Re: [PATCH v2] commit: remove padding from struct commit

[Junio C Hamano <gitster@pobox.com>]

Kristofer Karlsson <krka@spotify.com> writes:

> On Fri, 13 Nov 2026 at 15:57, Derrick Stolee <stolee@gmail.com> wrote:
>>
>> On 11/13/2026 8:30 AM, Kristofer Karlsson via GitGitGadget wrote:
>> > From: Kristofer Karlsson <krka@spotify.com>
>>
>> >  struct commit {
>> > +     /*
>> > +      * Must be the first member, so that a "struct object *" can be
>> > +      * cast to a "struct commit *".
>> > +      */
>> >       struct object object;
>>
>> I think your comment is backwards: We cast a commit into an object
>> and it works. We can't cast an arbitrary object to a commit.

I had the exact reaction.  In our "cheap C inheritance" pattern
downcasting commit to object can be done unconditionally, but
upcasting needs to be done with out of band knowledge (because these
struct types do not have "type" signature).  So

    ... so that a "struct commit *" can be cast to a "struct object
    *".  The code casts in other direction only when the object is
    known to be a commit.