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From: "redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug libstdc++/114821] _M_realloc_append should use memcpy instead
 of loop to copy data when possible
Date: Tue, 23 Apr 2024 08:30:03 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D114821

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Using memcpy on any std::pair is undefined behaviour because it's not trivi=
ally
copyable.=20

That's not because it has a copy constructor, its copy constructor is defau=
lted
and so trivial if the data members are trivially copy constructible:
      constexpr pair(const pair&) =3D default;    ///< Copy constructor

It's because it has a non-trivial assignment operator:

      /// Copy assignment operator
      constexpr pair&
      operator=3D(const pair& __p)
      noexcept(_S_nothrow_assignable<const _T1&, const _T2&>())
      requires (_S_assignable<const _T1&, const _T2&>())
      {
        first =3D __p.first;
        second =3D __p.second;
        return *this;
      }


I think this exact point was discussed when Marc introduced the relocate
optimizations. We could maybe cheat and say that we know it's safe to memcpy
std::pair<int, int> even though the language says it's undefined, because we
know what our std::pair implementation looks like.=