Allow registering "runtime classes"

core/extension/gdextension.cpp

@@ -205,6 +205,7 @@ public:
 	virtual Variant call(Object *p_object, const Variant **p_args, int p_arg_count, Callable::CallError &r_error) const override {
 #ifdef TOOLS_ENABLED
 		ERR_FAIL_COND_V_MSG(!valid, Variant(), vformat("Cannot call invalid GDExtension method bind '%s'. It's probably cached - you may need to restart Godot.", name));
+		ERR_FAIL_COND_V_MSG(p_object && p_object->is_extension_placeholder(), Variant(), vformat("Cannot call GDExtension method bind '%s' on placeholder instance.", name));
 #endif
 		Variant ret;
 		GDExtensionClassInstancePtr extension_instance = is_static() ? nullptr : p_object->_get_extension_instance();
@@ -218,6 +219,7 @@ public:
 	virtual void validated_call(Object *p_object, const Variant **p_args, Variant *r_ret) const override {
 #ifdef TOOLS_ENABLED
 		ERR_FAIL_COND_MSG(!valid, vformat("Cannot call invalid GDExtension method bind '%s'. It's probably cached - you may need to restart Godot.", name));
+		ERR_FAIL_COND_MSG(p_object && p_object->is_extension_placeholder(), vformat("Cannot call GDExtension method bind '%s' on placeholder instance.", name));
 #endif
 		ERR_FAIL_COND_MSG(vararg, "Vararg methods don't have validated call support. This is most likely an engine bug.");
 		GDExtensionClassInstancePtr extension_instance = is_static() ? nullptr : p_object->_get_extension_instance();
@@ -249,6 +251,7 @@ public:
 	virtual void ptrcall(Object *p_object, const void **p_args, void *r_ret) const override {
 #ifdef TOOLS_ENABLED
 		ERR_FAIL_COND_MSG(!valid, vformat("Cannot call invalid GDExtension method bind '%s'. It's probably cached - you may need to restart Godot.", name));
+		ERR_FAIL_COND_MSG(p_object && p_object->is_extension_placeholder(), vformat("Cannot call GDExtension method bind '%s' on placeholder instance.", name));
 #endif
 		ERR_FAIL_COND_MSG(vararg, "Vararg methods don't have ptrcall support. This is most likely an engine bug.");
 		GDExtensionClassInstancePtr extension_instance = p_object->_get_extension_instance();
@@ -341,10 +344,11 @@ public:
 
