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

2254. Standard-layout classes and bit-fields

Section: 11.4  [class.mem]     Status: CD5     Submitter: Richard Smith     Date: 2016-03-23

[Accepted as a DR at the June, 2018 (Rapperswil) meeting.]

According to 11.4 [class.mem] paragraph 25,

If a standard-layout class object has any non-static data members, its address is the same as the address of its first non-static data member. Otherwise, its address is the same as the address of its first base class subobject (if any). [Note: There might therefore be unnamed padding within a standard-layout struct object, but not at its beginning, as necessary to achieve appropriate alignment. —end note] [Note: The object and its first subobject are pointer-interconvertible (6.8.4 [basic.compound], 7.6.1.9 [expr.static.cast]). —end note]

This wording does not consider the case when the first non-static data member is a bit-field, which cannot have its address taken.

Proposed resolution, February, 2018: [SUPERSEDED]

Change 11.4 [class.mem] paragraph 25 as follows:

If a standard-layout class object has any non-static data members, its address is the same as the address of its first non-static data member if that member is not a bit-field. Otherwise, its Its address is also the same as the address of each of its first base class subobjects (if any). [Note: There might therefore be unnamed padding within a standard-layout struct object inserted by an implementation, but not at its beginning, as necessary to achieve appropriate alignment. —end note] [Note: The object and its first subobject are pointer-interconvertible (6.8.4 [basic.compound], 7.6.1.9 [expr.static.cast]). —end note]

Notes from the March, 2018 meeting:

It was pointed out that the definition of pointer interconvertibility in 6.7.6.3 [basic.stc.thread] paragraph 4 refers to “the first base class subobject” of the object and must also be updated to reflect the above proposed resolution.

Proposed resolution (March, 2018):

  1. Change 6.8.4 [basic.compound] bullet 4.3 as follows:

    2. Two objects a and b are pointer-interconvertible if:

  2. Change 11.4 [class.mem] paragraph 25 as follows:

    3. If a standard-layout class object has any non-static data members, its address is the same as the address of its first non-static data member if that member is not a bit-field. Otherwise, its Its address is also the same as the address of each of its first base class subobjects (if any). [Note: There might therefore be unnamed padding within a standard-layout struct object inserted by an implementation, but not at its beginning, as necessary to achieve appropriate alignment. —end note] [Note: The object and its first subobject are pointer-interconvertible (6.8.4 [basic.compound], 7.6.1.9 [expr.static.cast]). —end note]