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264 lines
8.0 KiB
264 lines
8.0 KiB
4 years ago
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## Object Wrappers
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The `ObjectWrap` class can be used to make wrapped C++ objects and a factory of wrapped objects.
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- <a href="#api_nan_object_wrap"><b><code>Nan::ObjectWrap</code></b></a>
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<a name="api_nan_object_wrap"></a>
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### Nan::ObjectWrap()
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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.
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Definition:
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```c++
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class ObjectWrap {
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public:
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ObjectWrap();
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virtual ~ObjectWrap();
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template <class T>
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static inline T* Unwrap(v8::Local<v8::Object> handle);
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inline v8::Local<v8::Object> handle();
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inline Nan::Persistent<v8::Object>& persistent();
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protected:
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inline void Wrap(v8::Local<v8::Object> handle);
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inline void MakeWeak();
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/* Ref() marks the object as being attached to an event loop.
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* Refed objects will not be garbage collected, even if
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* all references are lost.
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*/
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virtual void Ref();
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/* Unref() marks an object as detached from the event loop. This is its
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* default state. When an object with a "weak" reference changes from
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* attached to detached state it will be freed. Be careful not to access
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* the object after making this call as it might be gone!
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* (A "weak reference" means an object that only has a
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* persistent handle.)
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*
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* DO NOT CALL THIS FROM DESTRUCTOR
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*/
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virtual void Unref();
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int refs_; // ro
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};
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```
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See the Node documentation on [Wrapping C++ Objects](https://nodejs.org/api/addons.html#addons_wrapping_c_objects) for more details.
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### This vs. Holder
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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.
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The `Signature` installed by [`Nan::SetPrototypeMethod()`](methods.md#api_nan_set_prototype_method) does ensure that `info.Holder()` is just such an instance.
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In Node 0.12 and later, `info.This()` will also be of such a type, since otherwise the invocation will get rejected.
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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.
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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.
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On the other hand, calling `Unwrap` in an [accessor](methods.md#api_nan_set_accessor) should not use `Holder()` if the accessor is defined on the prototype.
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So either define your accessors on the instance template,
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or use `This()` after verifying that it is indeed a valid object.
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### Examples
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#### Basic
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```c++
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class MyObject : public Nan::ObjectWrap {
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public:
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static NAN_MODULE_INIT(Init) {
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v8::Local<v8::FunctionTemplate> tpl = Nan::New<v8::FunctionTemplate>(New);
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tpl->SetClassName(Nan::New("MyObject").ToLocalChecked());
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tpl->InstanceTemplate()->SetInternalFieldCount(1);
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Nan::SetPrototypeMethod(tpl, "getHandle", GetHandle);
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Nan::SetPrototypeMethod(tpl, "getValue", GetValue);
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constructor().Reset(Nan::GetFunction(tpl).ToLocalChecked());
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Nan::Set(target, Nan::New("MyObject").ToLocalChecked(),
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Nan::GetFunction(tpl).ToLocalChecked());
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}
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private:
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explicit MyObject(double value = 0) : value_(value) {}
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~MyObject() {}
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static NAN_METHOD(New) {
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if (info.IsConstructCall()) {
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double value = info[0]->IsUndefined() ? 0 : Nan::To<double>(info[0]).FromJust();
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MyObject *obj = new MyObject(value);
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obj->Wrap(info.This());
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info.GetReturnValue().Set(info.This());
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} else {
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const int argc = 1;
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v8::Local<v8::Value> argv[argc] = {info[0]};
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v8::Local<v8::Function> cons = Nan::New(constructor());
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info.GetReturnValue().Set(Nan::NewInstance(cons, argc, argv).ToLocalChecked());
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}
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}
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static NAN_METHOD(GetHandle) {
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MyObject* obj = Nan::ObjectWrap::Unwrap<MyObject>(info.Holder());
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info.GetReturnValue().Set(obj->handle());
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}
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static NAN_METHOD(GetValue) {
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MyObject* obj = Nan::ObjectWrap::Unwrap<MyObject>(info.Holder());
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info.GetReturnValue().Set(obj->value_);
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}
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static inline Nan::Persistent<v8::Function> & constructor() {
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static Nan::Persistent<v8::Function> my_constructor;
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return my_constructor;
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}
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double value_;
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};
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NODE_MODULE(objectwrapper, MyObject::Init)
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```
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To use in Javascript:
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```Javascript
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var objectwrapper = require('bindings')('objectwrapper');