 #ifndef DISABLE_DEPRECATED
 void GDExtension::_register_extension_class(GDExtensionClassLibraryPtr p_library, GDExtensionConstStringNamePtr p_class_name, GDExtensionConstStringNamePtr p_parent_class_name, const GDExtensionClassCreationInfo *p_extension_funcs) {
-	const GDExtensionClassCreationInfo2 class_info2 = {
+	const GDExtensionClassCreationInfo3 class_info3 = {
 		p_extension_funcs->is_virtual, // GDExtensionBool is_virtual;
 		p_extension_funcs->is_abstract, // GDExtensionBool is_abstract;
 		true, // GDExtensionBool is_exposed;
+		false, // GDExtensionBool is_runtime;
 		p_extension_funcs->set_func, // GDExtensionClassSet set_func;
 		p_extension_funcs->get_func, // GDExtensionClassGet get_func;
 		p_extension_funcs->get_property_list_func, // GDExtensionClassGetPropertyList get_property_list_func;
@@ -369,15 +373,45 @@ void GDExtension::_register_extension_class(GDExtensionClassLibraryPtr p_library
 	const ClassCreationDeprecatedInfo legacy = {
 		p_extension_funcs->notification_func,
 	};
-	_register_extension_class_internal(p_library, p_class_name, p_parent_class_name, &class_info2, &legacy);
+	_register_extension_class_internal(p_library, p_class_name, p_parent_class_name, &class_info3, &legacy);
+}
+
+void GDExtension::_register_extension_class2(GDExtensionClassLibraryPtr p_library, GDExtensionConstStringNamePtr p_class_name, GDExtensionConstStringNamePtr p_parent_class_name, const GDExtensionClassCreationInfo2 *p_extension_funcs) {
+	const GDExtensionClassCreationInfo3 class_info3 = {
+		p_extension_funcs->is_virtual, // GDExtensionBool is_virtual;
+		p_extension_funcs->is_abstract, // GDExtensionBool is_abstract;
+		p_extension_funcs->is_exposed, // GDExtensionBool is_exposed;
+		false, // GDExtensionBool is_runtime;
+		p_extension_funcs->set_func, // GDExtensionClassSet set_func;
+		p_extension_funcs->get_func, // GDExtensionClassGet get_func;
+		p_extension_funcs->get_property_list_func, // GDExtensionClassGetPropertyList get_property_list_func;
+		p_extension_funcs->free_property_list_func, // GDExtensionClassFreePropertyList free_property_list_func;
+		p_extension_funcs->property_can_revert_func, // GDExtensionClassPropertyCanRevert property_can_revert_func;
+		p_extension_funcs->property_get_revert_func, // GDExtensionClassPropertyGetRevert property_get_revert_func;
+		p_extension_funcs->validate_property_func, // GDExtensionClassValidateProperty validate_property_func;
+		p_extension_funcs->notification_func, // GDExtensionClassNotification2 notification_func;
+		p_extension_funcs->to_string_func, // GDExtensionClassToString to_string_func;
+		p_extension_funcs->reference_func, // GDExtensionClassReference reference_func;
+		p_extension_funcs->unreference_func, // GDExtensionClassUnreference unreference_func;
+		p_extension_funcs->create_instance_func, // GDExtensionClassCreateInstance create_instance_func; /* this one is mandatory */
+		p_extension_funcs->free_instance_func, // GDExtensionClassFreeInstance free_instance_func; /* this one is mandatory */
+		p_extension_funcs->recreate_instance_func, // GDExtensionClassRecreateInstance recreate_instance_func;
+		p_extension_funcs->get_virtual_func, // GDExtensionClassGetVirtual get_virtual_func;
+		p_extension_funcs->get_virtual_call_data_func, // GDExtensionClassGetVirtualCallData get_virtual_call_data_func;
+		p_extension_funcs->call_virtual_with_data_func, // GDExtensionClassCallVirtualWithData call_virtual_func;
+		p_extension_funcs->get_rid_func, // GDExtensionClassGetRID get_rid;
+		p_extension_funcs->class_userdata, // void *class_userdata;
+	};
+
+	_register_extension_class_internal(p_library, p_class_name, p_parent_class_name, &class_info3);
 }
 #endif // DISABLE_DEPRECATED
 
-void GDExtension::_register_extension_class2(GDExtensionClassLibraryPtr p_library, GDExtensionConstStringNamePtr p_class_name, GDExtensionConstStringNamePtr p_parent_class_name, const GDExtensionClassCreationInfo2 *p_extension_funcs) {
+void GDExtension::_register_extension_class3(GDExtensionClassLibraryPtr p_library, GDExtensionConstStringNamePtr p_class_name, GDExtensionConstStringNamePtr p_parent_class_name, const GDExtensionClassCreationInfo3 *p_extension_funcs) {
 	_register_extension_class_internal(p_library, p_class_name, p_parent_class_name, p_extension_funcs);
 }
 
-void GDExtension::_register_extension_class_internal(GDExtensionClassLibraryPtr p_library, GDExtensionConstStringNamePtr p_class_name, GDExtensionConstStringNamePtr p_parent_class_name, const GDExtensionClassCreationInfo2 *p_extension_funcs, const ClassCreationDeprecatedInfo *p_deprecated_funcs) {
+void GDExtension::_register_extension_class_internal(GDExtensionClassLibraryPtr p_library, GDExtensionConstStringNamePtr p_class_name, GDExtensionConstStringNamePtr p_parent_class_name, const GDExtensionClassCreationInfo3 *p_extension_funcs, const ClassCreationDeprecatedInfo *p_deprecated_funcs) {
 	GDExtension *self = reinterpret_cast<GDExtension *>(p_library);
 
 	StringName class_name = *reinterpret_cast<const StringName *>(p_class_name);
@@ -402,10 +436,15 @@ void GDExtension::_register_extension_class_internal(GDExtensionClassLibraryPtr
 
