Passing objects into callback function from C++ code

Question

I'm writing Agueas [1] addon for Node.js

For now I have synchronous code, C++ class looks like this:

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

protected:
    augeas * m_aug;
    LibAugeas();
    ~LibAugeas();

    static Handle<Value> New(const Arguments& args);

    static Handle<Value> get        (const Arguments& args);
    static Handle<Value> set        (const Arguments& args);
    static Handle<Value> setm       (const Arguments& args);
    // other methods
};

Usage of this class in JS:

var lib = require('...');
var aug = new lib.Augeas(...);
aug.set(...);
aug.get(...);
// etc

I'm going to impelement asynchronous code.

The bottleneck is creating augeas object (aug_init) while all or some lenses and files are being loaded and parsed. So the idea is creating augeas object asynchronously, and then pass created JS object in a callback function:

Pure C thread: call aug_init(), aug_load() to get augeas handle.
When ready, use augeas handle to creat JS object (see the first snippet)
Pass created JS object to callback function.

Usage might be as such:

lib.heracles(function(aug) {
        if (!aug.error()) {
            console.log('Hello!');

            // async save:
            aug.save(function(err, msg) {
                console.log(msg);
                });
        } else {
            console.log('Sad, but true :-(');
        }
    }
);

And finally, my problem: I do not know how to create JS object in C++ :-)

Constructor static Handle<Value> New(const Arguments& args); returns args.This(), but when I'm in C++ code I do not have args and also can't wrap object.

So, how do I create JS object in C++? Please, don't break my heart saying it is not possible :-)

[1] http://augeas.net

user933161 · Accepted Answer · 2012-04-24 17:38:59Z

1

Ok, thanks to everyone :-) I've found the right way. Here is a static method which creates an JS object wrapping given augeas handle. Then I can pass this object to callback function from C++ code.

Local<Object> LibAugeas::New(augeas *aug)
{
    LibAugeas *obj = new LibAugeas();
    obj->m_aug = aug;

    Handle<ObjectTemplate> tpl = ObjectTemplate::New();
    tpl->SetInternalFieldCount(1); // one field for LibAugeas* pointer (via obj->Wrap())

#define _OBJ_NEW_METHOD(m) NODE_SET_METHOD(tpl, #m, m)
    _OBJ_NEW_METHOD(get);
    _OBJ_NEW_METHOD(set);
    _OBJ_NEW_METHOD(setm);
    _OBJ_NEW_METHOD(rm);
    _OBJ_NEW_METHOD(mv);
    _OBJ_NEW_METHOD(save);
    _OBJ_NEW_METHOD(nmatch);
    _OBJ_NEW_METHOD(insert);
    _OBJ_NEW_METHOD(error);

    Local<Object> O = tpl->NewInstance();
    obj->Wrap(O);
    return O;
}

answered Apr 24, 2012 at 17:38

user933161

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1 Comment

user933161 Over a year ago

Update: object tenplate may be created once, also FunctionTemplate may be reused.

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