[PATCH v3] libstdc++: Make ref_view<R> statically sized if R has static size
Jonathan Wakely
jwakely@redhat.com
Tue May 26 09:33:23 GMT 2026
On Wed, 20 May 2026 at 13:18 +0200, Tomasz Kamiński wrote:
>This patch introduces ranges::__static_size<_Range> helper functions,
>that returns ranges::size(__rg) for __statically_sized_range.
>This function is then used for ref_view<R>::size if R has static size,
>avoiding derefence of pointer value that is not know at compile time.
"dereference" and "known"
>Similary for ref_view<R>::empty() we compare the size with zero,
>if it is know statically.
"known"
>
>This implements revelant part of P3928R0: static_sized_range by Hewill Kang.
"relevant"
>As standard does not specify when constexpr functions are usable at compile
>time, such implementation are allowed (but not mandated) by current draft.
>
>libstdc++-v3/ChangeLog:
>
> * include/bits/ranges_base.h (ranges::__static_size): Define.
> * include/std/ranges (ref_view::size()): For ranges with static
> size return ranges::__static_size<_Range> == 0.
> (ref_view::empty): For ranges with static size, compare it
> against zero.
> * testsuite/23_containers/inplace_vector/cons/from_iota_neg.cc:
> Expect errors from ref_view uses.
> * testsuite/23_containers/inplace_vector/cons/from_range_neg.cc:
> Expect errors from ref_view uses.
>
>Reviewed-by: Patrick Palka <ppalka@redhat.com>
>Signed-off-by: Tomasz Kamiński <tkaminsk@redhat.com>
>---
>v3 uses __conjures integral_constant<size_t, for all ranges whose size
>fits insize of size_t, regardless of they size_t.
>
> libstdc++-v3/include/bits/ranges_base.h | 14 ++++++++++++++
> libstdc++-v3/include/std/ranges | 18 ++++++++++++++++--
> .../inplace_vector/cons/from_iota_neg.cc | 6 ++----
> .../inplace_vector/cons/from_range_neg.cc | 10 ++++------
> 4 files changed, 36 insertions(+), 12 deletions(-)
>
>diff --git a/libstdc++-v3/include/bits/ranges_base.h b/libstdc++-v3/include/bits/ranges_base.h
>index fea4100eb09..bcfaa56fcab 100644
>--- a/libstdc++-v3/include/bits/ranges_base.h
>+++ b/libstdc++-v3/include/bits/ranges_base.h
>@@ -562,6 +562,20 @@ namespace ranges
> template<typename _Tp>
> concept __static_sized_range = sized_range<_Tp> && requires (_Tp& __t)
> { static_cast<char(*)[size_t(ranges::size(__t) >= 0)]>(nullptr); };
>+
>+ template<__static_sized_range _Tp>
>+ consteval range_size_t<_Tp>
>+ __static_size()
>+ {
>+ auto __conjure = [](_Tp& __t)
I think Patrick asked for the names _Range and __r, which I think
would be a small improvement, but it would make the return statements
even longer.
OK for trunk with or without that change.