 #ifdef TOOLS_ENABLED
 	Extension *extension = nullptr;
+	bool is_runtime = (bool)p_extension_funcs->is_runtime;
 	if (self->is_reloading && self->extension_classes.has(class_name)) {
 		extension = &self->extension_classes[class_name];
 		if (!parent_extension && parent_class_name != extension->gdextension.parent_class_name) {
-			ERR_FAIL_MSG(vformat("GDExtension class '%s' attempt to change parent type from '%s' to '%s' on hot reload. Restart Godot for this change to take effect.", class_name, extension->gdextension.parent_class_name, parent_class_name));
+			ERR_FAIL_MSG(vformat("GDExtension class '%s' cannot change parent type from '%s' to '%s' on hot reload. Restart Godot for this change to take effect.", class_name, extension->gdextension.parent_class_name, parent_class_name));
+		}
+		if (extension->gdextension.is_runtime != is_runtime) {
+			ERR_PRINT(vformat("GDExtension class '%s' cannot change to/from runtime class on hot reload. Restart Godot for this change to take effect.", class_name));
+			is_runtime = extension->gdextension.is_runtime;
 		}
 		extension->is_reloading = false;
 	} else {
@@ -434,6 +473,9 @@ void GDExtension::_register_extension_class_internal(GDExtensionClassLibraryPtr
 	extension->gdextension.is_virtual = p_extension_funcs->is_virtual;
 	extension->gdextension.is_abstract = p_extension_funcs->is_abstract;
 	extension->gdextension.is_exposed = p_extension_funcs->is_exposed;
+#ifdef TOOLS_ENABLED
+	extension->gdextension.is_runtime = is_runtime;
+#endif
 	extension->gdextension.set = p_extension_funcs->set_func;
 	extension->gdextension.get = p_extension_funcs->get_func;
 	extension->gdextension.get_property_list = p_extension_funcs->get_property_list_func;
@@ -840,8 +882,9 @@ void GDExtension::initialize_gdextensions() {
 
 #ifndef DISABLE_DEPRECATED
 	register_interface_function("classdb_register_extension_class", (GDExtensionInterfaceFunctionPtr)&GDExtension::_register_extension_class);
-#endif // DISABLE_DEPRECATED
 	register_interface_function("classdb_register_extension_class2", (GDExtensionInterfaceFunctionPtr)&GDExtension::_register_extension_class2);
+#endif // DISABLE_DEPRECATED
+	register_interface_function("classdb_register_extension_class3", (GDExtensionInterfaceFunctionPtr)&GDExtension::_register_extension_class3);
 	register_interface_function("classdb_register_extension_class_method", (GDExtensionInterfaceFunctionPtr)&GDExtension::_register_extension_class_method);
 	register_interface_function("classdb_register_extension_class_virtual_method", (GDExtensionInterfaceFunctionPtr)&GDExtension::_register_extension_class_virtual_method);
 	register_interface_function("classdb_register_extension_class_integer_constant", (GDExtensionInterfaceFunctionPtr)&GDExtension::_register_extension_class_integer_constant);
@@ -1060,7 +1103,10 @@ void GDExtension::prepare_reload() {
 
 				state.push_back(Pair<String, Variant>(P.name, value));
 			}
-			E.value.instance_state[obj_id] = state;
+			E.value.instance_state[obj_id] = {
+				state, // List<Pair<String, Variant>> properties;
+				obj->is_extension_placeholder(), // bool is_placeholder;
+			};
 		}
 	}
 }
@@ -1135,25 +1181,29 @@ void GDExtension::finish_reload() {
 	for (KeyValue<StringName, Extension> &E : extension_classes) {
 		// Loop over 'instance_state' rather than 'instance' because new instances
 		// may have been created when re-initializing the extension.
-		for (const KeyValue<ObjectID, List<Pair<String, Variant>>> &S : E.value.instance_state) {
+		for (const KeyValue<ObjectID, Extension::InstanceState> &S : E.value.instance_state) {
 			Object *obj = ObjectDB::get_instance(S.key);
 			if (!obj) {
 				continue;
 			}
 
+			if (S.value.is_placeholder) {
+				obj->reset_internal_extension(ClassDB::get_placeholder_extension(E.value.gdextension.class_name));
+			} else {
 				obj->reset_internal_extension(&E.value.gdextension);
 			}
 		}
+	}
 
