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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++/113099] locale without RTTI uses dynamic_cast before
 gcc 13.2 or has ODR violation since gcc 13.2
Date: Sun, 24 Dec 2023 15:27:09 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D113099

--- Comment #8 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Peter Dimov from comment #7)
> You don't necessarily need dynamic_cast because facets are always install=
ed
> and obtained by their exact type, not via a reference to base.

Is that true? std::use_facet<X> will return a reference to a facet with X::=
id
but it could actually be something derived from X. e.g. a user can install
their own facet derived from std::ctype<char>, which overrides some members.
Code that does std::use_facet<std::ctype<char>>(loc) will get the user's fa=
cet,
but via reference to base.

> You can store
> the Facet* as given (like shared_ptr does), and return it.
>=20
> The only reason dynamic_cast is needed here is because you can't static_c=
ast
> from facet* to Facet* when virtual inheritance. But you are not required =
to
> store facet* in the actual container; you can store the original Facet* as
> void*.

An implementation could do that, but I don't think libstdc++ can do it now
without an ABI change.=