Brian Love
Angular + TypeScript Developer in Denver, CO

Object Wrappers

Object Wrappers

The ObjectWrap class can be used to make wrapped C++ objects and a factory of wrapped objects.

### Nan::ObjectWrap()

A reimplementation of node::ObjectWrap that adds some API not present in older versions of Node. Should be preferred over node::ObjectWrap in all cases for consistency.

Definition:

class ObjectWrap {
 public:
  ObjectWrap();

  virtual ~ObjectWrap();

  template <class T>
  static inline T* Unwrap(v8::Local<v8::Object> handle);

  inline v8::Local<v8::Object> handle();

  inline Nan::Persistent<v8::Object>& persistent();

 protected:
  inline void Wrap(v8::Local<v8::Object> handle);

  inline void MakeWeak();

  /* Ref() marks the object as being attached to an event loop.
   * Refed objects will not be garbage collected, even if
   * all references are lost.
   */
  virtual void Ref();

  /* Unref() marks an object as detached from the event loop.  This is its
   * default state.  When an object with a "weak" reference changes from
   * attached to detached state it will be freed. Be careful not to access
   * the object after making this call as it might be gone!
   * (A "weak reference" means an object that only has a
   * persistant handle.)
   *
   * DO NOT CALL THIS FROM DESTRUCTOR
   */
  virtual void Unref();

  int refs_;  // ro
};

See the Node documentation on Wrapping C++ Objects for more details.

This vs. Holder

When calling Unwrap, it is important that the argument is indeed some JavaScript object which got wrapped by a Wrap call for this class or any derived class. The Signature installed by Nan::SetPrototypeMethod() does ensure that info.Holder() is just such an instance. In Node 0.12 and later, info.This() will also be of such a type, since otherwise the invocation will get rejected. However, in Node 0.10 and before it was possible to invoke a method on a JavaScript object which just had the extension type in its prototype chain. In such a situation, calling Unwrap on info.This() will likely lead to a failed assertion causing a crash, but could lead to even more serious corruption.

On the other hand, calling Unwrap in an accessor should not use Holder() if the accessor is defined on the prototype. So either define your accessors on the instance template, or use This() after verifying that it is indeed a valid object.

Examples

Basic

class MyObject : public Nan::ObjectWrap {
 public:
  static NAN_MODULE_INIT(Init) {
    v8::Local<v8::FunctionTemplate> tpl = Nan::New<v8::FunctionTemplate>(New);
    tpl->SetClassName(Nan::New("MyObject").ToLocalChecked());
    tpl->InstanceTemplate()->SetInternalFieldCount(1);

    SetPrototypeMethod(tpl, "getHandle", GetHandle);
    SetPrototypeMethod(tpl, "getValue", GetValue);

    constructor().Reset(Nan::GetFunction(tpl).ToLocalChecked());
    Nan::Set(target, Nan::New("MyObject").ToLocalChecked(),
      Nan::GetFunction(tpl).ToLocalChecked());
  }

 private:
  explicit MyObject(double value = 0) : value_(value) {}
  ~MyObject() {}

  static NAN_METHOD(New) {
    if (info.IsConstructCall()) {
      double value = info[0]->IsUndefined() ? 0 : Nan::To<double>(info[0]).FromJust();
      MyObject *obj = new MyObject(value);
      obj->Wrap(info.This());
      info.GetReturnValue().Set(info.This());
    } else {
      const int argc = 1;
      v8::Local<v8::Value> argv[argc] = {info[0]};
      v8::Local<v8::Function> cons = Nan::New(constructor());
      info.GetReturnValue().Set(cons->NewInstance(argc, argv));
    }
  }

  static NAN_METHOD(GetHandle) {
    MyObject* obj = Nan::ObjectWrap::Unwrap<MyObject>(info.Holder());
    info.GetReturnValue().Set(obj->handle());
  }

  static NAN_METHOD(GetValue) {
    MyObject* obj = Nan::ObjectWrap::Unwrap<MyObject>(info.Holder());
    info.GetReturnValue().Set(obj->value_);
  }

  static inline Nan::Persistent<v8::Function> & constructor() {
    static Nan::Persistent<v8::Function> my_constructor;
    return my_constructor;
  }

  double value_;
};

NODE_MODULE(objectwrapper, MyObject::Init)

To use in Javascript:

var objectwrapper = require('bindings')('objectwrapper');

var obj = new objectwrapper.MyObject(5);
console.log('Should be 5: ' + obj.getValue());

