Deriving from a custom class that implements Godot virtual methods fails to compile #854
Comments
Calinou
added
bug
|
Ok I was actually on the right track, something was wrong in the way I passed the So this is a simplified example of the trick: // In the real code, this is `Node`
struct A {
virtual void _ready() {
std::cout << "A::foo()" << std::endl;
}
};
struct B : public A {
void _ready() override {
std::cout << "A::foo()" << std::endl;
}
};
struct C : public B {
};
template <typename T, typename TBase>
void register_virtuals() {
if constexpr(!std::is_same_v<decltype(&T::_ready), decltype(&TBase::_ready)>) {
// The inside is not even compiled if the condition is false
auto call__ready = [](void *p_instance, const void*p_args, void *p_ret) -> void {
// I didn't even define `call_with_ptr_args` btw
call_with_ptr_args(reinterpret_cast<T *>(p_instance), &T::_ready, p_args, p_ret);
};
// [...] bind_virtual_method(call__ready)...
}
}
register_virtuals<C, B>();The idea is to check if the class defines a |
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Quick note because errors aren't obvious: if you write |
With GodotCpp
masterbranch.Simple example:
A setup like this one produces the following compiling errors:
The only workaround I could find so far was to implement those virtual methods again in all derived classes, making them call the parent method. This is inconvenient and becomes a nightmare in cases like this https://github.com/Zylann/godot_voxel/blob/gdextension/editor/vox/vox_scene_importer.h
MRP:
extension_test_issue854_virtual_methods.zip
Note: change the path to GodotCpp in
SConstruct.It seems to happen because
GDCLASSinternally definesregister_virtuals(), which callsBaseClass::register_virtuals<CurrentClass>(). And in there, it does this:When
Tis the derived class, it does not define_ready. Only its base class does. But for some reason, that makesBIND_VIRTUAL_METHODfail to compile.I thought that's because the
ifchecks the wrong condition. Indeed, whenTderives from a custom class,Node::_readyis not the same function asT::_ready. ButTdoes not actually define_ready, its custom base class does. So we should not try to bind the method again if it's the same as the base class.So I tried this:
Where
Bis the actual base class, so a more informed check can be done.Unfortunately, the fact there is an
ifdoes not prevent the code inside from being compiled. The error remains exactly the same: it can't compile the inside of theif.Here with the macro expanded:
So what I dont understand is, why all the methods I don't override are compiling just fine, while they should be in the same situation?
With
if constexpr, if the condition doesn't pass, the code inside isn't even compiled. But here it looks like it tries to compile it anyways.Funny enough, if I replace the condition with
It compiles fine. When I try
!std::is_same_v<decltype(&B::_ready), decltype(&T::_ready)>, I tested and expected it to befalsetoo for the derived class, but somehow discarding isn't working.I even tried somthing similar on
https://godbolt.org/and it worked...The text was updated successfully, but these errors were encountered: