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From: "fdumont 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 06:23:16 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D96088

--- Comment #1 from Fran=C3=A7ois Dumont <fdumont at gcc dot gnu.org> ---
The core issue here is that unordered_map key type is std::string while you
insert const char* which explains the temporary.

In f2 you use insert(Pair&&) method so a temporary is generated but then mo=
ved
into the container. In f1 we try to check for insertion before generating t=
he
value_type and to do so we use hash<std::string> that generates an addition=
al
temporary.

I'll check if we can be smarter here. A nice improvement would be to change
std::hash<std::string> operator signature to:

      size_t
      operator()(const string_view& __str) const noexcept

but that's a Standard modification.=