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From: "panigstein at hotmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/108993] Value initialization does not occur for derived
 class , for gcc versions > 5
Date: Thu, 20 Apr 2023 19:32:03 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D108993

--- Comment #7 from Pablo Anigstein <panigstein at hotmail dot com> ---
(In reply to Jonathan Wakely from comment #5)
> (In reply to Jonathan Wakely from comment #3)
> > (In reply to Pablo Anigstein from comment #2)
> > > (In reply to Andrew Pinski from comment #1)
> > > > Hmm,
> > > >  I noticed that since GCC 7 with -std=3Dc++17, the b.x is not initi=
alized at
> > > > all. So the question I have is there a difference between C++ stand=
ards here?
> >=20
> > Derived is an aggregate in C++17, so b{} does aggregate init, not value=
 init.
>=20
> And that means its Base subobject is copy-initialized from {} which means=
 we
> get a value-initialized object, so it's correct that b.x is not initializ=
ed
> in C++17 (which is what is shown in your godbolt link, because you didn't
> specify any -std option to override the -std=3Dgnu++17 default).
>=20
> With -std=3Dc++14 it looks like b.x is always set to zero, if I'm reading=
 the
> assembly output correctly (but I'm probably not).

Here is an updated example: https://godbolt.org/z/YePjhxKE4. Note that now
Derived is not an aggregate for any standard version due to the private mem=
ber.
Still zero-initialization does not happen for the Base sub-object for the c=
ase
where Base has a user-provided default constructor when compiling with -O1 =
and
above.=