Re: [PATCH] libstdc++/ranges: Use perfect forwarding in _Pipe and _Partial ctors

[Jonathan Wakely <jwakely@redhat.com>]

On Wed, 10 Jan 2024 at 21:40, Patrick Palka <ppalka@redhat.com> wrote:
>
> Tested on x86_64-pc-linux-gnu, does this look OK for trunk?
>
> -- >8 --
>
> This avoids redundant moves when composing and partially applying range
> adaptor objects.
>
> Note that the new constraints on _Partial's constructor templates are
> needed so that it's not inadvertently chosen over the copy constructor
> when constructing a _Partial object from a non-const _Partial lvalue.
>
> libstdc++-v3/ChangeLog:
>
>         * include/std/ranges (views::__adaptor::operator|): Perform
>         perfect forwarding of arguments.
>         (views::__adaptor::_Partial::_Partial): Likewise.
>         (views::__adaptor::_Pipe::__Pipe): Likewise.
> ---
>  libstdc++-v3/include/std/ranges | 65 ++++++++++++++++++++-------------
>  1 file changed, 39 insertions(+), 26 deletions(-)
>
> diff --git a/libstdc++-v3/include/std/ranges b/libstdc++-v3/include/std/ranges
> index 81a857502e3..0734daa42bf 100644
> --- a/libstdc++-v3/include/std/ranges
> +++ b/libstdc++-v3/include/std/ranges
> @@ -957,8 +957,11 @@ namespace views::__adaptor
>      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)}; }
> +    operator|(_Lhs&& __lhs, _Rhs&& __rhs)
> +    {
> +      return _Pipe<decay_t<_Lhs>, decay_t<_Rhs>>{std::forward<_Lhs>(__lhs),
> +                                                std::forward<_Rhs>(__rhs)};
> +    }
>
>    // The base class of every range adaptor non-closure.
>    //
> @@ -1004,10 +1007,12 @@ namespace views::__adaptor
>      {
>        tuple<_Args...> _M_args;
>
> -      constexpr
> -      _Partial(_Args... __args)
> -       : _M_args(std::move(__args)...)
> -      { }
> +      template<typename... _Ts>
> +       requires (sizeof...(_Ts) == sizeof...(_Args))

Do we also need a !same_as constraint here? If sizeof...(_Args) == 1
then this could be chosen instead of a constructor, no?

Or is
> +       constexpr
> +       _Partial(_Ts&&... __args)
> +         : _M_args(std::forward<_Ts>(__args)...)
> +       { }
>
>        // Invoke _Adaptor with arguments __r, _M_args... according to the
>        // value category of this _Partial object.
> @@ -1046,10 +1051,12 @@ namespace views::__adaptor
>      {
>        _Arg _M_arg;
>
> -      constexpr
> -      _Partial(_Arg __arg)
> -       : _M_arg(std::move(__arg))
> -      { }
> +      template<typename _Tp>
> +       requires (!same_as<remove_cvref_t<_Tp>, _Partial>)
> +       constexpr
> +       _Partial(_Tp&& __arg)
> +         : _M_arg(std::forward<_Tp>(__arg))
> +       { }
>
>        template<typename _Range>
>         requires __adaptor_invocable<_Adaptor, _Range, const _Arg&>
> @@ -1079,10 +1086,12 @@ namespace views::__adaptor
>      {
>        tuple<_Args...> _M_args;
>
> -      constexpr
> -      _Partial(_Args... __args)
> -       : _M_args(std::move(__args)...)
> -      { }
> +      template<typename... _Ts>
> +       requires (sizeof...(_Ts) == sizeof...(_Args))
> +       constexpr
> +       _Partial(_Ts&&... __args)
> +         : _M_args(std::forward<_Ts>(__args)...)
> +       { }
>
>        // Invoke _Adaptor with arguments __r, const _M_args&... regardless
>        // of the value category of this _Partial object.
> @@ -1109,10 +1118,12 @@ namespace views::__adaptor
>      {
>        _Arg _M_arg;
>
> -      constexpr
> -      _Partial(_Arg __arg)
> -       : _M_arg(std::move(__arg))
> -      { }
> +      template<typename _Tp>
> +       requires (!same_as<remove_cvref_t<_Tp>, _Partial>)
> +       constexpr
> +       _Partial(_Tp&& __arg)
> +         : _M_arg(std::forward<_Tp>(__arg))
> +       { }
>
>        template<typename _Range>
>         requires __adaptor_invocable<_Adaptor, _Range, const _Arg&>
> @@ -1135,10 +1146,11 @@ namespace views::__adaptor
>        [[no_unique_address]] _Lhs _M_lhs;
>        [[no_unique_address]] _Rhs _M_rhs;
>
> -      constexpr
> -      _Pipe(_Lhs __lhs, _Rhs __rhs)
> -       : _M_lhs(std::move(__lhs)), _M_rhs(std::move(__rhs))
> -      { }
> +      template<typename _Tp, typename _Up>
> +       constexpr
> +       _Pipe(_Tp&& __lhs, _Up&& __rhs)
> +         : _M_lhs(std::forward<_Tp>(__lhs)), _M_rhs(std::forward<_Up>(__rhs))
> +       { }
>
>        // Invoke _M_rhs(_M_lhs(__r)) according to the value category of this
>        // range adaptor closure object.
> @@ -1172,10 +1184,11 @@ namespace views::__adaptor
>        [[no_unique_address]] _Lhs _M_lhs;
>        [[no_unique_address]] _Rhs _M_rhs;
>
> -      constexpr
> -      _Pipe(_Lhs __lhs, _Rhs __rhs)
> -       : _M_lhs(std::move(__lhs)), _M_rhs(std::move(__rhs))
> -      { }
> +      template<typename _Tp, typename _Up>
> +       constexpr
> +       _Pipe(_Tp&& __lhs, _Up&& __rhs)
> +         : _M_lhs(std::forward<_Tp>(__lhs)), _M_rhs(std::forward<_Up>(__rhs))
> +       { }
>
>        template<typename _Range>
>         requires __pipe_invocable<const _Lhs&, const _Rhs&, _Range>
> --
> 2.43.0.283.ga54a84b333
>