On Fri, 14 Apr 2023, Patrick Palka wrote:
> Using the CRTP idiom for this base class avoids bloating the size of a
> pipeline when adding distinct empty range adaptor closure objects to it,
> as detailed in section 4.1 of P2387R3.
>
> But it means we can no longer define its operator| overloads as hidden
> friends, since each instantiation of _RangeAdaptorClosure would then
> introduce its own logically different hidden friends. So for example
> during overload resolution for the outer pipe operator in
>
> :x | (views::reverse | views::join)
>
> we'd have to consider 6 different hidden operator| friends:
>
> 2 from _RangeAdaptorClosure<_Reverse>
> 2 from _RangeAdaptorClosure<_Join>
> 2 from _RangeAdaptorClosure<_Pipe<_Reverse, _Join>>
>
> which is wasteful and can even cause hard errors in some cases. So we
> instead define the operator| overloads at namespace scope in an isolated
> namespace.
On second thought, since this doesn't fix a bug or add new functionality
it seems more like GCC 14 material. The size reduction is nice but it's
probably not a big deal in practice since adaptor pipelines are usually
very transient objects that don't get passed around as function
arguments etc.
But perhaps the second patch implementing range_adaptor_closure would be
desirable for GCC 13? I'll post an updated standalone version of that
patch for separate consideration.
>
> PR libstdc++/108827
>
> libstdc++-v3/ChangeLog:
>
> * include/std/ranges (__adaptor::_RangeAdaptorClosure): Move ...
> (__adaptor::__closure::_RangeAdaptorClosure): ... here and turn
> it into a CRTP class template. Move hidden operator| friends
> into namespace scope and adjust their constraints. Add a
> using-declaration for this at __adaptor::_RangeAdaptorClosure.
> (__closure::__is_range_adaptor_closure_fn): Define.
> (__closure::__is_range_adaptor_closure): Define.
> (__adaptor::_Partial): Adjust use of _RangeAdaptorClosure.
> (__adaptor::_Pipe): Likewise.
> (views::_All): Likewise.
> (views::_Join): Likewise.
> (views::_Common): Likewise.
> (views::_Reverse): Likewise.
> (views::_Elements): Likewise.
> (views::_Adjacent): Likewise.
> (views::_AsRvalue): Likewise.
> (views::_Enumerate): Likewise.
> (views::_AsConst): Likewise.
> * testsuite/std/ranges/adaptors/all.cc: Reintroduce
> static_assert expecting that adding empty range adaptor
> closure objects to a pipeline doesn't increase the size of a
> pipeline.
> ---
> libstdc++-v3/include/std/ranges | 69 +++++++++++--------
> .../testsuite/std/ranges/adaptors/all.cc | 7 --
> 2 files changed, 42 insertions(+), 34 deletions(-)
>
> diff --git a/libstdc++-v3/include/std/ranges b/libstdc++-v3/include/std/ranges
> index 283d757faa4..531ec6f68b3 100644
> --- a/libstdc++-v3/include/std/ranges
> +++ b/libstdc++-v3/include/std/ranges
> @@ -872,30 +872,45 @@ namespace views::__adaptor
> template<typename _Lhs, typename _Rhs>
> struct _Pipe;
>
> - // The base class of every range adaptor closure.
> - //
> - // The derived class should define the optional static data member
> - // _S_has_simple_call_op to true if the behavior of this adaptor is
> - // independent of the constness/value category of the adaptor object.
> - struct _RangeAdaptorClosure
> + namespace __closure
> {
> + // The base class of every range adaptor closure.
> + //
> + // The derived class should define the optional static data member
> + // _S_has_simple_call_op to true if the behavior of this adaptor is
> + // independent of the constness/value category of the adaptor object.
> + template<typename _Derived>
> + struct _RangeAdaptorClosure
> + { };
> +
> + template<typename _Tp, typename _Up>
> + requires (!same_as<_Tp, _RangeAdaptorClosure<_Up>>)
> + void __is_range_adaptor_closure_fn
> + (const _Tp&, const _RangeAdaptorClosure<_Up>&); // not defined
> +
> + template<typename _Tp>
> + concept __is_range_adaptor_closure
> + = requires (_Tp __t) { __closure::__is_range_adaptor_closure_fn(__t,
> __t); };
> +
> // range | adaptor is equivalent to adaptor(range).
> template<typename _Self, typename _Range>
> - requires derived_from<remove_cvref_t<_Self>, _RangeAdaptorClosure>
> + requires __is_range_adaptor_closure<_Self>
> && __adaptor_invocable<_Self, _Range>
> - friend constexpr auto
> + constexpr auto
> operator|(_Range&& __r, _Self&& __self)
> { return std::forward<_Self>(__self)(std::forward<_Range>(__r)); }
>
> // Compose the adaptors __lhs and __rhs into a pipeline, returning
> // another range adaptor closure object.
