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@nodejs

# Addons#

Addons are dynamically linked shared objects. They can provide glue to C and C++ libraries. The API (at the moment) is rather complex, involving knowledge of several libraries:

- V8 JavaScript, a C++ library. Used for interfacing with JavaScript: creating objects, calling functions, etc. Documented mostly in the `v8.h` header file (`deps/v8/include/v8.h` in the Node source tree), which is also available online.

- libuv, C event loop library. Anytime one needs to wait for a file descriptor to become readable, wait for a timer, or wait for a signal to received one will need to interface with libuv. That is, if you perform any I/O, libuv will need to be used.

- Internal Node libraries. Most importantly is the `node::ObjectWrap` class which you will likely want to derive from.

- Others. Look in `deps/` for what else is available.

Node statically compiles all its dependencies into the executable. When compiling your module, you don't need to worry about linking to any of these libraries.

## Hello world#

To get started let's make a small Addon which is the C++ equivalent of the following Javascript code:

    exports.hello = function() { return 'world'; };

First we create a file `hello.cc`:

    #include <node.h>
    #include <v8.h>

    using namespace v8;

    Handle<Value> Method(const Arguments& args) {
      HandleScope scope;
      return scope.Close(String::New("world"));
    }

    void init(Handle<Object> target) {
      target->Set(String::NewSymbol("hello"),
          FunctionTemplate::New(Method)->GetFunction());
    }
    NODE_MODULE(hello, init)

Note that all Node addons must export an initialization function:

    void Initialize (Handle<Object> target);
    NODE_MODULE(module_name, Initialize)

There is no semi-colon after `NODE_MODULE` as it's not a function (see `node.h`).

The `module_name` needs to match the filename of the final binary (minus the .node suffix).

The source code needs to be built into `hello.node`, the binary Addon. To do this we create a file called `wscript` which is python code and looks like this:

    srcdir = '.'
    blddir = 'build'
    VERSION = '0.0.1'

    def set_options(opt):
      opt.tool_options('compiler_cxx')

    def configure(conf):
      conf.check_tool('compiler_cxx')
      conf.check_tool('node_addon')

    def build(bld):
      obj = bld.new_task_gen('cxx', 'shlib', 'node_addon')
      obj.target = 'hello'
      obj.source = 'hello.cc'

Running `node-waf configure build` will create a file `build/default/hello.node` which is our Addon.

`node-waf` is just WAF, the python-based build system. `node-waf` is provided for the ease of users.

You can now use the binary addon in a Node project `hello.js` by pointing `require` to the recently built module:

    var addon = require('./build/Release/hello');

    console.log(addon.hello()); // 'world'

Please see patterns below for further information or

https://github.com/pietern/hiredis-node for an example in production.

## Addon patterns#

Below are some addon patterns to help you get started. Consult the online v8 reference for help with the various v8 calls, and v8's Embedder's Guide for an explanation of several concepts used such as handles, scopes, function templates, etc.

To compile these examples, create the `wscript` file below and run `node-waf configure build`:

    srcdir = '.'
    blddir = 'build'
    VERSION = '0.0.1'

    def set_options(opt):
      opt.tool_options('compiler_cxx')

    def configure(conf):
      conf.check_tool('compiler_cxx')
      conf.check_tool('node_addon')

    def build(bld):
      obj = bld.new_task_gen('cxx', 'shlib', 'node_addon')
      obj.target = 'addon'
      obj.source = ['addon.cc']

In cases where there is more than one `.cc` file, simply add the file name to the `obj.source` array, e.g.:

    obj.source = ['addon.cc', 'myexample.cc']

### Function arguments#

The following pattern illustrates how to read arguments from JavaScript function calls and return a result. This is the main and only needed source `addon.cc`:

    #define BUILDING_NODE_EXTENSION
    #include <node.h>

    using namespace v8;

    Handle<Value> Add(const Arguments& args) {
      HandleScope scope;

      if (args.Length() < 2) {
        ThrowException(Exception::TypeError(String::New("Wrong number of arguments")));
        return scope.Close(Undefined());
      }

      if (!args[0]->IsNumber() || !args[1]->IsNumber()) {
        ThrowException(Exception::TypeError(String::New("Wrong arguments")));
        return scope.Close(Undefined());
      }

