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From: "redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/110195] defaulted constructor does not respect the private
 accessor
Date: Fri, 09 Jun 2023 20:52:43 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D110195

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |INVALID
             Status|UNCONFIRMED                 |RESOLVED

--- Comment #6 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to jack from comment #2)
> (In reply to Andrew Pinski from comment #1)
> > Before C++20, Single{} didn't call the constructor so this behavior is
> > expected.
>=20
> Could you explain why it didn't call the constructor before c++20? C++
> standard rules or compiler make it this way?

The standard.

class Single
{
private:
    Single() =3D default;
};

In C++17 Single is an aggregate, and Single{} is aggregate-initialization,
which initializes each member in turn, without calling a constructor. Since=
 it
doesn't use the constructor, it doesn't matter if it's private.

In C++20 it's not an aggregate, so Single{} does value-initialization using=
 the
constructor, which is private.

The C++17 rule is:

An aggregate is an array or a class (Clause 12) with
=E2=80=94 no user-provided, explicit, or inherited constructors (15.1),
=E2=80=94 no private or protected non-static data members (Clause 14),
=E2=80=94 no virtual functions (13.3), and
=E2=80=94 no virtual, private, or protected base classes (13.1).

The type above meets this definition. It has a user-declared constructor, b=
ut
not a user-provided constructor.

In C++20 the rule changed to:

An aggregate is an array or a class (Clause 11) with
 =E2=80=94 no user-declared or inherited constructors (11.4.5),
 =E2=80=94 no private or protected direct non-static data members (11.8),
 =E2=80=94 no private or protected direct base classes (11.8.3), and
 =E2=80=94 no virtual functions (11.7.3) or virtual base classes (11.7.2).

Now the user-declared constructor means it's not an aggregate.


For the other classes:

class Single
{
private:
    explicit Single() =3D default;
};

Not an aggregate in any version of C++ because it has an explicit user-decl=
ared
constructor.

class Single
{
private:
    Single() {}
};

Not an aggregate in any version of C++ because it has a user-provided
constructor.

class Single
{
private:
    Single() =3D default;

    int a;
};

Not an aggregate in any version of C++ because it has a private member.

In all these cases the type is not an aggregate, so Single{} always does
value-initialization and always uses the constructor.


GCC is doing exactly what the standard requires.=