When is an internal field destroyed?

[Richard]

In the following sample code from: https://v8.dev/docs/embed#accessing-dynamic-variables

Point* p = ...;
v8::Local<v8::Object> obj = point_templ->NewInstance();
obj->SetInternalField(0, v8::External::New(isolate, p));

Is there a way to know when `p` has been finished with? I would like to know when/how to delete the allocated memory.

[Ben Noordhuis]

See v8::Global::SetWeak(), it lets you register a callback that's
invoked when the object is reclaimed by the garbage collector.

Caveat emptor, garbage collection is non-deterministic; it might
simply never run with short-lived scripts.

[alex...@gmail.com]

To flesh out Ben's answer a bit, you'd want a Persistent reference to obj which you could SetWeak. I guess maybe that's obvious but never hurts to state the obvious. The Persistent reference could be inside Point or a base class of Point (and others) if you have a menagerie of object types to manage.

If you're concerned about the non-deterministic nature of the cleanup that Ben pointed out, you can make Point (and any other such objects) extend String::ExternalStringResource. Heap tear down will call Dispose for every ExternalString in the Isolate. In addition, if obj is collected, and the presumed only reference to p (the interal field) is eliminated, the ExternalString will be collected and Dispose called for p. If all you want Dispose to do is delete p, you don't have to specify a Dispose override.

It might have been nice if V8 had a similar mechanism for plain External objects as it's a bit weird having your objects extend ExternalStringResource but on the grand scheme of weird it's not a big deal.

Note that the determistic aspect of  String::ExternalStringResource is only really important if your process creates and destroys multiple isolates as a matter of course so you don't want to wait for process termination to clean everything up.

Have fun!