      Local<Number> num = Number::New(args[0]->NumberValue() +
          args[1]->NumberValue());
      return scope.Close(num);
    }

    void Init(Handle<Object> target) {
      target->Set(String::NewSymbol("add"),
          FunctionTemplate::New(Add)->GetFunction());
    }

    NODE_MODULE(addon, Init)

You can test it with the following JavaScript snippet:

    var addon = require('./build/Release/addon');

    console.log( 'This should be eight:', addon.add(3,5) );

### Callbacks#

You can pass JavaScript functions to a C++ function and execute them from there. Here's `addon.cc`:

    #define BUILDING_NODE_EXTENSION
    #include <node.h>

    using namespace v8;

    Handle<Value> RunCallback(const Arguments& args) {
      HandleScope scope;

      Local<Function> cb = Local<Function>::Cast(args[0]);
      const unsigned argc = 1;
      Local<Value> argv[argc] = { Local<Value>::New(String::New("hello world")) };
      cb->Call(Context::GetCurrent()->Global(), argc, argv);

      return scope.Close(Undefined());
    }

    void Init(Handle<Object> target) {
      target->Set(String::NewSymbol("runCallback"),
          FunctionTemplate::New(RunCallback)->GetFunction());
    }

    NODE_MODULE(addon, Init)

To test it run the following JavaScript snippet:

    var addon = require('./build/Release/addon');

    addon.runCallback(function(msg){
      console.log(msg); // 'hello world'
    });

### Object factory#

You can create and return new objects from within a C++ function with this `addon.cc` pattern, which returns an object with property `msg` that echoes the string passed to `createObject()`:

    #define BUILDING_NODE_EXTENSION
    #include <node.h>

    using namespace v8;

    Handle<Value> CreateObject(const Arguments& args) {
      HandleScope scope;

      Local<Object> obj = Object::New();
      obj->Set(String::NewSymbol("msg"), args[0]->ToString());

      return scope.Close(obj);
    }

    void Init(Handle<Object> target) {
      target->Set(String::NewSymbol("createObject"),
          FunctionTemplate::New(CreateObject)->GetFunction());
    }

    NODE_MODULE(addon, Init)

To test it in JavaScript:

    var addon = require('./build/Release/addon');

    var obj1 = addon.createObject('hello');
    var obj2 = addon.createObject('world');
    console.log(obj1.msg+' '+obj2.msg); // 'hello world'

### Function factory#

This pattern illustrates how to create and return a JavaScript function that wraps a C++ function:

    #define BUILDING_NODE_EXTENSION
    #include <node.h>

    using namespace v8;

    Handle<Value> MyFunction(const Arguments& args) {
      HandleScope scope;
      return scope.Close(String::New("hello world"));
    }

    Handle<Value> CreateFunction(const Arguments& args) {
      HandleScope scope;

      Local<FunctionTemplate> tpl = FunctionTemplate::New(MyFunction);
      Local<Function> fn = tpl->GetFunction();
      fn->SetName(String::NewSymbol("theFunction")); // omit this to make it anonymous

      return scope.Close(fn);
    }

    void Init(Handle<Object> target) {
      target->Set(String::NewSymbol("createFunction"),
          FunctionTemplate::New(CreateFunction)->GetFunction());
    }

    NODE_MODULE(addon, Init)

To test:

    var addon = require('./build/Release/addon');

    var fn = addon.createFunction();
    console.log(fn()); // 'hello world'

### Wrapping C++ objects#

Here we will create a wrapper for a C++ object/class `MyObject` that can be instantiated in JavaScript through the `new` operator. First prepare the main module `addon.cc`:

    #define BUILDING_NODE_EXTENSION
    #include <node.h>
    #include "myobject.h"

    using namespace v8;

    void InitAll(Handle<Object> target) {
      MyObject::Init(target);
    }

    NODE_MODULE(addon, InitAll)

Then in `myobject.h` make your wrapper inherit from `node::ObjectWrap`:

    #ifndef MYOBJECT_H
    #define MYOBJECT_H

    #include <node.h>

    class MyObject : public node::ObjectWrap {
     public:
      static void Init(v8::Handle<v8::Object> target);

     private:
      MyObject();
      ~MyObject();

      static v8::Handle<v8::Value> New(const v8::Arguments& args);
      static v8::Handle<v8::Value> PlusOne(const v8::Arguments& args);
      double counter_;
    };

    #endif

And in `myobject.cc` implement the various methods that you want to expose. Here we expose the method `plusOne` by adding it to the constructor's prototype:

