I finally figured it out. I had to cobble my solution from piecing together other samples. In the final snippet you hang the data on the This() pointer but in the Google example they use the Holder(). I was trying to get the data from the Holder which has no slot set up for the data and thereby throws and exception.

The problem I’ve been having has been inconsistency among the various samples. You example is helpful but contributes to the inconsistency especially for newbies. I recalled you or your colleagues are working on a V8 Cookbook. That would be very helpful and help advance adoption of the V8 engine.

thx

In my defense, this article was meant to be used for illustration purposes; the reader is supposed to be familiar with v8 and how it works; also, at the time, it was based on the definition of Point that was in the embedder’s guide; this definition may have since changed, I really don’t know. In any case, this was simply meant to expand on that in a rather abstract way. Still, it does look like I was a bit too lazy.

As for v8::Handle::Cast… I seem to remember it existing. It may be a mistake (like the constructPoint appears to be), but perhaps, if indeed it is not in V8, perhaps it used to be and was removed?

I’m sorry for the trouble you’ve had. Keep in mind that this was my first blog post, and is over a year and a half old.

Alex

It is very frustrating reading your examples because you didn’t do a good job of explaining the introduction of variables. It also would be helpful to post the entire working code. Had you did that the snippets would have been correct and compilable. Can you please address the aforementioned issued and correct the snippets. That would be a big help to me to be able to get a working sample.

The reason why it isn’t straightforward to use the same function as constructor and for creating wrappers is probably that you don’t need it in a browser — there you do ‘document.createElement(“div”)’ rather than ‘new DivElement()’. so there’s never been a need for it in v8. However, by using a small hack it is possible.

As you say, the problem when using the constructor for making wrappers for existing points is that it creates a new Point instance. One way to work around this for ‘wrapPoint’ to pass an argument to the constructor that tells it that there already is an instance: the External value containing the point.

v8::Handle wrapPoint(Point *value) { v8::HandleScope scope; v8::Handle cons = pointConstructor->GetFunction(); v8::Handle external = v8::External::New(value); v8::Handle result = cons->NewInstance(1, &external); return scope.Close(result); }

Within the constructor you check if the first argument is an external, and if it is you don’t construct a Point:

v8::Handle constructorCall(const v8::Arguments& args) { v8::HandleScope scope; v8::Handle external; if (args[0]->IsExternal()) { external = v8::Handle::Cast(args[0]); } else { int x = args[0]->Int32Value(); int y = args[1]->Int32Value(); Point *p = new Point(x, y); external = v8::External::New(p); } args.This()->SetInternalField(0, wrapper); return args.This(); }

Note that now you can use the instance created by the constructor.

Note that I actually prevent it from being called without new:

// throw if called without `new'
if (!args.IsConstructCall()) 
    return ThrowException(String::New("Cannot call constructor as function"));

You could actually use it like so: “var point = Point(3, 4)”; whereas “new Point(3, 4)” will - create a new object - invoke Point(3, 4) on it - which will create and return a totally different object via wrapObject, becoming the result of the “new” expression - the object in step 1) is garbage collected

The fact that you can return something else than “this” from a JS constructor is surprising and I’m glad I finally understood what was itching me about your example.

I’m thinking of making one about persistent pointers, covering both the general overview (including the difference between Clear and Dispose that was cleared up for us) and the MakeWeak functionality. What do you think?

Alex

This is an issue that caused me many ours of debugging and hair pulling. I’m sure it will be very helpful to others.

Thanks for the correction! Alex