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From: "dprokoptsev at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug libstdc++/103755] {has,use}_facet() and iostream constructor
 performance
Date: Sun, 19 Dec 2021 03:44:50 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D103755

--- Comment #6 from Dmitry Prokoptsev <dprokoptsev at gmail dot com> ---
That would also work, I suppose (it even outperforms my original approach b=
y a
tiny bit -- 33 ns for v2 vs 36 for my original implementation).

There are a few build errors in the alternative implementations, however:
is_convertible<locale::facet*, _Facet*>::value is false, as it tries to
downcast the facet -- did you mean is_base_of<>, which would also better al=
ign
with the static assert message?

Also, despite the always_inline attribute of __try_use_facet, my attempts to
build the library with -O0 -ggdb and link my benchmark yielded a bunch of
errors like=20

    ld:
/home/av/prgs/gcc/debug/x86_64-pc-linux-gnu/libstdc++-v3/src/.libs/libstdc+=
+.so:
undefined reference to `std::num_get<char, std::istreambuf_iterator<char,
std::char_traits<char> > > const* std::__try_use_facet<std::num_get<char,
std::istreambuf_iterator<char, std::char_traits<char> > > >(std::locale
const&)'

Suggest explicitly instantiating __try_use_facet where has_facet() and
use_facet() is instantiated.

See attached fix for build problems I discovered.=