[cxx-abi-dev] Question about tail padding

[scott douglass]

At 01:57 PM 16-08-02 +0100, you wrote:
>On Fri, 16 Aug 2002 12:00:54 +0100, scott douglass <sdouglass at arm.com> wrote:
>
>> If the tail padding must not be changed this will be quite inefficient
>> when there is a trailing bit-field -- it makes you wish there were
>> separate whole-object op= and a sub-object op=.
>
>dsize for a class is measured in bytes, so bit-fields shouldn't make a
>difference.

I don't think the implicit copy assignment for B can copy all dsize bytes in this case:

struct B {
    virtual void f(); // B is non-POD
    int bf1:3;
};

struct D : B {
    D();
    int bf2:3;
};

Won't bf1 & bf2 be in the same byte?  And doing this:

    B b;
    D d;
    (B&)d = b; // must not clobber 'd.bf2'

A related thought:  I think constructors, including copy constructors, don't have to worrry about preseving tail padding because after they finish none of the derived fields are valid anyway.