Instantiate & pass C++ object to Node.js function V8

Question

I would like to be able to instantiate an instance of a C++ class and pass it as an argument to a JavaScript callback. The class inherits from ObjectWrap, and is available to and used in Node.js.

I am able to pass instances of the same class to the callback if said instance came from JavaScript.

This is a basic example of what I need to do:

MyClass *instance = new MyClass();

// If was passed as the first param to this method, commit is set as below, this code works.
// MyClass *instance = ObjectWrap::Unwrap<MyClass>(args[0]->ToObject());

// Assign some property values to commit
Handle<Value> argv[] = { instance->handle_ };
ar->callback->Call(Context::GetCurrent()->Global(), 1, argv);

This doesn't work, instead it segfaults. gdb tells me:

Program received signal EXC_BAD_ACCESS, Could not access memory.

How might I modify my code to allow me to pass my instance to the callback?

Update: the class in question may be viewed on Github: commit.cc.

Why are you trying to pass the whole class? Surely you can find a cleaner way to communicate between node and C++? Maybe you can design your project so the C++ program outputs the data you need to stdin and your js program takes it from stdin. — Plato
– Plato, Commented Mar 4, 2013 at 14:08
Because the module wraps the functionality of a library. What you suggest is not an option. — Michael Robinson
– Michael Robinson, Commented Mar 4, 2013 at 18:38
You should instanciate your object from C++ with MyClass::New that you probably defined as the Constructor. Otherwise, you handle_ reference is not initiated. — Floby
– Floby, Commented Mar 4, 2013 at 19:28
Thanks @Floby, in my use case, what should I pass as &args? github.com/tbranyen/nodegit/blob/master/src/commit.cc#L105? — Michael Robinson
– Michael Robinson, Commented Mar 4, 2013 at 19:56
@Floby in this case there aren't any, I'm instantiating this object form the afterwork method for uv_queue_work — Michael Robinson
– Michael Robinson, Commented Mar 5, 2013 at 12:41

Michael Robinson · Accepted Answer · 2013-03-05 19:05:30Z

2

The answer is that I was doing it wrong.

The correct code:

Local<Value> instance = MyClass::constructor_template->NewInstance();

// Assign some property values to commit
Handle<Value> argv[] = { instance };
ar->callback->Call(Context::GetCurrent()->Global(), 1, argv);

To get the MyClass instance:

MyClass *instanceOfMyClass = ObjectWrap::Unwrap(instance);

answered Mar 5, 2013 at 19:05

Michael Robinson

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