Custom GDNative class as class variable triggers crash

[SnailBones]

I have one GDNative object listed as a property of another. When returning a pointer to the property or casting said pointer to a reference, the engine crashes.

However, if I initialize a new instance and copy the values over, it works as intended. Unfortunately, my workaround creates a drain on performance. Perhaps Godot only has access to the heap?

C++ code:

Grid *Model::getState()
// Ref<Grid> Model::getState()
{
    // This fails:
    return next_state;

    // This also fails:
    Ref<Grid> ref = Ref<Grid>(next_state);
    return ref;

    // This works:
    Grid *v = Grid::_new();
    v->init(width, height); // Coping object
    v->imitate(next_state); 
    return v;
}

Backtrace:

[0] RaiseException
[1] _CxxThrowException (d:\agent\_work\2\s\src\vctools\crt\vcruntime\src\eh\throw.cpp:133)
[2] __RTDynamicCast (d:\agent\_work\2\s\src\vctools\crt\vcruntime\src\eh\rtti.cpp:291)
[3] Object::cast_to<Reference> (C:\Users\Aidan\Downloads\godot-master\godot-master\core\object.h:610)
[4] godot_variant_new_object (C:\Users\Aidan\Downloads\godot-master\godot-master\modules\gdnative\gdnative\variant.cpp:175)
[5] godot::Variant::Variant (C:\Users\Aidan\Documents\Fun Stuff\Godot\gdnative_cpp_example\godot-cpp\src\core\Variant.cpp:122)
[6] godot::_WrappedMethod<godot::Model,godot::Grid *>::apply<> (C:\Users\Aidan\Documents\Fun Stuff\Godot\Cells d2\godot-cpp\include\core\Godot.hpp:183)
[7] godot::__wrapped_method<godot::Model,godot::Grid *> (C:\Users\Aidan\Documents\Fun Stuff\Godot\Cells d2\godot-cpp\include\core\Godot.hpp:210)
[8] NativeScriptInstance::call (C:\Users\Aidan\Downloads\godot-master\godot-master\modules\gdnative\nativescript\nativescript.cpp:719)
[9] Object::call (C:\Users\Aidan\Downloads\godot-master\godot-master\core\object.cpp:912)
[10] Variant::call_ptr (C:\Users\Aidan\Downloads\godot-master\godot-master\core\variant_call.cpp:1074)
[11] GDScriptFunction::call (C:\Users\Aidan\Downloads\godot-master\godot-master\modules\gdscript\gdscript_function.cpp:1079)
[12] GDScriptInstance::call_multilevel (C:\Users\Aidan\Downloads\godot-master\godot-master\modules\gdscript\gdscript.cpp:1180)
[13] Node::_notification (C:\Users\Aidan\Downloads\godot-master\godot-master\scene\main\node.cpp:60)
[14] Node::_notificationv (C:\Users\Aidan\Downloads\godot-master\godot-master\scene\main\node.h:46)
[15] CanvasItem::_notificationv (C:\Users\Aidan\Downloads\godot-master\godot-master\scene\2d\canvas_item.h:166)
[16] Control::_notificationv (C:\Users\Aidan\Downloads\godot-master\godot-master\scene\gui\control.h:51)
[17] Object::notification (C:\Users\Aidan\Downloads\godot-master\godot-master\core\object.cpp:945)
[18] SceneTree::_notify_group_pause (C:\Users\Aidan\Downloads\godot-master\godot-master\scene\main\scene_tree.cpp:959)
[19] SceneTree::idle (C:\Users\Aidan\Downloads\godot-master\godot-master\scene\main\scene_tree.cpp:515)
[20] Main::iteration (C:\Users\Aidan\Downloads\godot-master\godot-master\main\main.cpp:1920)
[21] OS_Windows::run (C:\Users\Aidan\Downloads\godot-master\godot-master\platform\windows\os_windows.cpp:3010)
[22] widechar_main (C:\Users\Aidan\Downloads\godot-master\godot-master\platform\windows\godot_windows.cpp:162)
[23] _main (C:\Users\Aidan\Downloads\godot-master\godot-master\platform\windows\godot_windows.cpp:184)
[24] main (C:\Users\Aidan\Downloads\godot-master\godot-master\platform\windows\godot_windows.cpp:196)
[25] __scrt_common_main_seh (d:\agent\_work\2\s\src\vctools\crt\vcstartup\src\startup\exe_common.inl:288)
[26] BaseThreadInitThunk
-- END OF BACKTRACE --

[BastiaanOlij]

The problem with structuring it like this is that Godot looses the ability to reference count the object and destroys it before its ready.

You need to define next_state as a Ref as well and then call instance on it to create it:

Ref<Grid> next_state;

...

// instantiate our object properly initialising our refcount.
next_state.instance()

...

Ref<Grid> Model::get_state() {
    return next_state;
}

When you did Ref<Grid> ref = Ref<Grid>(next_state); you are creating a new reference counter object with reference count set to 1. Once that goes out of scope next_state will be destructed even though you are still using it, hence the crash.

[Zylann]

Is this issue fixed? (tbh there are others like this which don't seem to be issues with Godot-Cpp but more like questions/troubleshootings which are still hanging around)