> template<typename _Lhs, typename _Rhs>
> - requires derived_from<_Lhs, _RangeAdaptorClosure>
> - && derived_from<_Rhs, _RangeAdaptorClosure>
> - friend constexpr auto
> + requires __is_range_adaptor_closure<_Lhs>
> + && __is_range_adaptor_closure<_Rhs>
> + constexpr auto
> operator|(_Lhs __lhs, _Rhs __rhs)
> { return _Pipe<_Lhs, _Rhs>{std::move(__lhs), std::move(__rhs)}; }
> - };
> + } // namespace __closure
> +
> + using __closure::_RangeAdaptorClosure;
>
> // The base class of every range adaptor non-closure.
> //
> @@ -937,7 +952,7 @@ namespace views::__adaptor
> // A range adaptor closure that represents partial application of
> // the range adaptor _Adaptor with arguments _Args.
> template<typename _Adaptor, typename... _Args>
> - struct _Partial : _RangeAdaptorClosure
> + struct _Partial : _RangeAdaptorClosure<_Partial<_Adaptor, _Args...>>
> {
> tuple<_Args...> _M_args;
>
> @@ -978,7 +993,7 @@ namespace views::__adaptor
> // A lightweight specialization of the above primary template for
> // the common case where _Adaptor accepts a single extra argument.
> template<typename _Adaptor, typename _Arg>
> - struct _Partial<_Adaptor, _Arg> : _RangeAdaptorClosure
> + struct _Partial<_Adaptor, _Arg> :
> _RangeAdaptorClosure<_Partial<_Adaptor, _Arg>>
> {
> _Arg _M_arg;
>
> @@ -1011,7 +1026,7 @@ namespace views::__adaptor
> template<typename _Adaptor, typename... _Args>
> requires __adaptor_has_simple_extra_args<_Adaptor, _Args...>
> && (is_trivially_copyable_v<_Args> && ...)
> - struct _Partial<_Adaptor, _Args...> : _RangeAdaptorClosure
> + struct _Partial<_Adaptor, _Args...> :
> _RangeAdaptorClosure<_Partial<_Adaptor, _Args...>>
> {
> tuple<_Args...> _M_args;
>
> @@ -1041,7 +1056,7 @@ namespace views::__adaptor
> template<typename _Adaptor, typename _Arg>
> requires __adaptor_has_simple_extra_args<_Adaptor, _Arg>
> && is_trivially_copyable_v<_Arg>
> - struct _Partial<_Adaptor, _Arg> : _RangeAdaptorClosure
> + struct _Partial<_Adaptor, _Arg> :
> _RangeAdaptorClosure<_Partial<_Adaptor, _Arg>>
> {
> _Arg _M_arg;
>
> @@ -1066,7 +1081,7 @@ namespace views::__adaptor
> // A range adaptor closure that represents composition of the range
> // adaptor closures _Lhs and _Rhs.
> template<typename _Lhs, typename _Rhs>
> - struct _Pipe : _RangeAdaptorClosure
> + struct _Pipe : _RangeAdaptorClosure<_Pipe<_Lhs, _Rhs>>
> {
> [[no_unique_address]] _Lhs _M_lhs;
> [[no_unique_address]] _Rhs _M_rhs;
> @@ -1102,7 +1117,7 @@ namespace views::__adaptor
> template<typename _Lhs, typename _Rhs>
> requires __closure_has_simple_call_op<_Lhs>
> && __closure_has_simple_call_op<_Rhs>
> - struct _Pipe<_Lhs, _Rhs> : _RangeAdaptorClosure
> + struct _Pipe<_Lhs, _Rhs> : _RangeAdaptorClosure<_Pipe<_Lhs, _Rhs>>
> {
> [[no_unique_address]] _Lhs _M_lhs;
> [[no_unique_address]] _Rhs _M_rhs;
> @@ -1264,7 +1279,7 @@ namespace views::__adaptor
> concept __can_owning_view = requires {
> owning_view{std::declval<_Range>()}; };
> } // namespace __detail
>
> - struct _All : __adaptor::_RangeAdaptorClosure
> + struct _All : __adaptor::_RangeAdaptorClosure<_All>
> {
> template<typename _Range>
> static constexpr bool
> @@ -3048,7 +3063,7 @@ namespace views::__adaptor
> = requires { join_view<all_t<_Range>>{std::declval<_Range>()}; };
> } // namespace __detail
>
> - struct _Join : __adaptor::_RangeAdaptorClosure
> + struct _Join : __adaptor::_RangeAdaptorClosure<_Join>
> {
> template<viewable_range _Range>
> requires __detail::__can_join_view<_Range>
