Smart pointers and inheritance

[שחר לנגבהיים‎]

Hi,
I have a couple of structs objects, inheriting from a base struct, and passed using smart pointers. I've tried to bind them using embind, but when I look at the object in the debugger I see that it isn't a smart pointer but a raw pointer, and when I try to pass the object to another object, I get an error that the wrong type of object is passed. 

c++ code:

struct RenderModel {};

EMSCRIPTEN_BINDINGS(RenderModelBinding) {

emscripten::class_<RenderModel>("RenderModel")

.constructor<>()

.smart_ptr<std::shared_ptr<RenderModel>>("RenderModel");

}

struct GroupRenderModel : RenderModel {

void addModel(std::shared_ptr<wre_model::RenderModel> model) {

models.push_back(model);

}

std::vector<std::shared_ptr<wre_model::RenderModel>> models;

};

EMSCRIPTEN_BINDINGS(GroupRenderModelBinding) {

emscripten::class_<GroupRenderModel>("GroupRenderModel")

.constructor<>()

.function("addModel", &GroupRenderModel::addModel)

.smart_ptr<std::shared_ptr<GroupRenderModel>>("GroupRenderModel");

}

struct TextureRenderModel : public RenderModel {};

EMSCRIPTEN_BINDINGS(TextureRenderModelBinding) {

emscripten::class_<TextureRenderModel>("TextureRenderModel")

.constructor<>()

.smart_ptr<std::shared_ptr<TextureRenderModel>>("TextureRenderModel");

}

JS code:

const groupLayer = new Module.GroupRenderModel();

const textureLayer = new Module.TextureRenderModel();

groupLayer.addModel(textureLayer);

When checking in the debugger, I see that groupLayer's pointer is:

1. ptrType: RegisteredPointer
1. destructorFunction: null
2. isConst: false
3. isReference: false
4. isSmartPointer: false
5. name: "GroupRenderModel*"

textureLayer's pointer is:

1. ptrType: RegisteredPointer
1. destructorFunction: null
2. isConst: false
3. isReference: false
4. isSmartPointer: false
5. name: "TextureRenderModel*"

And the error printed when calling setModel is:

1. BindingError {name: "BindingError", message: "Expected null or instance of RenderModel, got an instance of TextureRenderModel", stack: "BindingError: Expected null or instance of RenderM…ttp://localhost:51840/client.e31bb0bc.js:68387:15"}
2. message: "Expected null or instance of RenderModel, got an instance of TextureRenderModel"
1. name: "BindingError"
2. stack: "BindingError: Expected null or instance of RenderModel, got an instance of TextureRenderModel↵ at BindingError.<anonymous> (http://localhost:51840/render/appWASM.js:5207:24)↵ at new BindingError (eval at createNamedFunction (http://localhost:51840/render/appWASM.js:1:1), <anonymous>:4:34)↵ at throwBindingError (http://localhost:51840/render/appWASM.js:5225:13)↵ at upcastPointer (http://localhost:51840/render/appWASM.js:5569:15)↵ at RegisteredPointer.genericPointerToWireType [as toWireType] (http://localhost:51840/render/appWASM.js:5622:13)↵ at GroupRenderModel$addModel [as addModel] (eval at new_ (http://localhost:51840/render/appWASM.js:1:1), <anonymous>:9:26)↵ at CompositionCanvas.updateRenderEngine (http://localhost:51840/client.e31bb0bc.js:68401:18)↵ at http://localhost:51840/client.e31bb0bc.js:68387:15"
3. __proto__: Error

Using smart_ptr_constructor changes the value in isSmartPointer, but the same binding error is returned.

struct RenderModel {};

EMSCRIPTEN_BINDINGS(RenderModelBinding) {

emscripten::class_<RenderModel>("RenderModel")

.smart_ptr_constructor<std::shared_ptr<RenderModel>>("RenderModel",

&std::make_shared<RenderModel>);

