How do you free a wrapped C++ object when associated Javascript object is garbage collected in V8?

Question

V8's documentation explains how to create a Javascript object that wraps a C++ object. The Javascript object holds on to a pointer to a C++ object instance. My question is, let's say you create the C++ object on the heap, how can you get a notification when the Javascript object is collected by the gc, so you can free the heap allocated C++ object?

Answer 1

The trick is to create a Persistent handle (second bullet point from the linked-to API reference: "Persistent handles are not held on a stack and are deleted only when you specifically remove them. ... Use a persistent handle when you need to keep a reference to an object for more than one function call, or when handle lifetimes do not correspond to C++ scopes."), and call MakeWeak() on it, passing a callback function that will do the necessary cleanup ("A persistent handle can be made weak, using Persistent::MakeWeak, to trigger a callback from the garbage collector when the only references to an object are from weak persistent handles." -- that is, when all "regular" handles have gone out of scope and when the garbage collector is about to delete the object).

The Persistent::MakeWeak method signature is:

void MakeWeak(void* parameters, WeakReferenceCallback callback); 

Where WeakReferenceCallback is defined as a pointer-to-function taking two parameters:

typedef void (*WeakReferenceCallback)(Persistent<Object> object,                                       void* parameter); 

These are found in the v8.h header file distributed with V8 as the public API.

You would want the function you pass to MakeWeak to clean up the Persistent<Object> object parameter that will get passed to it when it's called as a callback. The void* parameter parameter can be ignored (or the void* parameter can point to a C++ structure that holds the objects that need cleaning up):

void CleanupV8Point(Persistent<Object> object, void*) {     // do whatever cleanup on object that you're looking for     object.destroyCppObjects(); }  Parameter<ObjectTemplate> my_obj(ObjectTemplate::New());  // when the Javascript part of my_obj is about to be collected // we'll have V8 call CleanupV8Point(my_obj) my_obj.MakeWeak(NULL, &CleanupV8Point);