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From: "glisse at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug libstdc++/96088] Range insertion into unordered_map is less
 effective than a loop with insertion
Date: Thu, 09 Jul 2020 10:50:58 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D96088

--- Comment #3 from Marc Glisse <glisse at gcc dot gnu.org> ---

(In reply to Jonathan Wakely from comment #2)
> Or use unordered_map<string, int, hash<string_view>, equal_to<>> which
> should perform better.

Good idea.

> We haven't implemented http://wg21.link/p0919r3 and http://wg21.link/p169=
0r1
> yet, I wonder if those would help, especially if we make the internal
> helpers available pre-C++20. That could allow the range insertion to use =
the
> heteregenous lookup, to avoid creating temporaries. I'm not sure if that
> would be conforming though. Heterogeneous lookup is observably different,
> and not conforming in C++17.

Restricting it to a few standard types like string should not be observable.

> Adding hash<string>::operator()(string_view) is an interesting idea for t=
he
> standard though.

Indeed. If we want to, I think it is possible to add some overloads for when
the argument is exactly const char* or string_view, which should remain
conforming and provide a significant part of the benefits.=