Multiple inheritance with Embind?

[donalffons]

Hi!

The topic of multiple inheritance with Embind has been discussed in #6382 and I understand that this is currently not supported. However, I was wondering if there is a workaround or if there is a chance that this sort of feature might be supported at some point...

Here is an example of my use case:

struct BaseA {
  int i;
};
struct BaseB {
  int j;
};
struct Derived : public BaseA, public BaseB {
  int k;
};
void funcBaseA(BaseA& a) {
  cout << "funcBaseA " << a.i++ << "\n";
}
void funcBaseB(BaseB& b) {
  cout << "funcBaseB " << b.j++ << "\n";
}
void funcDerived(Derived& d) {
  cout << "funcDerived " << d.k++ << "\n";
}

I would like to be able to call all functions above with an instance of type Derived. One naive workaround I have come up with, is to reinterpret_cast the underlying pointer in my Embind binding definitions.

EMSCRIPTEN_BINDINGS(MyBindings) {
  class_<BaseA>("BaseA").constructor();
  class_<BaseB>("BaseB").constructor();
  class_<Derived>("Derived").constructor();
  function(
      "funcBaseA",
      optional_override(
          [](val v) {
            BaseA& a = *reinterpret_cast<BaseA*>(v["$$"]["ptr"].as<unsigned long>(allow_raw_pointers()));
            return funcBaseA(a);
          }));
  function(
      "funcBaseB",
      optional_override(
          [](val v) {
            BaseB& b = *reinterpret_cast<BaseB*>(v["$$"]["ptr"].as<unsigned long>(allow_raw_pointers()));
            return funcBaseB(b);
          }));
  function(
      "funcDerived",
      optional_override(
          [](val v) {
            Derived& d = *reinterpret_cast<Derived*>(v["$$"]["ptr"].as<unsigned long>(allow_raw_pointers()));
            return funcDerived(d);
          }));
}

That works...

const d = new Module.Derived();
Module.funcBaseA(d);
Module.funcBaseB(d);
Module.funcDerived(d);

...but unfortunately I'm losing type safety and error messages. And I guess that this is probably also hurting runtime performance?!

If anyone else has ideas on this issue, I would greatly appreciate if you would share them.