    #define BUILDING_NODE_EXTENSION
    #include <node.h>
    #include "myobject.h"

    using namespace v8;

    MyObject::MyObject() {};
    MyObject::~MyObject() {};

    void MyObject::Init(Handle<Object> target) {
      // Prepare constructor template
      Local<FunctionTemplate> tpl = FunctionTemplate::New(New);
      tpl->SetClassName(String::NewSymbol("MyObject"));
      tpl->InstanceTemplate()->SetInternalFieldCount(1);
      // Prototype
      tpl->PrototypeTemplate()->Set(String::NewSymbol("plusOne"),
          FunctionTemplate::New(PlusOne)->GetFunction());

      Persistent<Function> constructor = Persistent<Function>::New(tpl->GetFunction());
      target->Set(String::NewSymbol("MyObject"), constructor);
    }

    Handle<Value> MyObject::New(const Arguments& args) {
      HandleScope scope;

      MyObject* obj = new MyObject();
      obj->counter_ = args[0]->IsUndefined() ? 0 : args[0]->NumberValue();
      obj->Wrap(args.This());

      return args.This();
    }

    Handle<Value> MyObject::PlusOne(const Arguments& args) {
      HandleScope scope;

      MyObject* obj = ObjectWrap::Unwrap<MyObject>(args.This());
      obj->counter_ += 1;

      return scope.Close(Number::New(obj->counter_));
    }

Test it with:

    var addon = require('./build/Release/addon');

    var obj = new addon.MyObject(10);
    console.log( obj.plusOne() ); // 11
    console.log( obj.plusOne() ); // 12
    console.log( obj.plusOne() ); // 13

### Factory of wrapped objects#

This is useful when you want to be able to create native objects without explicitly instantiating them with the `new` operator in JavaScript, e.g.

    var obj = addon.createObject();
    // instead of:
    // var obj = new addon.Object();

Let's register our `createObject` method in `addon.cc`:

    #define BUILDING_NODE_EXTENSION
    #include <node.h>
    #include "myobject.h"

    using namespace v8;

    Handle<Value> CreateObject(const Arguments& args) {
      HandleScope scope;
      return scope.Close(MyObject::NewInstance(args));
    }

    void InitAll(Handle<Object> target) {
      MyObject::Init();

      target->Set(String::NewSymbol("createObject"),
          FunctionTemplate::New(CreateObject)->GetFunction());
    }

    NODE_MODULE(addon, InitAll)

In `myobject.h` we now introduce the static method `NewInstance` that takes care of instantiating the object (i.e. it does the job of `new` in JavaScript):

    #define BUILDING_NODE_EXTENSION
    #ifndef MYOBJECT_H
    #define MYOBJECT_H

    #include <node.h>

    class MyObject : public node::ObjectWrap {
     public:
      static void Init();
      static v8::Handle<v8::Value> NewInstance(const v8::Arguments& args);

     private:
      MyObject();
      ~MyObject();

      static v8::Persistent<v8::Function> constructor;
      static v8::Handle<v8::Value> New(const v8::Arguments& args);
      static v8::Handle<v8::Value> PlusOne(const v8::Arguments& args);
      double counter_;
    };

    #endif

The implementation is similar to the above in `myobject.cc`:

    #define BUILDING_NODE_EXTENSION
    #include <node.h>
    #include "myobject.h"

    using namespace v8;

    MyObject::MyObject() {};
    MyObject::~MyObject() {};

    Persistent<Function> MyObject::constructor;

    void MyObject::Init() {
      // Prepare constructor template
      Local<FunctionTemplate> tpl = FunctionTemplate::New(New);
      tpl->SetClassName(String::NewSymbol("MyObject"));
      tpl->InstanceTemplate()->SetInternalFieldCount(1);
      // Prototype
      tpl->PrototypeTemplate()->Set(String::NewSymbol("plusOne"),
          FunctionTemplate::New(PlusOne)->GetFunction());

      constructor = Persistent<Function>::New(tpl->GetFunction());
    }

    Handle<Value> MyObject::New(const Arguments& args) {
      HandleScope scope;

