On Sat, Nov 4, 2017 at 9:29 AM, Bruce MacNaughton <bmacnaugh...@gmail.com>
wrote:
> I am new to Nan, V8, and C++ (so if I haven't put a big enough target on
> my back I don't know what else I can add). I've written a lot of JavaScript
> and, in the past, C, assembler, and kernel mode code, so hopefully the
> bulls-eye is a little smaller now.
>
> I'm working with an existing code base and am trying to understand why
> things were done the way they were. It uses Nan to create an addon for
> nodejs. I'm hoping someone here can help me understand some pieces that
> escape me.
>
Nan is really a nodejs thing, and not V8... so this is sort of the wrong
place for these questions...
>
> 1. The code sets internal field count for each class - sometimes to 1 and
> sometimes to 2 - but never invokes "setInternalField()" or
> "getInternalField()". Is there some reason, possibly historical, that
> "setInternalFieldCount()" needed to be called to set a value? The way I
> have interpreted what I've read is that my code needs to set and get the
> value explicitly, so setting a value but never storing anything there makes
> no sense to me.
>
> // Prepare constructor template
> v8::Local<v8::FunctionTemplate> ctor = Nan::New<v8::FunctionTemplate>
> (New);
> ctor->InstanceTemplate()->SetInternalFieldCount(2);
> ctor->SetClassName(Nan::New("MyClass").ToLocalChecked());
>
>
Does it use Wrap and/or as classes subclassed with ObjectWrap? Wrap uses
internal field 0 to store the class so it can be later unwrapped from the
V8 object.
https://github.com/nodejs/node/blob/master/src/node_object_wrap.h#L75 (near
that is also SetWeak reference)
> 2. Given that I'm storing something in internal fields, my understanding
> is that I need to free any resources (memory, etc.) that are used by the
> internal field if the object is GC'd. Doing that in the destructor seems to
> be the right way to handle that. Is that all there is to it?
>
> the destructor is really too late, at the point the destructor is called,
the Object holding it would have also disappeared.... If the destructor is
getting called, it's probably because of an ObjectWrapped thing
disappearing, which internally stores the object in the class as a
Persistent<> that is SetWeak()'d. SetWeak takes a callback which is called
when the object is GC'd.
> 3. What difference does it make to v8 if the internal field is an aligned
> pointer or not? Is the ability to set/get aligned pointers a consistency
> check so assumptions can be made? Does the interface check the alignment?
> (Not critical for me, I don't think, but I'd like to understand.)
>
>
I dooubt it matters... basically internal fields seem to be user-data
fields that store the value so your user code can later retrieve it.
Internally I wouldn't expect V8 to ever actually do anything with those
fields. Since they are usually pointers that are stored, aligned buffers
will be more optimal.
>
>
>
>
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