Factory of wrapped objects

class MyFactoryObject : public Nan::ObjectWrap {
 public:
  static NAN_MODULE_INIT(Init) {
    v8::Local<v8::FunctionTemplate> tpl = Nan::New<v8::FunctionTemplate>(New);
    tpl->InstanceTemplate()->SetInternalFieldCount(1);

    Nan::SetPrototypeMethod(tpl, "getValue", GetValue);

    constructor().Reset(Nan::GetFunction(tpl).ToLocalChecked());
  }

  static NAN_METHOD(NewInstance) {
    v8::Local<v8::Function> cons = Nan::New(constructor());
    double value = info[0]->IsNumber() ? Nan::To<double>(info[0]).FromJust() : 0;
    const int argc = 1;
    v8::Local<v8::Value> argv[1] = {Nan::New(value)};
    info.GetReturnValue().Set(Nan::NewInstance(cons, argc, argv).ToLocalChecked());
  }

  // Needed for the next example:
  inline double value() const {
    return value_;
  }

 private:
  explicit MyFactoryObject(double value = 0) : value_(value) {}
  ~MyFactoryObject() {}

  static NAN_METHOD(New) {
    if (info.IsConstructCall()) {
      double value = info[0]->IsNumber() ? Nan::To<double>(info[0]).FromJust() : 0;
      MyFactoryObject * obj = new MyFactoryObject(value);
      obj->Wrap(info.This());
      info.GetReturnValue().Set(info.This());
    } else {
      const int argc = 1;
      v8::Local<v8::Value> argv[argc] = {info[0]};
      v8::Local<v8::Function> cons = Nan::New(constructor());
      info.GetReturnValue().Set(Nan::NewInstance(cons, argc, argv).ToLocalChecked());
    }
  }

  static NAN_METHOD(GetValue) {
    MyFactoryObject* obj = ObjectWrap::Unwrap<MyFactoryObject>(info.Holder());
    info.GetReturnValue().Set(obj->value_);
  }

  static inline Nan::Persistent<v8::Function> & constructor() {
    static Nan::Persistent<v8::Function> my_constructor;
    return my_constructor;
  }

  double value_;
};

NAN_MODULE_INIT(Init) {
  MyFactoryObject::Init(target);
  Nan::Set(target,
    Nan::New<v8::String>("newFactoryObjectInstance").ToLocalChecked(),
    Nan::GetFunction(
      Nan::New<v8::FunctionTemplate>(MyFactoryObject::NewInstance)).ToLocalChecked()
  );
}

NODE_MODULE(wrappedobjectfactory, Init)

To use in Javascript:

var wrappedobjectfactory = require('bindings')('wrappedobjectfactory');

var obj = wrappedobjectfactory.newFactoryObjectInstance(10);
console.log('Should be 10: ' + obj.getValue());

Passing wrapped objects around

Use the MyFactoryObject class above along with the following:

static NAN_METHOD(Sum) {
  Nan::MaybeLocal<v8::Object> maybe1 = Nan::To<v8::Object>(info[0]);
  Nan::MaybeLocal<v8::Object> maybe2 = Nan::To<v8::Object>(info[1]);

  // Quick check:
  if (maybe1.IsEmpty() || maybe2.IsEmpty()) {
    // return value is undefined by default
    return;
  }

  MyFactoryObject* obj1 =
    Nan::ObjectWrap::Unwrap<MyFactoryObject>(maybe1.ToLocalChecked());
  MyFactoryObject* obj2 =
    Nan::ObjectWrap::Unwrap<MyFactoryObject>(maybe2.ToLocalChecked());

  info.GetReturnValue().Set(Nan::New<v8::Number>(obj1->value() + obj2->value()));
}

NAN_MODULE_INIT(Init) {
  MyFactoryObject::Init(target);
  Nan::Set(target,
    Nan::New<v8::String>("newFactoryObjectInstance").ToLocalChecked(),
    Nan::GetFunction(
      Nan::New<v8::FunctionTemplate>(MyFactoryObject::NewInstance)).ToLocalChecked()
  );
  Nan::Set(target,
    Nan::New<v8::String>("sum").ToLocalChecked(),
    Nan::GetFunction(Nan::New<v8::FunctionTemplate>(Sum)).ToLocalChecked()
  );
}

NODE_MODULE(myaddon, Init)

To use in Javascript:

var myaddon = require('bindings')('myaddon');

var obj1 = myaddon.newFactoryObjectInstance(5);
var obj2 = myaddon.newFactoryObjectInstance(10);
console.log('sum of object values: ' + myaddon.sum(obj1, obj2));