Simple embind example doesn't seem to work

[k1nder10]

main.cpp

#include <emscripten/bind.h>

struct Point {
  int x;
  int y;
};

EMSCRIPTEN_BINDINGS(MainModule) {
  emscripten::value_object<Point>("Point")
  .field("x", &Point::x)
  .field("y", &Point::y)
  ;
}

int main() {}

my_js.js

var Module = require('./main.js')

Module.onRuntimeInitialized = async function(){
  var point = new Module.Point();
}

Build command: emcc main.cpp -o main.js -lembind
emsdk version is 3.1.13

Runtime error:
TypeError: Module.Point is not a constructor
at Object.Module.onRuntimeInitialized (C:\projects\opengl_wasm\bind_small_example\my_js.js:7:15)
at doRun (C:\projects\opengl_wasm\bind_small_example\main.js:3138:71)
at run (C:\projects\opengl_wasm\bind_small_example\main.js:3155:5)
at runCaller (C:\projects\opengl_wasm\bind_small_example\main.js:3060:19)
at removeRunDependency (C:\projects\opengl_wasm\bind_small_example\main.js:1404:7)
at receiveInstance (C:\projects\opengl_wasm\bind_small_example\main.js:1592:5)
at receiveInstantiationResult (C:\projects\opengl_wasm\bind_small_example\main.js:1610:5)

[k1nder10]

I tried to expose Point as emscripten::class and it works fine. But neither emscripten::value_object nor emscripten::value_array works

class_<Point>("Point")
  .constructor<>();

[brendandahl]

I don't think value_object is meant to be used with new. There are some examples using value_object in the tests. It uses a function to get one:

emscripten/tests/embind/embind_test.cpp

Lines 2579 to 2582 in df69d56

	struct OrderedStruct {
	FirstElement first;
	SecondElement second;
	};

emscripten/tests/embind/embind_test.cpp

Lines 2588 to 2590 in df69d56

	OrderedStruct getOrderedStruct() {
	return OrderedStruct();
	}

emscripten/tests/embind/embind.test.js

Lines 2408 to 2412 in df69d56

	var os = cm.getOrderedStruct();
	assert.instanceof(os.first, cm.FirstElement);
	assert.instanceof(os.second, cm.SecondElement);
	os.first.delete();
	os.second.delete();

[sbc100]

I'm afraid I'm not much of an embind expert, but its seems like the world of JS library functions and the world of embind are separate. As far as I know JS library function only take basic types (and pointers).. So I don't think you can make a JS library function that takes a Point object directly... but I could be wrong.

[brendandahl]

Passing by value isn't really wired up that way to work between JS<->C++ and doesn't really behave how it would in a pure C/C++ program. value_object is more a helper for creating a way to send a non mutable object between JS/C++. If you did want to mix more JS/C++ you could do something like:

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

using namespace emscripten;

// in C++:
struct Point { 
  int x;
  int y;

  int getX() const {
    return x;
  }
  void setX(int aX) {
    x = aX;
  }

  int getY() const {
    return y;
  }
  void setY(int aY) {
    y = aY;
  }

  static Point* load(int ptr) {
    return (Point*) ptr;
  }
};

extern "C" extern void fill_point(Point* p);
extern "C" extern Point* get_point();

int main() {
  Point* p = get_point();
  printf("Point1 x=%d y=%d\n", p->x, p->y);
  fill_point(p);
  printf("Point2 x=%d y=%d\n", p->x, p->y);
  delete p;
  return 0;
}

EMSCRIPTEN_BINDINGS(order) {
    class_<Point>("Point")
        .constructor()
        .property("x", &Point::getX, &Point::setX)
        .property("y", &Point::getY, &Point::setY)
        .class_function("load", &Point::load, allow_raw_pointers());
}

mergeInto(LibraryManager.library, {
    get_point: function() {
        let p = new Module.Point();
        p.x = 5;
        p.y = 10;

        return p.$$.ptr;
    },
    fill_point: function(ptr) {
        let p = Module.Point.load(ptr);
        p.x = 4;
        p.y = 2;
    }
  });

and this would output:

Point1 x=5 y=10
Point2 x=4 y=2

There may be a cleaner way to do this, I'm still getting use to embind as well.

[k1nder10]

Yeah, it seems like they can't be used together. Though I made up the following example, I don't know whether it should work.
Here we use pointers, which are allowed in JS libraries. However, the code works as if fillPoint was never called. Is it expected?

// point.cpp
extern "C" {
struct Point {
 int x;
 int y;
};

extern void fillPoint(Point *p); // to be implemented in JS

Point createPoint() { 
 return {};
}
}

EMSCRIPTEN_BINDINGS(PointModule) {
 emscripten::value_object<Point>("Point")
     .field("x", &Point::x)
     .field("y", &Point::y);

 emscripten::function("createPoint", &createPoint);
}

int main() {
 Point p{1, 2};
 fillPoint(&p);
 std::cout << p.x << ' ' << p.y << std::endl; // prints 1, 2
}
 
// point.js
mergeInto(LibraryManager.library, {
 fillPoint__sig: 'vp',
 fillPoint: function(point) {
   let p = Module.createPoint();
   p.x = 5;
   p.y = 10;
   point = p;
 }
});

emcc point.cpp --js-library point.js --bind -o point_out.html

[k1nder10]

Ok. typeof() of a C++ pointer says it is a number. Then I understand why it doesn't work. But then the question is how can I write to a C++ pointer in JS?

 fillPoint: function(point) {
   console.log(typeof(point)); // number
   let p = Module.createPoint();
   console.log(typeof(p)); // object
   p.x = 5;
   p.y = 10;
   point = p; // assigning object to number
   // Question: how to treat 'point' variable as object ?? how to dereference 'point' variable?
 }

[k1nder10]

@brendandahl
I didn't see your comment while writing those two above. Thank you very much!