 	// Now that all the classes are back, restore the state.
 	for (KeyValue<StringName, Extension> &E : extension_classes) {
-		for (const KeyValue<ObjectID, List<Pair<String, Variant>>> &S : E.value.instance_state) {
+		for (const KeyValue<ObjectID, Extension::InstanceState> &S : E.value.instance_state) {
 			Object *obj = ObjectDB::get_instance(S.key);
 			if (!obj) {
 				continue;
 			}
 
-			for (const Pair<String, Variant> &state : S.value) {
+			for (const Pair<String, Variant> &state : S.value.properties) {
 				obj->set(state.first, state.second);
 			}
 		}
@@ -1161,7 +1211,7 @@ void GDExtension::finish_reload() {
 
 	// Finally, let the objects know that we are done reloading them.
 	for (KeyValue<StringName, Extension> &E : extension_classes) {
-		for (const KeyValue<ObjectID, List<Pair<String, Variant>>> &S : E.value.instance_state) {
+		for (const KeyValue<ObjectID, Extension::InstanceState> &S : E.value.instance_state) {
 			Object *obj = ObjectDB::get_instance(S.key);
 			if (!obj) {
 				continue;

core/extension/gdextension.h

@@ -59,7 +59,12 @@ class GDExtension : public Resource {
 		bool is_reloading = false;
 		HashMap<StringName, GDExtensionMethodBind *> methods;
 		HashSet<ObjectID> instances;
-		HashMap<ObjectID, List<Pair<String, Variant>>> instance_state;
+
+		struct InstanceState {
+			List<Pair<String, Variant>> properties;
+			bool is_placeholder = false;
+		};
+		HashMap<ObjectID, InstanceState> instance_state;
 #endif
 	};
 
@@ -73,9 +78,10 @@ class GDExtension : public Resource {
 
 #ifndef DISABLE_DEPRECATED
 	static void _register_extension_class(GDExtensionClassLibraryPtr p_library, GDExtensionConstStringNamePtr p_class_name, GDExtensionConstStringNamePtr p_parent_class_name, const GDExtensionClassCreationInfo *p_extension_funcs);
-#endif // DISABLE_DEPRECATED
 	static void _register_extension_class2(GDExtensionClassLibraryPtr p_library, GDExtensionConstStringNamePtr p_class_name, GDExtensionConstStringNamePtr p_parent_class_name, const GDExtensionClassCreationInfo2 *p_extension_funcs);
-	static void _register_extension_class_internal(GDExtensionClassLibraryPtr p_library, GDExtensionConstStringNamePtr p_class_name, GDExtensionConstStringNamePtr p_parent_class_name, const GDExtensionClassCreationInfo2 *p_extension_funcs, const ClassCreationDeprecatedInfo *p_deprecated_funcs = nullptr);
+#endif // DISABLE_DEPRECATED
+	static void _register_extension_class3(GDExtensionClassLibraryPtr p_library, GDExtensionConstStringNamePtr p_class_name, GDExtensionConstStringNamePtr p_parent_class_name, const GDExtensionClassCreationInfo3 *p_extension_funcs);
+	static void _register_extension_class_internal(GDExtensionClassLibraryPtr p_library, GDExtensionConstStringNamePtr p_class_name, GDExtensionConstStringNamePtr p_parent_class_name, const GDExtensionClassCreationInfo3 *p_extension_funcs, const ClassCreationDeprecatedInfo *p_deprecated_funcs = nullptr);
 	static void _register_extension_class_method(GDExtensionClassLibraryPtr p_library, GDExtensionConstStringNamePtr p_class_name, const GDExtensionClassMethodInfo *p_method_info);
 	static void _register_extension_class_virtual_method(GDExtensionClassLibraryPtr p_library, GDExtensionConstStringNamePtr p_class_name, const GDExtensionClassVirtualMethodInfo *p_method_info);
 	static void _register_extension_class_integer_constant(GDExtensionClassLibraryPtr p_library, GDExtensionConstStringNamePtr p_class_name, GDExtensionConstStringNamePtr p_enum_name, GDExtensionConstStringNamePtr p_constant_name, GDExtensionInt p_constant_value, GDExtensionBool p_is_bitfield);

core/extension/gdextension_interface.cpp

@@ -1379,7 +1379,7 @@ static GDExtensionMethodBindPtr gdextension_classdb_get_method_bind(GDExtensionC
 