> @@ -3842,7 +3857,7 @@ namespace views::__adaptor
> = requires { common_view{std::declval<_Range>()}; };
> } // namespace __detail
>
> - struct _Common : __adaptor::_RangeAdaptorClosure
> + struct _Common : __adaptor::_RangeAdaptorClosure<_Common>
> {
> template<viewable_range _Range>
> requires __detail::__already_common<_Range>
> @@ -3963,7 +3978,7 @@ namespace views::__adaptor
> = requires { reverse_view{std::declval<_Range>()}; };
> } // namespace __detail
>
> - struct _Reverse : __adaptor::_RangeAdaptorClosure
> + struct _Reverse : __adaptor::_RangeAdaptorClosure<_Reverse>
> {
> template<viewable_range _Range>
> requires __detail::__is_reverse_view<remove_cvref_t<_Range>>
> @@ -4363,7 +4378,7 @@ namespace views::__adaptor
> } // namespace __detail
>
> template<size_t _Nm>
> - struct _Elements : __adaptor::_RangeAdaptorClosure
> + struct _Elements : __adaptor::_RangeAdaptorClosure<_Elements<_Nm>>
> {
> template<viewable_range _Range>
> requires __detail::__can_elements_view<_Nm, _Range>
> @@ -5484,7 +5499,7 @@ namespace views::__adaptor
> }
>
> template<size_t _Nm>
> - struct _Adjacent : __adaptor::_RangeAdaptorClosure
> + struct _Adjacent : __adaptor::_RangeAdaptorClosure<_Adjacent<_Nm>>
> {
> template<viewable_range _Range>
> requires (_Nm == 0) || __detail::__can_adjacent_view<_Nm, _Range>
> @@ -8609,7 +8624,7 @@ namespace views::__adaptor
> concept __can_as_rvalue_view = requires {
> as_rvalue_view(std::declval<_Tp>()); };
> }
>
> - struct _AsRvalue : __adaptor::_RangeAdaptorClosure
> + struct _AsRvalue : __adaptor::_RangeAdaptorClosure<_AsRvalue>
> {
> template<viewable_range _Range>
> requires __detail::__can_as_rvalue_view<_Range>
> @@ -8918,7 +8933,7 @@ namespace views::__adaptor
> = requires { enumerate_view<all_t<_Tp>>(std::declval<_Tp>()); };
> }
>
> - struct _Enumerate : __adaptor::_RangeAdaptorClosure
> + struct _Enumerate : __adaptor::_RangeAdaptorClosure<_Enumerate>
> {
> template<viewable_range _Range>
> requires __detail::__can_enumerate_view<_Range>
> @@ -9004,7 +9019,7 @@ namespace views::__adaptor
> concept __can_as_const_view = requires {
> as_const_view(std::declval<_Range>()); };
> }
>
> - struct _AsConst : __adaptor::_RangeAdaptorClosure
> + struct _AsConst : __adaptor::_RangeAdaptorClosure<_AsConst>
> {
> template<viewable_range _Range>
> constexpr auto
> diff --git a/libstdc++-v3/testsuite/std/ranges/adaptors/all.cc
> b/libstdc++-v3/testsuite/std/ranges/adaptors/all.cc
> index 57cb542174b..6bbaf73a20a 100644
> --- a/libstdc++-v3/testsuite/std/ranges/adaptors/all.cc
> +++ b/libstdc++-v3/testsuite/std/ranges/adaptors/all.cc
> @@ -105,12 +105,6 @@ static_assert(std::is_empty_v<decltype(views::common
> | views::common
> | views::keys
> | views::reverse)>);
> -#if 0
> -// Adding empty range adaptor closure objects to a pipeline used to not
> -// increase the size of the pipeline, but now that our range adaptor closure
> -// objects derive from a common empty base class, [[no_unique_address]] can
> no
> -// longer make two empty adjacent range adaptor closure objects occupy the
> same
> -// data member address.
> static_assert(sizeof(decltype(views::take(5) | views::drop(5)))
> == sizeof(decltype(views::take(5)
> | views::join
> @@ -119,7 +113,6 @@ static_assert(sizeof(decltype(views::take(5) |
> views::drop(5)))
> | views::keys
> | views::drop(5)
> | views::reverse)));
> -#endif
>
> template<auto all = views::all>
> void
> --
> 2.40.0.335.g9857273be0
>
>
