Hi Ludo' :) On Mon 25 Jan 2010 14:44, l...@gnu.org (Ludovic Courtès) writes:
> Andy Wingo <wi...@pobox.com> writes: > >> - GC_REGISTER_DISPLACEMENT (2*sizeof(scm_t_bits)); /* for the self data >> pointer */ >> - GC_REGISTER_DISPLACEMENT (2*sizeof(scm_t_bits) >> - + scm_tc3_struct); /* for the vtable data >> pointer */ >> + /* The first word of a struct is equal to `SCM_STRUCT_DATA (vtable) + >> + scm_tc3_struct', and `SCM_STRUCT_DATA (vtable)' is 2 words after >> VTABLE by >> + default. */ >> + GC_REGISTER_DISPLACEMENT (2 * sizeof (scm_t_bits) + scm_tc3_struct); >> >> I'm pretty sure the first case is still needed, for instance >> reallocation. It is, as the comment indicates, "for the self data >> pointer" -- the second word of the object, which normally points to the >> third word of the object, but for reallocated instances points to the >> third word of *another* object. > > As an excuse, the comment was a bit too terse for me to understand, but > I get your point now. However, it’s still unclear to me how an ‘s’ > field can point to something else than “self”. Hm, I think you're right; the wording was not only terse but misleading. It is "for the data pointer, which might point to another object's data", or something like that. Nothing to do with `s' fields. >> See my article on redefining classes for more info. > > By ‘self’ here, did you mean what’s known as ‘data’ on the diagrams at > <http://wingolog.org/archives/2009/11/09/class-redefinition-in-guile>? Yes indeed. > PS: It would be really cool if this blog post were part of the “Data > Representation” node of the manual. :-) Heh, yeah. That whole chapter needs revision... Andy -- http://wingolog.org/
