V8 FunctionTemplate Class Instance

Question

I have the following class:

class PluginManager
{
public:
    Handle<Value> Register(const Arguments& args);
    Handle<ObjectTemplate> GetObjectTemplate();
};  

I want the Register method to be accessible from JavaScript. I add it to the global object like this:

PluginManager pluginManagerInstance;

global->Set(String::New("register"), FunctionTemplate::New(pluginManagerInstance.Register)); 

It throws the following error:

'PluginManager::Register': function call missing argument list; use '&PluginManager::Register' to create a pointer to member

I tried to do that, but it doesn't work either. And it's not correct, because I want it to call the Register method of the pluginManagerInstance.

Except for making the Register method static or global, any ideas?

Thanks.

Answer 1

You're trying to bind two things at once: the instance and the method to invoke on it, and have it look like a function pointer. That unfortunately doesn't work in C++. You can only bind a pointer to a plain function or a static method. So image you add a static "RegisterCB" method and register it as the callback:

static Handle<Value> RegisterCB(const Arguments& args);
...FunctionTemplate::New(&PluginManager::RegisterCB)...

Now where do you get the pluginManagerInstance from? For this purpose, most callback-registration apis in V8 have an additional "data" parameter that will get passed back to the callback. So does FunctionTemplate::New. So you actually want to bind it like this:

...FunctionTemplate::New(&PluginManager::RegisterCB,
                         External::Wrap(pluginManagerInstance))...

The data is then available through args.Data() and you can delegate to the actual method:

return ((PluginManager*)External::Unwrap(args.Data())->Register(args);

This can surely be made a little easier with some macro.

Answer 2

You will likely need to make it static. Don't forget member functions take a hidden this parameter as the first argument. Because of this, they rarely work well as function pointer prototypes.

Answer 3

For an example take a look at the code in this tutorial. The same method mernst suggests above is used to send a pointer to this object, to the log function.

in header:

    virtual void log(const string &str);
    static Handle<Value> logCallback(const Arguments &args);

    Local<FunctionTemplate> makeStaticCallableFunc(InvocationCallback func);
    Local<External> classPtrToExternal();

    ////////////////////////////////////////////////////////////////////////
    //
    // Converts an External to a V8TutorialBase pointer. This assumes that the
    // data inside the v8::External is a "this" pointer that was wrapped by
    // makeStaticCallableFunc
    //
    // \parameter data Shoudld be v8::Arguments::Data()
    //
    // \return "this" pointer inside v8::Arguments::Data() on success, NULL otherwise
    //
    ////////////////////////////////////////////////////////////////////////        
    template <typename T>
    static T *externalToClassPtr(Local<Value> data)
    {
        if(data.IsEmpty())
            cout<<"Data empty"<<endl;
        else if(!data->IsExternal())
            cout<<"Data not external"<<endl;
        else
            return static_cast<T *>(External::Unwrap(data));

        //If function gets here, one of the checks above failed
        return NULL;
    }

implementation:

////////////////////////////////////////////////////////////////////////
//
// Wrap a callback function into a FunctionTemplate, providing the "this"
// pointer to the callback when v8 calls the callback func
//
// \parameter func Static callback to be used in FunctionTemplate
//
// \return Local<FunctionTemplate> containing func
//
////////////////////////////////////////////////////////////////////////
Local<FunctionTemplate> V8TutorialBase::makeStaticCallableFunc(InvocationCallback func)
{
    HandleScope scope;
    Local<FunctionTemplate> funcTemplate = FunctionTemplate::New(func, classPtrToExternal());
    return scope.Close(funcTemplate);
}

////////////////////////////////////////////////////////////////////////
//
// Makes the "this" pointer be an external so that it can be accessed by
// the static callback functions
//
// \return Local<External> containing the "this" pointer
////////////////////////////////////////////////////////////////////////
Local<External> V8TutorialBase::classPtrToExternal()
{
    HandleScope scope;
    return scope.Close(External::New(reinterpret_cast<void *>(this)));
}

Handle<Value> V8TutorialBase::logCallback(const Arguments &args)
{
    HandleScope scope;

    .....

    V8TutorialBase *objPtr = externalToClassPtr<V8TutorialBase>(args.Data());
    String::Utf8Value val(Local<String>::Cast(args[0]));
    objPtr->log(*val);    // log is a non static member function 
    // or you can directly do anything that you would do in a member function using the objPtr

    return v8::Null();
}

Answer 4

If you want to call that method, you have to add parentheses:

lobal->Set( String::New("register")
          , FunctionTemplate::New(pluginManagerInstance.Register()) );
                                                                ^^

If you want to take its address, you have to add a &:

lobal->Set( String::New("register")
          , FunctionTemplate::New(&PluginManager::Register) );
                                  ^

(Which is exactly what the error message says.)