[PATCH v2 2/2] libstdc++: Make ref_view<R> statically sized if R is statically sized
Hewill Kang
hewillk@gmail.com
Thu Mar 19 07:58:52 GMT 2026
>
> But one is a type and other a value, so I do not think the parallel is
> right. You cannot pass range_size_t
> via forwarding reference, and cause additional symbol and 4B of data to
> be emitted to binary. And even
> if it is not, handling and removing this symbol will increase compile time
> cost for no benefit.
That makes sense. But I can add the discussion for this, thanks.
We can still mandate the same thing as for assign_range, i.e. that
> ranges::static_size<RG>() <= N,
> not suggesting to check current capacity.
If that's the case, then insert_range should also be done in the same way.
Tomasz Kaminski <tkaminsk@redhat.com> 於 2026年3月19日週四 下午3:41寫道：
>
>
> On Thu, Mar 19, 2026 at 8:35 AM Hewill Kang <hewillk@gmail.com> wrote:
>
>> you should also require that integer_class types are structural
>>
>> I have already submitted an LWG for this today.
>>
>> I think consteval ranges::static_size<Rg>() function would be better
>>> that variable, this is not something
>>> we want to emit in binary (because someone took reference to it)
>>
>> range_static_size_v basically follows the naming convention of the
>> currently available range_size_t.
>>
> But one is a type and other a value, so I do not think the parallel is
> right. You cannot pass range_size_t
> via forwarding reference, and cause additional symbol and 4B of data to
> be emitted to binary. And even
> if it is not, handling and removing this symbol will increase compile time
> cost for no benefit.
>
>
>>
>> inplace_vector could also use mandates on append_range, you cannot append
>>> more than capacity
>>
>> I don't know how this can be mandates, because the current size of the
>> inplace_vector itself is not a constant expression.
>>
> We can still mandate the same thing as for assign_range, i.e. that
> ranges::static_size<RG>() <= N,
> not suggesting to check current capacity.
>
>
>>
>> Add some discussion regarding the breaks (not a Precondition to Mandate)
>>> that the paper is adding:
>>
>> Yes, that will be more complete, thank you.
>>
>> Tomasz Kaminski <tkaminsk@redhat.com> 於 2026年3月19日週四 下午3:18寫道：
>>
>>>
>>>
>>> On Wed, Mar 18, 2026 at 5:55 PM Hewill Kang <hewillk@gmail.com> wrote:
>>>
>>>> I wrote a similar paper, namely
>>>> https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2025/p3928r0.html
>>>> If you want to enhance ref_view::size, then I think ref_view::empty
>>>> should also be enhanced, just like the paper did.
>>>>
>>> Thanks, here are some suggestions:
>>> * you should also require that integer_class types are structural
>>> * I think consteval ranges::static_size<Rg>() function would be better
>>> that variable, this is not something
>>> we want to emit in binary (because someone took reference to it)
>>> * inplace_vector could also use mandates on append_range, you cannot
>>> append more than capacity
>>>
>>> Add some discussion regarding the breaks (not a Precondition to Mandate)
>>> that the paper is adding:
>>> * span CTAD deduces sized range
>>> * join_view, join_with_view is now sized sometimes
>>> * split_view
>>>
>>>>
>>>> Tomasz Kamiński <tkaminsk@redhat.com> 於 2026年3月19日週四 上午12:31寫道：
>>>>
>>>>> If the referenced range is statically sized, instead of referencing a
>>>>> pointer whose value is not know at compile time, we return
>>>>> integral_constant<range_difference_t<_Rg>, ranges::size(__rg)>::value.
>>>>>
>>>>> libstdc++-v3/ChangeLog:
>>>>>
>>>>> * include/std/ranges (ref_view::size()): Return constant for
>>>>> statically
>>>>> sized ranges, even if pointed object is unknown.
>>>>> *
>>>>> testsuite/23_containers/inplace_vector/cons/from_range_neg.cc:
>>>>> Expect errors from ref_view of array examples.
>>>>> ---
>>>>> v2 uses integral_constant to produce a value of ranges size.
>>>>>
>>>>> libstdc++-v3/include/std/ranges | 18 +++++++++++++++++-
>>>>> .../inplace_vector/cons/from_range_neg.cc | 6 ++----
>>>>> 2 files changed, 19 insertions(+), 5 deletions(-)
>>>>>
>>>>> diff --git a/libstdc++-v3/include/std/ranges
>>>>> b/libstdc++-v3/include/std/ranges
>>>>> index 95e0109a10c..71ab0d674c9 100644
>>>>> --- a/libstdc++-v3/include/std/ranges
>>>>> +++ b/libstdc++-v3/include/std/ranges
>>>>> @@ -1395,7 +1395,23 @@ namespace views::__adaptor
>>>>>
>>>>> constexpr auto
>>>>> size() const requires sized_range<_Range>
>>>>> - { return ranges::size(*_M_r); }
>>>>> + {
>>>>> +#if __cplusplus > 202302L
>>>>> + if constexpr (__static_sized_range<_Range>)
>>>>> + {
>>>>> + auto __conjure_size_constant
>>>>> + = [](_Range& __rg)
>>>>> + {
>>>>> + return integral_constant<
>>>>> + range_difference_t<_Range>,
>>>>> + ranges::size(__rg)>();
>>>>> + };
>>>>> + return decltype(__conjure_size_constant(*_M_r))::value;
>>>>> + }
>>>>> + 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_range_neg.cc
>>>>> b/libstdc++-v3/testsuite/23_containers/inplace_vector/cons/from_range_neg.cc
>>>>> index cf87129da51..15fd7b5c285 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
>>>>> @@ -57,14 +57,12 @@ test_all()
>>>>> test_one(a1); // { dg-error "from here" }
>>>>> test_one(s1); // { dg-error "from here" }
>>>>> test_one(s5 | std::views::adjacent<5> | std::views::elements<0>);
>>>>> // { dg-error "from here" }
>>>>> - // ref_view is not statically sized due pointer dereference
>>>>> - test_one(a5 | std::views::adjacent<7> | std::views::elements<0>);
>>>>> + test_one(a5 | 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" }
>>>>> test_five(s7 | std::views::adjacent<3> | std::views::elements<0>);
>>>>> // { dg-error "from here" }
>>>>> - // ref_view is not statically sized due pointer dereference
>>>>> - test_five(a7 | std::views::adjacent<3> | std::views::elements<0>);
>>>>> + test_five(a7 | std::views::adjacent<3> | std::views::elements<0>);
>>>>> // { dg-error "from here" }
>>>>> }
>>>>>
>>>>> // { dg-error "static assertion failed" "" { target *-*-* } 0 }
>>>>> --
>>>>> 2.53.0
>>>>>
>>>>>
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