      MyObject* obj = new MyObject();
      obj->counter_ = args[0]->IsUndefined() ? 0 : args[0]->NumberValue();
      obj->Wrap(args.This());

      return args.This();
    }

    Handle<Value> MyObject::NewInstance(const Arguments& args) {
      HandleScope scope;

      const unsigned argc = 1;
      Handle<Value> argv[argc] = { args[0] };
      Local<Object> instance = constructor->NewInstance(argc, argv);

      return scope.Close(instance);
    }

    Handle<Value> MyObject::PlusOne(const Arguments& args) {
      HandleScope scope;

      MyObject* obj = ObjectWrap::Unwrap<MyObject>(args.This());
      obj->counter_ += 1;

      return scope.Close(Number::New(obj->counter_));
    }

Test it with:

    var addon = require('./build/Release/addon');

    var obj = addon.createObject(10);
    console.log( obj.plusOne() ); // 11
    console.log( obj.plusOne() ); // 12
    console.log( obj.plusOne() ); // 13

    var obj2 = addon.createObject(20);
    console.log( obj2.plusOne() ); // 21
    console.log( obj2.plusOne() ); // 22
    console.log( obj2.plusOne() ); // 23

### Passing wrapped objects around#

In addition to wrapping and returning C++ objects, you can pass them around by unwrapping them with Node's `node::ObjectWrap::Unwrap` helper function. In the following `addon.cc` we introduce a function `add()` that can take on two `MyObject` objects:

    #define BUILDING_NODE_EXTENSION
    #include <node.h>
    #include "myobject.h"

    using namespace v8;

    Handle<Value> CreateObject(const Arguments& args) {
      HandleScope scope;
      return scope.Close(MyObject::NewInstance(args));
    }

    Handle<Value> Add(const Arguments& args) {
      HandleScope scope;

      MyObject* obj1 = node::ObjectWrap::Unwrap<MyObject>(
          args[0]->ToObject());
      MyObject* obj2 = node::ObjectWrap::Unwrap<MyObject>(
          args[1]->ToObject());

      double sum = obj1->Val() + obj2->Val();
      return scope.Close(Number::New(sum));
    }

    void InitAll(Handle<Object> target) {
      MyObject::Init();

      target->Set(String::NewSymbol("createObject"),
          FunctionTemplate::New(CreateObject)->GetFunction());

      target->Set(String::NewSymbol("add"),
          FunctionTemplate::New(Add)->GetFunction());
    }

    NODE_MODULE(addon, InitAll)

To make things interesting we introduce a public method in `myobject.h` so we can probe private values after unwrapping the object:

    #define BUILDING_NODE_EXTENSION
    #ifndef MYOBJECT_H
    #define MYOBJECT_H

    #include <node.h>

    class MyObject : public node::ObjectWrap {
     public:
      static void Init();
      static v8::Handle<v8::Value> NewInstance(const v8::Arguments& args);
      double Val() const { return val_; }

     private:
      MyObject();
      ~MyObject();

      static v8::Persistent<v8::Function> constructor;
      static v8::Handle<v8::Value> New(const v8::Arguments& args);
      double val_;
    };

    #endif

The implementation of `myobject.cc` is similar as before:

    #define BUILDING_NODE_EXTENSION
    #include <node.h>
    #include "myobject.h"

    using namespace v8;

    MyObject::MyObject() {};
    MyObject::~MyObject() {};

    Persistent<Function> MyObject::constructor;

    void MyObject::Init() {
      // Prepare constructor template
      Local<FunctionTemplate> tpl = FunctionTemplate::New(New);
      tpl->SetClassName(String::NewSymbol("MyObject"));
      tpl->InstanceTemplate()->SetInternalFieldCount(1);

      constructor = Persistent<Function>::New(tpl->GetFunction());
    }

    Handle<Value> MyObject::New(const Arguments& args) {
      HandleScope scope;

      MyObject* obj = new MyObject();
      obj->val_ = args[0]->IsUndefined() ? 0 : args[0]->NumberValue();
      obj->Wrap(args.This());

      return args.This();
    }

    Handle<Value> MyObject::NewInstance(const Arguments& args) {
      HandleScope scope;

      const unsigned argc = 1;
      Handle<Value> argv[argc] = { args[0] };
      Local<Object> instance = constructor->NewInstance(argc, argv);

      return scope.Close(instance);
    }

Test it with:

    var addon = require('./build/Release/addon');

    var obj1 = addon.createObject(10);
    var obj2 = addon.createObject(20);
    var result = addon.add(obj1, obj2);

    console.log(result); // 30