 static GDExtensionObjectPtr gdextension_classdb_construct_object(GDExtensionConstStringNamePtr p_classname) {
 	const StringName classname = *reinterpret_cast<const StringName *>(p_classname);
-	return (GDExtensionObjectPtr)ClassDB::instantiate(classname);
+	return (GDExtensionObjectPtr)ClassDB::instantiate_no_placeholders(classname);
 }
 
 static void *gdextension_classdb_get_class_tag(GDExtensionConstStringNamePtr p_classname) {

core/extension/gdextension_interface.h

@@ -290,7 +290,7 @@ typedef struct {
 	GDExtensionClassGetVirtual get_virtual_func; // Queries a virtual function by name and returns a callback to invoke the requested virtual function.
 	GDExtensionClassGetRID get_rid_func;
 	void *class_userdata; // Per-class user data, later accessible in instance bindings.
-} GDExtensionClassCreationInfo; // Deprecated. Use GDExtensionClassCreationInfo2 instead.
+} GDExtensionClassCreationInfo; // Deprecated. Use GDExtensionClassCreationInfo3 instead.
 
 typedef struct {
 	GDExtensionBool is_virtual;
@@ -323,7 +323,41 @@ typedef struct {
 	GDExtensionClassCallVirtualWithData call_virtual_with_data_func;
 	GDExtensionClassGetRID get_rid_func;
 	void *class_userdata; // Per-class user data, later accessible in instance bindings.
-} GDExtensionClassCreationInfo2;
+} GDExtensionClassCreationInfo2; // Deprecated. Use GDExtensionClassCreationInfo3 instead.
+
+typedef struct {
+	GDExtensionBool is_virtual;
+	GDExtensionBool is_abstract;
+	GDExtensionBool is_exposed;
+	GDExtensionBool is_runtime;
+	GDExtensionClassSet set_func;
+	GDExtensionClassGet get_func;
+	GDExtensionClassGetPropertyList get_property_list_func;
+	GDExtensionClassFreePropertyList free_property_list_func;
+	GDExtensionClassPropertyCanRevert property_can_revert_func;
+	GDExtensionClassPropertyGetRevert property_get_revert_func;
+	GDExtensionClassValidateProperty validate_property_func;
+	GDExtensionClassNotification2 notification_func;
+	GDExtensionClassToString to_string_func;
+	GDExtensionClassReference reference_func;
+	GDExtensionClassUnreference unreference_func;
+	GDExtensionClassCreateInstance create_instance_func; // (Default) constructor; mandatory. If the class is not instantiable, consider making it virtual or abstract.
+	GDExtensionClassFreeInstance free_instance_func; // Destructor; mandatory.
+	GDExtensionClassRecreateInstance recreate_instance_func;
+	// Queries a virtual function by name and returns a callback to invoke the requested virtual function.
+	GDExtensionClassGetVirtual get_virtual_func;
+	// Paired with `call_virtual_with_data_func`, this is an alternative to `get_virtual_func` for extensions that
+	// need or benefit from extra data when calling virtual functions.
+	// Returns user data that will be passed to `call_virtual_with_data_func`.
+	// Returning `NULL` from this function signals to Godot that the virtual function is not overridden.
+	// Data returned from this function should be managed by the extension and must be valid until the extension is deinitialized.
+	// You should supply either `get_virtual_func`, or `get_virtual_call_data_func` with `call_virtual_with_data_func`.
+	GDExtensionClassGetVirtualCallData get_virtual_call_data_func;
+	// Used to call virtual functions when `get_virtual_call_data_func` is not null.
+	GDExtensionClassCallVirtualWithData call_virtual_with_data_func;
+	GDExtensionClassGetRID get_rid_func;
+	void *class_userdata; // Per-class user data, later accessible in instance bindings.
+} GDExtensionClassCreationInfo3;
 
 typedef void *GDExtensionClassLibraryPtr;
 
