This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 117b. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.
2025-08-11
2583. Common initial sequence should consider over-alignmentSection: 11.4.1 [class.mem.general] Status: C++23 Submitter: Brian Bi Date: 2022-05-03[Accepted as a DR at the November, 2022 meeting.]
Consider:
struct A {
int i;
char c;
};
struct B {
int i;
alignas(8) char c;
};
union U { A a; B b; };
On a lot of platforms, A and B do not have the same layout, yet 11.4.1 [class.mem.general] paragraph 23 does not consider differences in alignment in the rules for "common initial sequence":
The common initial sequence of two standard-layout struct (11.2 [class.prop]) types is the longest sequence of non-static data members and bit-fields in declaration order, starting with the first such entity in each of the structs, such that corresponding entities have layout-compatible types (6.9 [basic.types]), either both entities are declared with the no_unique_address attribute (9.13.11 [dcl.attr.nouniqueaddr]) or neither is, and either both entities are bit-fields with the same width or neither is a bit-field.
In the following example,
struct S0 {
alignas(16) char x[128];
int i;
};
struct alignas(16) S1 {
char x[128];
int i;
};
S0 and S1 have the same alignment, yet per the suggested rules below, they will not be layout-compatible.
Suggested resolution [SUPERSEDED]:
Change in 11.4.1 [class.mem.general] paragraphs 23-25 as follows (also add bullets):
The common initial sequence of two standard-layout struct (11.2 [class.prop]) types is the longest sequence of non-static data members and bit-fields in declaration order, starting with the first such entity in each of the structs, such that
- corresponding entities have layout-compatible types (6.9 [basic.types]),
- either both entities have alignment-specifiers that specify equivalent alignment or neither entity has an alignment-specifier (9.13.2 [dcl.align]),
- either both entities are declared with the no_unique_address attribute (9.13.11 [dcl.attr.nouniqueaddr]) or neither is, and
- either both entities are bit-fields with the same width or neither is a bit-field.
[...]
Two standard-layout struct (11.2 [class.prop]) types are layout-compatible classes if their common initial sequence comprises all members and bit-fields of both classes (6.9 [basic.types]) and either both types are declared with alignment-specifiers that specify equivalent alignment or neither type has an alignment-specifier.
Two standard-layout unions are layout-compatible if they have the same number of non-static data members and corresponding non-static data members (in any order)
- have layout-compatible types (6.9.1 [basic.types.general]) and
- either both have alignment-specifiers that specify equivalent alignment or neither has an alignment-specifier.
Proposed resolution (approved by CWG telecon 2022-08-26):
Change in 11.4.1 [class.mem.general] paragraphs 23-25 as follows (also add bullets):
The common initial sequence of two standard-layout struct (11.2 [class.prop]) types is the longest sequence of non-static data members and bit-fields in declaration order, starting with the first such entity in each of the structs, such that
- corresponding entities have layout-compatible types (6.9 [basic.types]),
- corresponding entities have the same alignment requirements (6.8.3 [basic.align]),
- either both entities are declared with the no_unique_address attribute (9.13.11 [dcl.attr.nouniqueaddr]) or neither is, and
- either both entities are bit-fields with the same width or neither is a bit-field.
[...]
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