CWG Issue 1672 (original) (raw)

This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 117a. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.

2025-04-13

1672. Layout compatibility with multiple empty bases

Section: 11.4 [class.mem]Status: CD4Submitter: Richard SmithDate: 2013-04-27

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

The layout compatibility rules of 11.4 [class.mem] paragraph 16 are phrased only in terms of non-static data members, ignoring the existence of base classes:

Two standard-layout struct (Clause 11 [class]) types are layout-compatible if they have the same number of non-static data members and corresponding non-static data members (in declaration order) have layout-compatible types (6.8 [basic.types]).

However, this means that in an example like

struct empty {}; struct A { char a; }; struct also_empty : empty {}; struct C : empty, also_empty { char c; }; union U { struct X { A a1, a2; } x; struct Y { C c1, c2; } y; } u;

u.x.a2.a and u.y.c2.c must have the same address, even though sizeof(A) would typically be 1 andsizeof(B) would need to be at least 2 to give theempty subobjects different addresses.

Proposed resolution (October, 2014):

Change Clause 11 [class] paragraph 7 as indicated and add the following as a new paragraph:

A class S is a standard-layout class is a class that if it:

• ...
• has no base classes of the same type as the first non-static data member element of the set M(S) of types (defined below) as a base class.109

M(X) is defined as follows:

• If X is a non-union class type, the set_M(X)_ is empty if X has no (possibly inherited (11.7 [class.derived])) non-static data members; otherwise, it consists of the type of the first non-static data member of X (where said member may be an anonymous union), X0, and the elements of M(X0).
• If X is a union type, the set M(X), where each Ui is the type of the i_th non-static data member of X, is the union of all_M(Ui) and the set containing all_Ui_.
• If X is a non-class type, the set_M(X)_ is empty.

[Note: M(X) is the set of the types of all non-base-class subobjects that are guaranteed in a standard-layout class to be at a zero offset in X. —_end note_]

(See also the related changes in the resolution forissue 1813.)