How to get JS object for class instance using Embind

[stevenwdv]

For an Embind class_, is there a way to get the current JavaScript object (so JS's this as an emscripten::val) from the body of a method called by Embind?

For example:

struct MyClass {
  void fun() {
    // How to get reference to object that `fun` was called on as emscripten::val?
  }
};

EMSCRIPTEN_BINDINGS(MyClass) {
  emscripten::class_<MyClass>("MyClass")
    .function("fun", &MyClass::fun);
}

[sbc100]

@brendandahl

[brendandahl]

    val valThis(this, allow_raw_pointers());
    val::global("console").call<void>("log", valThis);

[stevenwdv]

Thanks for your reaction! Unfortunately, this doesn't do what I want, as it creates a new JavaScript object instead.

Try this:

#include <cstdint>
#include <string>
#include <utility>
#include <iostream>

#include <emscripten/bind.h>
#include <emscripten/val.h>

using namespace emscripten;

struct MyClass {
  void fun() {
    {
      val valThis(this, allow_raw_pointers());
      valThis.set("extraProp", 123);
    }
    {
      val valThis(this, allow_raw_pointers());
      val::global("console").call<void>("log", val("extraProp is now:"), valThis["extraProp"]);
    }
  }
};

EMSCRIPTEN_BINDINGS(MyClass) {
  class_<MyClass>("MyClass")
    .function("fun", &MyClass::fun);
}

int main() {
  val(MyClass{}).call<void>("fun");
  //val::global("window").set("myObj", MyClass{});
}

em++ -lembind -o webtest.js main.cpp && node webtest.js

I'd want it to print extraProp is now: 123, but instead it prints extraProp is now: undefined. If you uncomment the last line and build as .html instead and open in the browser, you can verify that myObj.extraProp is indeed still undefined after calling myObj.fun().

[brendandahl]

The JS object representing the C++ object is just a thin wrapper. Embind doesn't cache the wrappers so it's not able to provide the same wrapper everytime the C++ object is passed into JS. Some alternatives:

1. Put the extraProp as a member in the C++ MyClass and add a binding for it.
2. Add a val member to MyClass and put the properties there.
3. Maintain your own map in JS of C++ object to some properties.