cv));
> +
> + std::optional<T> e;
> + auto cve = std::views::as_const(e);
> + static_assert(is_optional<decltype(cve)> == usesOptional);
> + static_assert(std::is_same_v<decltype(*cve.begin()), const
> std::remove_reference_t<T>&>);
> + VERIFY(std::ranges::empty(cve));
> +}
> +
> +template<bool usesOptional, typename T, typename U = std::remove_cv_t<T>>
> +constexpr void
> +test_reverse(std::type_identity_t<U> u)
> +{
> + std::optional<T> o(std::in_place, std::forward<U>(u));
> + auto rv = std::views::reverse(o);
> + static_assert(is_optional<decltype(rv)> == usesOptional);
> + static_assert(std::is_same_v<decltype(*rv.begin()), T&>);
> + VERIFY(!std::ranges::empty(rv));
> +
> + std::optional<T> e;
> + auto rve = std::views::reverse(e);
> + static_assert(is_optional<decltype(rve)> == usesOptional);
> + static_assert(std::is_same_v<decltype(*rve.begin()), T&>);
> + VERIFY(std::ranges::empty(rve));
> +}
> +
> +template<bool usesOptional, typename T, typename U = std::remove_cv_t<T>>
> +constexpr void
> +test_take(std::type_identity_t<U> u)
> +{
> + std::optional<T> o(std::in_place, std::forward<U>(u));
> + auto tvp = std::views::take(o, 3);
> + static_assert(is_optional<decltype(tvp)> == usesOptional);
> + static_assert(std::is_same_v<decltype(*tvp.begin()), T&>);
> + VERIFY(!std::ranges::empty(tvp));
> +
> + auto tvz = std::views::take(o, 0);
> + static_assert(is_optional<decltype(tvz)> == usesOptional);
> + static_assert(std::is_same_v<decltype(*tvz.begin()), T&>);
> + VERIFY(std::ranges::empty(tvz));
> +
> + std::optional<T> e;
> + auto tvep = std::views::take(e, 5);
> + static_assert(is_optional<decltype(tvep)> == usesOptional);
> + static_assert(std::is_same_v<decltype(*tvep.begin()), T&>);
> + VERIFY(std::ranges::empty(tvep));
> +
> + auto tvez = std::views::take(e, 0);
> + static_assert(is_optional<decltype(tvez)> == usesOptional);
> + static_assert(std::is_same_v<decltype(*tvez.begin()), T&>);
> + VERIFY(std::ranges::empty(tvez));
> +}
> +
> +template<bool usesOptional, typename T, typename U = std::remove_cv_t<T>>
> +constexpr void
> +test_drop(std::type_identity_t<U> u)
> +{
> + std::optional<T> o(std::in_place, std::forward<U>(u));
> + auto dvp = std::views::drop(o, 3);
> + static_assert(is_optional<decltype(dvp)> == usesOptional);
> + static_assert(std::is_same_v<decltype(*dvp.begin()), T&>);
> + VERIFY(std::ranges::empty(dvp));
> +
> + auto dvz = std::views::drop(o, 0);
> + static_assert(is_optional<decltype(dvz)> == usesOptional);
> + static_assert(std::is_same_v<decltype(*dvz.begin()), T&>);
> + VERIFY(!std::ranges::empty(dvz));
> +
> + std::optional<T> e;
> + auto dvep = std::views::drop(e, 5);
> + static_assert(is_optional<decltype(dvep)> == usesOptional);
> + static_assert(std::is_same_v<decltype(*dvep.begin()), T&>);
> + VERIFY(std::ranges::empty(dvep));
> +
> + auto dvez = std::views::drop(e, 0);
> + static_assert(is_optional<decltype(dvez)> == usesOptional);
> + static_assert(std::is_same_v<decltype(*dvez.begin()), T&>);
> + VERIFY(std::ranges::empty(dvez));
> +}
> +
> constexpr
> bool
> all_tests()
> @@ -175,6 +289,8 @@ all_tests()
> test(42);
> int i = 42;
> int arr[10]{};
> + NonMovable nm;
> + NonAssignable na;
> test(&i);
> test(std::string_view("test"));
> test(std::vector<int>{1, 2, 3, 4});
> @@ -185,6 +301,10 @@ all_tests()
> test<const int&>(i);
> test<int(&)[10]>(arr);
> test<const int(&)[10]>(arr);
> + test<NonMovable&>(nm);
> + test<const NonMovable&>(nm);
> + test<NonAssignable&>(na);
> + test<const NonAssignable&>(na);
> test_not_range<void(&)()>();
> test_not_range<void(&)(int)>();
> test_not_range<int(&)[]>();
> @@ -192,6 +312,37 @@ all_tests()
>
> range_chain_example();
>
> + test_as_const<false, int>(i);
> + test_as_const<false, const int>(i);
> + test_as_const<true, int&>(i);
> + test_as_const<true, const int&>(i);
> + test_as_const<false, NonMovable, int>(10);
> + test_as_const<false, const NonMovable, int>(10);
> + test_as_const<true, NonMovable&>(nm);
> + test_as_const<true, const NonMovable&>(nm);
> + test_as_const<false, NonAssignable>({});
> + test_as_const<false, const NonAssignable>({});
> + test_as_const<true, NonAssignable&>(na);
> + test_as_const<true, const NonAssignable&>(na);
> +
> +#define TEST_ADAPTOR(name) \
> + test_##name<true, int>(i); \
> + test_##name<false, const int>(i); \
> + test_##name<true, int&>(i); \
> + test_##name<true, const int&>(i); \
> + test_##name<false, NonMovable, int>(10); \
> + test_##name<false, const NonMovable, int>(10); \
> + test_##name<true, NonMovable&>(nm); \
> + test_##name<true, const NonMovable&>(nm); \
> + test_##name<false, NonAssignable>({}); \
> + test_##name<false, const NonAssignable>({}); \
> + test_##name<true, NonAssignable&>(na); \
> + test_##name<true, const NonAssignable&>(na)
> +
> + TEST_ADAPTOR(reverse);
> + TEST_ADAPTOR(take);
> + TEST_ADAPTOR(drop);
> +#undef TEST_ADAPTOR
> return true;
> }
>
> --
> 2.51.0
>