}

struct GroupRenderModel : public RenderModel {

void addModel(std::shared_ptr<wre_model::RenderModel> model) {

models.push_back(model);

}

std::vector<std::shared_ptr<wre_model::RenderModel>> models;

};

EMSCRIPTEN_BINDINGS(GroupRenderModelBinding) {

emscripten::class_<GroupRenderModel>("GroupRenderModel")

.function("addModel", &GroupRenderModel::addModel)

.smart_ptr_constructor<std::shared_ptr<GroupRenderModel>>(

"GroupRenderModel", &std::make_shared<GroupRenderModel>);

}

struct TextureRenderModel : public RenderModel {};

EMSCRIPTEN_BINDINGS(TextureRenderModelBinding) {

emscripten::class_<TextureRenderModel>("TextureRenderModel")

.smart_ptr_constructor<std::shared_ptr<TextureRenderModel>>(

"TextureRenderModel", &std::make_shared<TextureRenderModel>);

}

Am I making some mistake with my binding declaration, or does embind not support implicit casting?

1. 
   

[Shachar Langbeheim]

I see now that the same thing happens with raw pointers, and is not an issue of smart pointers.

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[Shachar Langbeheim]

OK, so this was me misreading the documentation. Just in case anyone else will be stuck with the same issue, here were my mistakes - 

a. When declaring a derived class, the base class needs to be declared along with the derived class, using class_<Derived, base<Base>>.

b. The reason that the objects returned with smart_ptr_constructor were defined as smart pointers, and the first version wasn't is that the smart_ptr declaration only allows usage of smart pointers, it doesn't automatically force their usage.

Sorry for the noise, I should've RTFM'd twice.

One last thing that I'm not sure about - how do I define smart_ptr_constructor with arguments? 

using this code 

struct TextRenderModel : public RenderModel {

TextRenderModel(float left, float top, float width, float height, const std::string &text,

int size, int xCoord, int yCoord)

: RenderModel(SkRect::MakeXYWH(left, top, width, height)),

text(text),

fontSize(size),

xCoord(xCoord),

yCoord(yCoord) {}

std::string text;

/// Size of the font;

int fontSize;

/// X offset of the drawn text.

int xCoord;

/// Y offset of the drawn text.

int yCoord;

};

EMSCRIPTEN_BINDINGS(TextRenderModelBinding) {

emscripten::class_<TextRenderModel, emscripten::base<TextRenderModel>>("TextRenderModel")

.smart_ptr_constructor<float, float, float, float, std::string, int, int, int>>

("TextRenderModel", &std::make_shared<TextRenderModel>);

}

prints this error:

/render/third_party/emsdk/fastcomp/emscripten/system/include/libcxx/memory:4271:5: error: static_assert failed due to requirement 'is_constructible<TextRenderModel>::value' "Can't construct object in make_shared"
    static_assert( is_constructible<_Tp, _Args...>::value, "Can't construct object in make_shared" );
    ^              ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/render/third_party/emsdk/fastcomp/emscripten/system/include/libcxx/memory:4656:29: note: in instantiation of function template specialization 'std::__2::shared_ptr<wre_model::TextRenderModel>::make_shared<>' requested here
    return shared_ptr<_Tp>::make_shared(_VSTD::forward<_Args>(__args)...);
                            ^
./src/model/TextRenderModel.hpp:41:33: note: in instantiation of function template specialization 'std::__2::make_shared<wre_model::TextRenderModel>' requested here
      ("TextRenderModel", &std::make_shared<TextRenderModel>);
                                ^
In file included from <built-in>:1:
In file included from ././src/pch.hpp:4:
/render/third_party/emsdk/fastcomp/emscripten/system/include/libcxx/memory:2089:66: error: no matching constructor for initialization of 'wre_model::TextRenderModel'
  _LIBCPP_INLINE_VISIBILITY constexpr __compressed_pair_elem() : __value_() {}