Benjamin Goldberg <[EMAIL PROTECTED]> writes: > I would like for Parrot to have some way of creating Weak References; I > think that this is probably a vital feature. > > The way I envision this is as follows. The following typedef and new > function would be added: > > typedef void (*pobject_died_cb)(INTERP, PMC* who_asked, > Pobj* weakref, void *callback_info); > void pobject_weakref(INTERP, pobject_died_cb callback, > Pobj* weakref, void *callback_info);
This might be sometimes useful. Keeping a container of active objects like filehandles or windows for example. In this case it can't mark the reference because this would make the object active forever. > Inside of a PMC*'s mark method, it registers callbacks (using the above > function) so that it will be informed about what to do if the object to > which it weakly refers to is found to not be alive at the end of the DOD > sweep. This does not need to go into the mark function. The weakref_callback code can be called inside the destroy function. Im not sure if it should be called befor or after the custom destroy-function is run. And is the order of the weakref_callbacks defined. > The pobject_weakref function first checks if the 'weakref' argument has > been marked as alive -- if so, nothing happens. Then, it adds the Pobj* > to a lookup table, pointing from the Pobj*, to a list of registered > callbacks for that Pobj*. This is far to complicated. Each object has a list of destroy-functions, one of them is the costom-destroy function the others are the weakref-callbacks. But one other thing, what happens if the object holding the weakref dies before the refrenced object? Then the callback-function will be called for a dead object. So pobject_weakref() needs to return a handle for the weakref and there needs to be a function weakref_destroy(weakref_handle *handle). Other issue is who owns the data_structure of the weakref? The referent, the referee, or will this be garbage-collected (which makes the weakref_handle a PObj* and weakref_destroy its custom destroy function. > After DOD finishes, the lookup table is walked; for each entry whose > Pobj* hasn't been marked as alive, the callbacks are called. > > The effect of this of course is that a WeakRef has no cost except during > Dead Object Detection. It only has a cost at object destroy-time. (If the weakrefs are garbagecollected they have an effect on DOD in the way that there are more objects to trace) > The first, perhaps most important use, would be to implement a > string-interning table. > > You'd have a global (per-interpreter) pmc which contains a hashtable of > cstrings to perlstrings; if the cstring is present, the corresponding > perlstring is returned; otherwise a new perlstring would be created and > added to the table, then returned. If any of the perlstrings go out of > scope, then their hash entries should disappear from the table. > Obivously, if the table contained normal references to the strings, then > they won't ever go out of scope. And if the table simply doesn't mark > them as alive, it wouldn't know when they're dead. But with weakrefs, > this is easy. this is only useful if a hashlookup is fast compared with string_make. bye boe -- Juergen Boemmels [EMAIL PROTECTED] Fachbereich Physik Tel: ++49-(0)631-205-2817 Universitaet Kaiserslautern Fax: ++49-(0)631-205-3906 PGP Key fingerprint = 9F 56 54 3D 45 C1 32 6F 23 F6 C7 2F 85 93 DD 47
