Hmm... as far as I know base::Optional is not currently used anywhere else
in the public v8 API correct?
An alternative here that avoids the exception issue could be:
1. For GetInternalFieldMaybe(), if the value is not set, it returns
Undefined rather than empty. Not ideal but certainly something we can work
with.
2. For GetAlignedPointerFromInternalField() we could use a pattern more
like To() ... that is:
bool GetAlignedPointerFromInternalField(int field, void* ptr)
Such that if the internal field is an aligned pointer, ptr is set and
true is returned, otherwise ptr is untouched and false is returned. With no
assert. It's less ideal but also something we can work with.
- James
On Wednesday, February 26, 2020 at 10:00:53 PM UTC-8, Yuki Shiino wrote:
>
> Blink-V8 bindings team expects (and is relying on) a convention that, if a
> Maybe/MaybeLocal is empty, then an exception is being thrown. This is very
> important when implementing V8 callback functions (IDL attribute callback,
> IDL operation callback, etc.).
>
> v8::MaybeLocal<v8::Value> maybe_value = Func();
> v8::Local<v8::Value> value;
> if (!maybe_value.ToLocal(&value)) {
> return; // Just returns because an exception must be thrown already.
> }
> // Use |value|.
>
>
> I personally would like V8 APIs to keep this convention. In case of
> GetAlignedPointerFromInternalField, I'm afraid that there wouldn't be a
> proper exception to be thrown. Maybe, base::Optional<T> would fit better?
>
> Cheers,
> Yuki Shiino
>
>
> 2020年2月27日(木) 8:20 James Snell <[email protected] <javascript:>>:
>
>> Currently existing value checks from GetInternalField and
>> GetAlignedPointerFromInternalField can be a bit awkward. The former returns
>> a Local<Value>, while the latter returns a void* and asserts if the value
>> is not an aligned pointer.
>>
>> I would like to explore adding a pair of alternatives that would return a
>> MaybeLocal<Value> and Maybe<T>, respectively... for instance:
>>
>> ```
>> MaybeLocal<Value> ret = obj->GetInternalFieldMaybe(0);
>> if (!ret.IsEmpty()) {
>> Local<Value> val = ret.ToLocalChecked();
>> // do something with val
>> }
>> ```
>>
>> and
>>
>> ```
>> Foo* foo = nullptr;
>> Maybe<Foo> ret = obj->GetAlignedPointerFromInternalFieldMaybe(0);
>> if (ret.To(foo))
>> // do something with foo
>> ```
>>
>> A key difference in behavior with the
>> GetAlignedPointerFromInternalFieldMaybe variant is that instead of
>> asserting when not an aligned pointer, it would return a Maybe with
>> IsNothing == true. One use case for this we have in Node.js is that we
>> currently use an internal field in the Promise object to store a reference
>> to a tracking object (part of our async context tracking mechanism). That
>> is stored using SetInternalField, but we have a check just prior to make
>> sure there's not an aligned pointer set in that field already, which will
>> assert if SetInternalField has already been called.
>>
>> Before going off and implementing, I wanted to (a) double check to make
>> sure this even made sense and (b) ask for alternative naming suggestions
>> because `GetInternalFieldMaybe()` really isn't that great unless I was
>> Carly Rae Jepsen.
>>
>> - James
>>
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>>
>
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