From: Patrick Palka <ppalka@redhat.com>
To: Feng Jisen <J1senn@outlook.com>
Cc: "libstdc++@gcc.gnu.org" <libstdc++@gcc.gnu.org>,
"gcc-patches@gcc.gnu.org" <gcc-patches@gcc.gnu.org>
Subject: Re: Remove redundant partial specialization in _Nth_type
Date: Fri, 3 Nov 2023 11:36:59 -0400 (EDT) [thread overview]
Message-ID: <781a0876-edcc-e839-046d-5e537fb1adfd@idea> (raw)
In-Reply-To: <PN2P287MB04959F8A5240E7CC19930B2991A3A@PN2P287MB0495.INDP287.PROD.OUTLOOK.COM>
[-- Attachment #1: Type: text/plain, Size: 2103 bytes --]
yn Sat, 28 Oct 2023, Feng Jisen wrote:
> This patch remove a redundant partial specialization in class _Nth_type.
> For the original metafunction _Nth_type code,
> # 0
> template<typename _Tp0, typename... _Rest> struct _Nth_type<0, _Tp0, _Rest...>
> { using type = _Tp0; };
> # 1
> template<typename _Tp0, typename _Tp1, typename... _Rest>
> struct _Nth_type<0, _Tp0, _Tp1, _Rest...>
> { using type = _Tp0; }; # 2
> template<typename _Tp0, typename _Tp1, typename _Tp2, typename... _Rest>
> struct _Nth_type<0, _Tp0, _Tp1, _Tp2, _Rest...>
> { using type = _Tp0; };
> # 3
> template<size_t _Np, typename _Tp0, typename _Tp1, typename _Tp2, typename... _Rest>
> #if __cpp_concepts
> requires (_Np >= 3)
> #endif
> struct _Nth_type<_Np, _Tp0, _Tp1, _Tp2, _Rest...>
> : _Nth_type<_Np - 3, _Rest...>
> { };
>
> we need partial specialization # 2 to deal with template argument <0, Tp0, Tp1, Tp2, ...>.
> Because without concepts, both # 0 and # 3 is legal and there is no partial order relationship between them.
> However, # 1 is redundant. For template argument <0, Tp0, Tp1>, #0 is instantiated and that's enough.
Thanks for the patch! This looks good to me.
>
> libstdc++-v3/ChangeLog:
>
> * include/bits/utility.h:(_Nth_type) Remove redundant partial specialization.
>
>
> diff --git a/libstdc++-v3/include/bits/utility.h b/libstdc++-v3/include/bits/utility.hindex bed94525642..8766dfbc15f 100644
> --- a/libstdc++-v3/include/bits/utility.h
> +++ b/libstdc++-v3/include/bits/utility.h
> @@ -258,10 +258,6 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> { };
>
> #if ! __cpp_concepts // Need additional specializations to avoid ambiguities.
> - template<typename _Tp0, typename _Tp1, typename... _Rest>
> - struct _Nth_type<0, _Tp0, _Tp1, _Rest...>
> - { using type = _Tp0; };
> -
> template<typename _Tp0, typename _Tp1, typename _Tp2, typename... _Rest>
> struct _Nth_type<0, _Tp0, _Tp1, _Tp2, _Rest...>
> { using type = _Tp0; };
> --
>
>
>
next prev parent reply other threads:[~2023-11-03 15:37 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-10-28 13:14 Feng Jisen
2023-11-03 15:36 ` Patrick Palka [this message]
2023-11-04 8:45 ` Jonathan Wakely
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