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From: Jonathan Wakely <>
To: Patrick Palka <>
Subject: Re: [PATCH 1/3] libstdc++: Separate construct/convertibility tests for std::tuple
Date: Tue, 23 Aug 2022 10:15:09 +0100	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

On Tue, 23 Aug 2022 at 02:35, Patrick Palka via Libstdc++
<> wrote:
> P2321R2 adds new conditionally explicit constructors to std::tuple which
> we'll concisely implement in a subsequent patch using explicit(bool), like
> in our C++20 std::pair implementation.  But before we can do that, this
> patch first adds members to _TupleConstraints that test for constructibility
> and convertibility separately; we'll use the first in the new constructors'
> constraints, and the second in their explicit specifier.
> In passing, this patch also redefines the existing predicates
> __is_ex/implicitly_constructible in terms of these new members.  This
> seems to reduce compile time and memory usage by about 10% for large


> tuples when using the relevant constructors constrained by
> _Explicit/_ImplicitCtor (since we no longer have to redundantly expand
> and process is_constructible<_Types, _UTypes>... twice for each pair of
> such constructors).  In order to retain maximal short circuiting, do
> this only when constexpr if is available.

Would we get similar benefits for C++11 and C++14 by doing:

       return __and_<__and_<is_constructible<_Types, _UTypes>...>,
                     __and_<is_convertible<_UTypes, _Types>...>

This is slightly more work in total, but if we have __and_<A,B> and
__and_<A,__not_<B>> then the A and B instantiations will be cached and
can be reused.

> Tested on x86_64-pc-linux-gnu, does this look OK for trunk?

Yes, thanks.

  parent reply	other threads:[~2022-08-23  9:15 UTC|newest]

Thread overview: 11+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-08-23  1:34 Patrick Palka
2022-08-23  1:34 ` [PATCH 2/3] libstdc++: Implement std::pair/tuple/misc enhancements from P2321R2 Patrick Palka
2022-08-23 12:03   ` Jonathan Wakely
2022-08-23 15:14     ` Patrick Palka
2022-08-23  1:35 ` [PATCH 3/3] libstdc++: Implement ranges::zip_view " Patrick Palka
2022-08-24 12:15   ` Jonathan Wakely
2022-08-26 20:05   ` Jonathan Wakely
2022-08-31 10:12     ` Jonathan Wakely
2022-08-23  9:15 ` Jonathan Wakely [this message]
2022-08-23 13:44   ` [PATCH 1/3] libstdc++: Separate construct/convertibility tests for std::tuple Patrick Palka
2022-08-23 14:53     ` Jonathan Wakely

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