Movatterモバイル変換

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

Proposed resolution (October, 2014):

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

AclassS is astandard-layoutclassis a class thatif it:

• ...

• has nobase classes of the same type as thefirst non-static data memberelement of thesetM(S) of types (defined below) asa base class.¹⁰⁹

M(X) is defined as follows:

• IfX is a non-union class type, the setM(X) is empty ifX has no (possibly inherited(11.7 [class.derived])) non-static data members;otherwise, it consists of the type of the first non-staticdata member ofX (where said member may be an anonymousunion),X0, and the elements ofM(X0).

• IfX is a union type, the setM(X),where eachU_i is the type of theithnon-static data member ofX, is the union of allM(U_i) and the set containing allU_i.

• IfX is a non-class type, the setM(X) is empty.

[Note:M(X) is the set of the types of allnon-base-class subobjects that are guaranteed in astandard-layout class to be at a zero offset inX.—end note]

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

The layout compatibility rules of 11.4 [class.mem] paragraph16 are phrased only in terms of non-static data members, ignoring theexistence of base classes:

Two standard-layout struct (Clause 11 [class]) types arelayout-compatible if they have the same number of non-static data membersand corresponding non-static data members (in declaration order) havelayout-compatible types (6.9 [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 andu.y.c2.c must have the same address,even thoughsizeof(A) would typically be 1 andsizeof(B) would need to be at least 2 to give theempty subobjects different addresses.