Thanks for pointing me in the right direction.
On Saturday, November 4, 2017 at 9:52:23 AM UTC-7, J Decker wrote:
>
>
>
> On Sat, Nov 4, 2017 at 9:29 AM, Bruce MacNaughton <bmacna...@gmail.com
> <javascript:>> 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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>
>
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