[SOLVED] c++ Java javascript chain ## Object Wrappers

## Object Wrappers

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

Nan::ObjectWrap

### 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:

`c++
class ObjectWrap {
public:
ObjectWrap();

virtual ~ObjectWrap();

template
static inline T* Unwrap(v8::Local handle);

inline v8::Local handle();

inline Nan::Persistent & persistent();

protected:
inline void Wrap(v8::Local 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](https://nodejs.org/api/addons.html#addons_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()`](methods.md#api_nan_set_prototype_method) 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](methods.md#api_nan_set_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

`c++
class MyObject : public Nan::ObjectWrap {
public:
static NAN_MODULE_INIT(Init) {
v8::Local tpl = Nan::New (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 (info[0]).FromJust();
MyObject *obj = new MyObject(value);
obj->Wrap(info.This());
info.GetReturnValue().Set(info.This());
} else {
const int argc = 1;
v8::Local argv[argc] = {info[0]};
v8::Local cons = Nan::New(constructor());
info.GetReturnValue().Set(cons->NewInstance(argc, argv));
}
}

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

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

static inline Nan::Persistent & constructor() {
static Nan::Persistent my_constructor;
return my_constructor;
}

double value_;
};

NODE_MODULE(objectwrapper, MyObject::Init)
`

To use in Javascript:

`Javascript
var objectwrapper = require(bindings)(objectwrapper);

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

#### Factory of wrapped objects

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

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

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

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

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

static inline Nan::Persistent & constructor() {
static Nan::Persistent my_constructor;
return my_constructor;
}

double value_;
};

NAN_MODULE_INIT(Init) {
MyFactoryObject::Init(target);
Nan::Set(target,
Nan::New (newFactoryObjectInstance).ToLocalChecked(),
Nan::GetFunction(
Nan::New (MyFactoryObject::NewInstance)).ToLocalChecked()
);
}

NODE_MODULE(wrappedobjectfactory, Init)
`

To use in Javascript:

`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:

`c++
static NAN_METHOD(Sum) {
Nan::MaybeLocal maybe1 = Nan::To (info[0]);
Nan::MaybeLocal maybe2 = Nan::To (info[1]);

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

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

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

NAN_MODULE_INIT(Init) {
MyFactoryObject::Init(target);
Nan::Set(target,
Nan::New (newFactoryObjectInstance).ToLocalChecked(),
Nan::GetFunction(
Nan::New (MyFactoryObject::NewInstance)).ToLocalChecked()
);
Nan::Set(target,
Nan::New (sum).ToLocalChecked(),
Nan::GetFunction(Nan::New (Sum)).ToLocalChecked()
);
}

NODE_MODULE(myaddon, Init)
`

To use in Javascript:

`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));
`