val::operator() with pointer as argument

[isc30]

I'm trying to avoid the need of JS users calling .delete() on the API by accepting lambdas as actions that work with the objects:

EMSCRIPTEN_BINDINGS()
{
    function("using_x", optional_override([](val action)
    {
        X x; // C++ constructs `x`
        action(x); // JS uses `x`
        // C++ destroys `x`
    }));
}

The idea is to use it like this:

Module.using_x(x => x.something()); // C++ destroys the object internally

I realized that embind is copying the object every time I pass it as a parameter, so I tried passing the ptr to x:

action(&x);

The compiler complains: "Implicitly binding raw pointers is illegal. Specify allow_raw_pointer<arg<?>>"

I see no way of specifying policy in this case.

Is it possible to pass pointers into val::operator()?
How can I achieve managing memory from C++ and calling JS actions like this in an easy way?
Am I wrong in my assumption that this design would help defining the proper ownership of those objects?

Thanks a lot

[isc30]

Okay, looks like I found a workaround:

passing an lvalue reference to the pointer instead of the pointer itself

X x;
X* ptr = &x;
action(ptr);

This works as expected, interesting...

[chadaustin]

That's... scary. I feel like that shouldn't work. :D What happens when JavaScript grabs a long-lived reference to the value and tries to run it later?

This sounds related to #7292 to me. cc @mejedi

[isc30]

Well, that would be a bad practice from the JS side (same as storing a reference to a deleted value in C++).

I don't consider it a bug. Being able to do this opens many doors when talking about memory management from the C++ side when exposing objects to js.

I wrote a small article about this topic and how you can benefit from such constructions: https://blog.codeisc.com/2018/09/15/xlnt-wasm-bindings.html#avoiding-exposed-ownership

[stale]

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