This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 116a. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.

2024-12-19

1813. Direct vs indirect bases in standard-layout classes

[Moved to DR at the November, 2014 meeting.]

One of the criteria for a standard-layout class in Clause 11 [class] paragraph 7 is:

• either has no non-static data members in the most derived class and at most one base class with non-static data members, or has no base classes with non-static data members,

In an example like

  struct B { int i; };
  struct C : B { };
  struct D : C { };

this could be read as indicating that D is not a standard-layout class, since it has two base classes, one direct and one indirect, that each have a non-static data member. The intent should be clarified.

See also issue 1881 for a related question about standard-layout classes.

Proposed resolution (June, 2014):

Change Clause 11 [class] paragraph 7 as follows:

A standard-layout class is a class that:

• has no non-static data members of type non-standard-layout class (or array of such types) or reference,

• has no virtual functions (11.7.3 [class.virtual]) and no virtual base classes (11.7.2 [class.mi]),

• has the same access control ( 11.8 [class.access]) for all non-static data members,

• has no non-standard-layout base classes,

• has at most one base class subobject of any given type,

• either has no non-static data members in the most derived class and at most one base class with non-static data members, or has no base classes with non-static data members has all non-static data members and bit-fields in the class and its base classes first declared in the same class, and

• has no base classes of the same type as the first non-static data member.¹⁰⁹

[Example:

   struct B { int i; };         // standard-layout class
   struct C : B { };            // standard-layout class
   struct D : C { };            // standard-layout class
   struct E : D { char : 4; };  // not a standard-layout class

   struct Q {};
   struct S : Q { };
   struct T : Q { };
   struct U : S, T { };         // not a standard-layout class

—end example]

This resolution also resolves issue 1881.

(See also the related changes in the resolution of issue 1672.)