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var obj = new objectwrapper.MyObject(5);
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console.log('Should be 5: ' + obj.getValue());
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```
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#### Factory of wrapped objects
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```c++
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class MyFactoryObject : public Nan::ObjectWrap {
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public:
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static NAN_MODULE_INIT(Init) {
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v8::Local<v8::FunctionTemplate> tpl = Nan::New<v8::FunctionTemplate>(New);
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tpl->InstanceTemplate()->SetInternalFieldCount(1);
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Nan::SetPrototypeMethod(tpl, "getValue", GetValue);
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constructor().Reset(Nan::GetFunction(tpl).ToLocalChecked());
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}
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static NAN_METHOD(NewInstance) {
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v8::Local<v8::Function> cons = Nan::New(constructor());
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double value = info[0]->IsNumber() ? Nan::To<double>(info[0]).FromJust() : 0;
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const int argc = 1;
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v8::Local<v8::Value> argv[1] = {Nan::New(value)};
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info.GetReturnValue().Set(Nan::NewInstance(cons, argc, argv).ToLocalChecked());
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}
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// Needed for the next example:
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inline double value() const {
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return value_;
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}
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private:
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explicit MyFactoryObject(double value = 0) : value_(value) {}
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~MyFactoryObject() {}
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static NAN_METHOD(New) {
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if (info.IsConstructCall()) {
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double value = info[0]->IsNumber() ? Nan::To<double>(info[0]).FromJust() : 0;
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MyFactoryObject * obj = new MyFactoryObject(value);
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obj->Wrap(info.This());
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info.GetReturnValue().Set(info.This());
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} else {
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const int argc = 1;
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v8::Local<v8::Value> argv[argc] = {info[0]};
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v8::Local<v8::Function> cons = Nan::New(constructor());
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info.GetReturnValue().Set(Nan::NewInstance(cons, argc, argv).ToLocalChecked());
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}
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}
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static NAN_METHOD(GetValue) {
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MyFactoryObject* obj = ObjectWrap::Unwrap<MyFactoryObject>(info.Holder());
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info.GetReturnValue().Set(obj->value_);
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}
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static inline Nan::Persistent<v8::Function> & constructor() {
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static Nan::Persistent<v8::Function> my_constructor;
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return my_constructor;
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}
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double value_;
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};
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NAN_MODULE_INIT(Init) {
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MyFactoryObject::Init(target);
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Nan::Set(target,
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Nan::New<v8::String>("newFactoryObjectInstance").ToLocalChecked(),
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Nan::GetFunction(
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Nan::New<v8::FunctionTemplate>(MyFactoryObject::NewInstance)).ToLocalChecked()
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);
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}
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NODE_MODULE(wrappedobjectfactory, Init)
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```
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To use in Javascript:
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```Javascript
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var wrappedobjectfactory = require('bindings')('wrappedobjectfactory');
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var obj = wrappedobjectfactory.newFactoryObjectInstance(10);
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console.log('Should be 10: ' + obj.getValue());
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```
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#### Passing wrapped objects around
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Use the `MyFactoryObject` class above along with the following:
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```c++
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static NAN_METHOD(Sum) {
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Nan::MaybeLocal<v8::Object> maybe1 = Nan::To<v8::Object>(info[0]);
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Nan::MaybeLocal<v8::Object> maybe2 = Nan::To<v8::Object>(info[1]);
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// Quick check:
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if (maybe1.IsEmpty() || maybe2.IsEmpty()) {
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// return value is undefined by default
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return;
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}
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MyFactoryObject* obj1 =
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Nan::ObjectWrap::Unwrap<MyFactoryObject>(maybe1.ToLocalChecked());
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MyFactoryObject* obj2 =
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Nan::ObjectWrap::Unwrap<MyFactoryObject>(maybe2.ToLocalChecked());
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info.GetReturnValue().Set(Nan::New<v8::Number>(obj1->value() + obj2->value()));
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}
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NAN_MODULE_INIT(Init) {
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MyFactoryObject::Init(target);
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Nan::Set(target,
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Nan::New<v8::String>("newFactoryObjectInstance").ToLocalChecked(),
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Nan::GetFunction(
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Nan::New<v8::FunctionTemplate>(MyFactoryObject::NewInstance)).ToLocalChecked()
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);
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Nan::Set(target,
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Nan::New<v8::String>("sum").ToLocalChecked(),
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Nan::GetFunction(Nan::New<v8::FunctionTemplate>(Sum)).ToLocalChecked()
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);
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}
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NODE_MODULE(myaddon, Init)
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```
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To use in Javascript:
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```Javascript
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var myaddon = require('bindings')('myaddon');
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var obj1 = myaddon.newFactoryObjectInstance(5);
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var obj2 = myaddon.newFactoryObjectInstance(10);
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console.log('sum of object values: ' + myaddon.sum(obj1, obj2));
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```
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