Never mind my last question, it didn't really make much sense. So, I have found some basic behaviour related to this, which really surprised me:
Local<Object> parent; // this will be a JS object Local<Value> slot; // this will be a new slot in parent Local<Object> object = // this will be the value we want to put in that slot => another object // store object in parent Local<Value> value = Local<Value>::Cast(object); parent->Set(slot, value); // remember its pointer and hash int h0 = object->GetIdentityHash(); Value* p0 = *value; // retrieve it back Local<Value> check_value = parent->Get(slot); Local<Object> check_object = Local<Object>::Cast(check_value); // remember its pointer and hash int h1 = check_object->GetIdentityHash(); Value* p1 = *check_value; My surprise is that p0 and p1 are not the same, although h0 and h1 are the same. So, the object I store in that slot is actually not identical to what I retrieve back from the slot. Is there any way to store that object so that what you get back is actually the same pointer? I'm pretty sure this must be possible, because when you do this in the Chrome console, JSON does detect the cyclic reference. Cheers! On Thursday, June 21, 2018 at 9:35:09 AM UTC+2, Gonzalo Diethelm wrote: > > Thanks, that is useful (not sure why it is unary * instead of unary &, but > meh). > > Do you know if there is any way to then go from the T* back into the > Local<T>? Something like: > > Handle<Object> object = Object::New(isolate); > Local<Object> ret = Local<Object>::Cast(object); > Object* ptr = *object; // this works, according to your comment > ... > ret = Local<Object>::Cast(ptr); // this doesn't work > > Cheers! > > On Thursday, June 21, 2018 at 9:13:16 AM UTC+2, Zac Hansen wrote: >> >> unary * on a local<T> gives a T*: >> https://v8.paulfryzel.com/docs/master/classv8_1_1_local.html#a3c96c0bc5db1288bad5769e8e54e26da >> >> On Wednesday, June 20, 2018 at 10:58:37 PM UTC-7, Gonzalo Diethelm wrote: >>> >>> I run the following JS code in the Chrome console: >>> >>> // Version 67.0.3396.87 (Official Build) (64-bit) >>> >>> var x = [1, 2, {"foo": 11}]; >>> x[2].bar = x; >>> >>> Now from C++ code, I get ahold of x as a Local<Object>, and wish to >>> traverse the whole structure; for the sake of the example, let's say I am >>> converting it into serialized data (I know I can use JSON.stringify() to do >>> this, serializing is just an example to clarify ideas). My question is, >>> how can I keep track of the nodes in the structure that I have already >>> seen, and their associated serialized value, so that I can avoid an >>> infinite traversal? >>> >>> It seems to me doing this would require a way to get a unique identity >>> for each node, so that the C++ code can do something similar to this: >>> >>> typedef map<NodeId, NodeData> NodeMap; >>> NodeMap seen; >>> ... >>> Local<Object> node = current.GetNextChild(); >>> NodeId id = node.GetUniqueId(); >>> NodeMap::iterator k = seen.find(id); >>> NodeData data; >>> if (k != seen.end()) { >>> // node already seen, reuse its serialization >>> data = k->first; >>> } else { >>> // first time we see node, serialize and remember >>> data = node.Serialize(); // recurses >>> seen[id] = data; >>> } >>> >>> The specific question is: what type could be NodeId, and how do I get >>> the equivalent of GetUniqueId()? >>> >>> I am very tempted to ask for a way to get a raw void* to each node, but >>> I guess any way of doing this is fine, as long as I can get a unique id >>> that is stable while I'm traversing the data. For these reasons, >>> GetIdentityHash() does not seem to fit the bill: "*The return value >>> will never be 0. Also, it is not guaranteed to be unique.*" >>> >>> Incidentally, If I try to use JSON.stringify for my data, I get this: >>> >>> JSON.stringify(x) >>> VM170:1 Uncaught TypeError: Converting circular structure to JSON >>> at JSON.stringify (<anonymous>) >>> at <anonymous>:1:6 >>> >>> This is taken care of here in the V8 code: >>> >>> JsonStringifier::Result JsonStringifier::StackPush(Handle<Object> object >>> ) { >>> ... >>> // member stack_ is: Handle<JSArray> stack_; >>> int length = Smi::ToInt(stack_->length()); >>> FixedArray* elements = FixedArray::cast(stack_->elements()); >>> for (int i = 0; i < length; i++) { >>> FixedArray* elements = FixedArray::cast(stack_->elements()); >>> if (elements->get(i) == *object) { >>> // boom >>> } >>> } >>> } >>> >>> So, operator*() in a Handle<Object> gives me a unique id? Which is the >>> type for this? Can I store that in a C++ map? Is it stable (enough)? >>> >>> Thanks! >>> >> -- -- v8-users mailing list v8-users@googlegroups.com http://groups.google.com/group/v8-users --- You received this message because you are subscribed to the Google Groups "v8-users" group. To unsubscribe from this group and stop receiving emails from it, send an email to v8-users+unsubscr...@googlegroups.com. For more options, visit https://groups.google.com/d/optout.
