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!
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