>+ {
>+ if constexpr (ranges::size(__t) <= size_t(-1))
I was going to say I would have used -1zu here, but maybe being
explicit about size_t makes sense, to match the next line:
>+ return integral_constant<size_t, size_t(ranges::size(__t))>{};
>+ else
>+ return integral_constant<range_size_t<_Tp>, ranges::size(__t)>{};
>+ };
>+ return range_size_t<_Tp>(decltype(__conjure(std::declval<_Tp&>()))::value);
>+ }
> #endif // C++26
>
> template<typename _Derived>
>diff --git a/libstdc++-v3/include/std/ranges b/libstdc++-v3/include/std/ranges
>index 7d8b37f8c5c..2f11cc2336f 100644
>--- a/libstdc++-v3/include/std/ranges
>+++ b/libstdc++-v3/include/std/ranges
>@@ -1382,11 +1382,25 @@ namespace views::__adaptor
>
> constexpr bool
> empty() const requires requires { ranges::empty(*_M_r); }
>- { return ranges::empty(*_M_r); }
>+ {
>+#if __cplusplus > 202302L
>+ if constexpr (__static_sized_range<_Range>)
>+ return ranges::__static_size<_Range>() == 0;
>+ else
>+#endif
>+ return ranges::empty(*_M_r);
>+ }
>
> constexpr auto
> size() const requires sized_range<_Range>
>- { return ranges::size(*_M_r); }
>+ {
>+#if __cplusplus > 202302L
>+ if constexpr (__static_sized_range<_Range>)
>+ return ranges::__static_size<_Range>();
>+ else
>+#endif
>+ return ranges::size(*_M_r);
>+ }
>
> constexpr auto
> data() const requires contiguous_range<_Range>
>diff --git a/libstdc++-v3/testsuite/23_containers/inplace_vector/cons/from_iota_neg.cc b/libstdc++-v3/testsuite/23_containers/inplace_vector/cons/from_iota_neg.cc
>index ae0b96175b7..6b66b5d97ed 100644
>--- a/libstdc++-v3/testsuite/23_containers/inplace_vector/cons/from_iota_neg.cc
>+++ b/libstdc++-v3/testsuite/23_containers/inplace_vector/cons/from_iota_neg.cc
>@@ -33,14 +33,12 @@ test_all()
> std::inplace_vector<int, 15> tr1(std::from_range, ref_view(m12));
>
> std::inplace_vector<int, 10> tm2(std::from_range, m12); // { dg-error "(from here|expansion of)" }
>- // ref_view is not statically sized due pointer dereference
>- std::inplace_vector<int, 10> tr2(std::from_range, ref_view(m12));
>+ std::inplace_vector<int, 10> tr2(std::from_range, ref_view(m12)); // { dg-error "(from here|expansion of)" }
>
> StaticIota<__int128, 0> mm;
>
> std::inplace_vector<int, 10> tm3(std::from_range, mm); // { dg-error "(from here|expansion of)" }
>- // ref_view is not statically sized due pointer dereference
>- std::inplace_vector<int, 10> tr3(std::from_range, ref_view(mm));
>+ std::inplace_vector<int, 10> tr3(std::from_range, ref_view(mm)); // { dg-error "(from here|expansion of)" }
> }
>
> // { dg-error "static assertion failed" "" { target *-*-* } 0 }
>diff --git a/libstdc++-v3/testsuite/23_containers/inplace_vector/cons/from_range_neg.cc b/libstdc++-v3/testsuite/23_containers/inplace_vector/cons/from_range_neg.cc
>index 48d5b4c56f5..ea187761e5e 100644
>--- a/libstdc++-v3/testsuite/23_containers/inplace_vector/cons/from_range_neg.cc
>+++ b/libstdc++-v3/testsuite/23_containers/inplace_vector/cons/from_range_neg.cc
>@@ -62,16 +62,14 @@ test_all()
>
> test_one(a1); // { dg-error "from here" }
> test_one(s1); // { dg-error "from here" }
>- // ref_view is not statically sized due pointer dereference
>- test_one(ref_view(a1));
>- test_one(a5 | std::views::adjacent<7> | std::views::elements<0>);
>+ test_one(ref_view(a1)); // { dg-error "from here" }
>+ test_one(a5 | std::views::adjacent<5> | std::views::elements<0>); // { dg-error "from here" }
> test_one(s5 | std::views::adjacent<5> | std::views::elements<0>); // { dg-error "from here" }
>
> test_five(a5); // { dg-error "from here" }
> test_five(s5); // { dg-error "from here" }
>- // ref_view is not statically sized due pointer dereference
>- test_five(ref_view(a5));
>- test_five(a7 | std::views::adjacent<3> | std::views::elements<0>);
>+ test_five(ref_view(a5)); // { dg-error "from here" }
>+ test_five(a7 | std::views::adjacent<3> | std::views::elements<0>); // { dg-error "from here" }
> test_five(s7 | std::views::adjacent<3> | std::views::elements<0>); // { dg-error "from here" }
> }
>
>--
>2.54.0
>
>
More information about the Libstdc++
mailing list