Accessing class variables within static function bound with FunctionTemplate

[Jack]

Not sure if this is the right place to ask for help, sorry.

I am using 
global->Set(String::NewFromUtf8(isolate, "print", NewStringType::kNormal). ToLocalChecked(), FunctionTemplate::New(isolate, V8Instance::js_print));
to create a print() function in JS from the following function:
static void js_print(const v8::FunctionCallbackInfo<v8::Value> &args);
Implementation:
void V8Instance::js_print(const v8::FunctionCallbackInfo<v8::Value> &args) {
    for (int i = 0; i < args.Length(); i++) {
        v8::HandleScope handle_scope(args.GetIsolate());
        v8::String::Utf8Value str(args[i]);
        std::cout << *str << std::endl;
    }
    std::cout << std::endl;
}


As you may be able to tell, both of these are happening inside a class, V8Instance. This works fine, but I have to make js_print a static function to use it in FunctionTemplate::New. 
My problem is that I need access to a variable which is a non-static member of V8Instance within the js_print function. Is there a way I can either: 

• Use js_print as a non-static class member function
• Pass a reference or pointer to my class variable to js_print such that is available whenever it is called

Thanks for any help in advance. :)

[Ben Noordhuis]

You can wrap a pointer to your instance in a v8::External that you
then pass as the |data| argument to v8::FunctionTemplate::New(). It's
available in the callback as args.Data().

[Jack]

On Tuesday, 2 August 2016 07:23:09 UTC+1, Ben Noordhuis wrote:

You can wrap a pointer to your instance in a v8::External that you
then pass as the |data| argument to v8::FunctionTemplate::New().  It's
available in the callback as args.Data().

 Thanks for the reply. Sorry for the trouble, but could someone show an example of creating an External object from a pointer to my class? I assume I need to cast the pointer to void * but I can't figure out how to create the object.

Thanks.

[Jack]

Ok I figured out I need to use:

Local<External> self = External::New(isolate, (void *) this);

However I can't figure out how to extract the pointer again in the function. How do I cast the Local<External> returned by args.Data() back to MyClass *

[Ben Noordhuis]

On Tue, Aug 2, 2016 at 1:44 PM, Jack <jack....@gmail.com> wrote:
> Ok I figured out I need to use:
> Local<External> self = External::New(isolate, (void *) this);
>
> However I can't figure out how to extract the pointer again in the function.
> How do I cast the Local<External> returned by args.Data() back to MyClass *

auto data = args.Data().As<v8::External>();
auto self = static_cast<MyClass*>(data->Value());

[Jack]

Great, thanks for all your help Ben! :)