@@ -2240,6 +2274,9 @@ typedef void (*GDExtensionInterfaceObjectSetInstance)(GDExtensionObjectPtr p_o,
  *
  * Gets the class name of an Object.
  *
+ * If the GDExtension wraps the Godot object in an abstraction specific to its class, this is the
+ * function that should be used to determine which wrapper to use.
+ *
  * @param p_object A pointer to the Object.
  * @param p_library A pointer the library received by the GDExtension's entry point function.
  * @param r_class_name A pointer to a String to receive the class name.
@@ -2486,7 +2523,7 @@ typedef void *(*GDExtensionInterfaceClassdbGetClassTag)(GDExtensionConstStringNa
 /**
  * @name classdb_register_extension_class
  * @since 4.1
- * @deprecated in Godot 4.2. Use `classdb_register_extension_class2` instead.
+ * @deprecated in Godot 4.2. Use `classdb_register_extension_class3` instead.
  *
  * Registers an extension class in the ClassDB.
  *
@@ -2502,6 +2539,7 @@ typedef void (*GDExtensionInterfaceClassdbRegisterExtensionClass)(GDExtensionCla
 /**
  * @name classdb_register_extension_class2
  * @since 4.2
+ * @deprecated in Godot 4.3. Use `classdb_register_extension_class3` instead.
  *
  * Registers an extension class in the ClassDB.
  *
@@ -2514,6 +2552,21 @@ typedef void (*GDExtensionInterfaceClassdbRegisterExtensionClass)(GDExtensionCla
  */
 typedef void (*GDExtensionInterfaceClassdbRegisterExtensionClass2)(GDExtensionClassLibraryPtr p_library, GDExtensionConstStringNamePtr p_class_name, GDExtensionConstStringNamePtr p_parent_class_name, const GDExtensionClassCreationInfo2 *p_extension_funcs);
 
+/**
+ * @name classdb_register_extension_class3
+ * @since 4.3
+ *
+ * Registers an extension class in the ClassDB.
+ *
+ * Provided struct can be safely freed once the function returns.
+ *
+ * @param p_library A pointer the library received by the GDExtension's entry point function.
+ * @param p_class_name A pointer to a StringName with the class name.
+ * @param p_parent_class_name A pointer to a StringName with the parent class name.
+ * @param p_extension_funcs A pointer to a GDExtensionClassCreationInfo2 struct.
+ */
+typedef void (*GDExtensionInterfaceClassdbRegisterExtensionClass3)(GDExtensionClassLibraryPtr p_library, GDExtensionConstStringNamePtr p_class_name, GDExtensionConstStringNamePtr p_parent_class_name, const GDExtensionClassCreationInfo3 *p_extension_funcs);
+
 /**
  * @name classdb_register_extension_class_method
  * @since 4.1

core/object/class_db.cpp

@@ -70,6 +70,138 @@ HashMap<StringName, ClassDB::ClassInfo> ClassDB::classes;
 HashMap<StringName, StringName> ClassDB::resource_base_extensions;
 HashMap<StringName, StringName> ClassDB::compat_classes;
 
+#ifdef TOOLS_ENABLED
+HashMap<StringName, ObjectGDExtension> ClassDB::placeholder_extensions;
+
+class PlaceholderExtensionInstance {
+	StringName class_name;
+	HashMap<StringName, Variant> properties;
+
+public:
+	PlaceholderExtensionInstance(const StringName &p_class_name) {
+		class_name = p_class_name;
+	}
+
+	~PlaceholderExtensionInstance() {}
+
+	void set(const StringName &p_name, const Variant &p_value) {
+		bool is_default_valid = false;
+		Variant default_value = ClassDB::class_get_default_property_value(class_name, p_name, &is_default_valid);
+
+		// If there's a default value, then we know it's a valid property.
+		if (is_default_valid) {
+			properties[p_name] = p_value;
+		}
+	}
+
+	Variant get(const StringName &p_name) {
+		const Variant *value = properties.getptr(p_name);
+		Variant ret;
+
+		if (value) {
+			ret = *value;
+		} else {
+			bool is_default_valid = false;
+			Variant default_value = ClassDB::class_get_default_property_value(class_name, p_name, &is_default_valid);
+			if (is_default_valid) {
+				ret = default_value;
+			}
+		}
+
+		return ret;
+	}
+
+	static GDExtensionBool placeholder_instance_set(GDExtensionClassInstancePtr p_instance, GDExtensionConstStringNamePtr p_name, GDExtensionConstVariantPtr p_value) {
+		PlaceholderExtensionInstance *self = (PlaceholderExtensionInstance *)p_instance;
+		const StringName &name = *(StringName *)p_name;
+		const Variant &value = *(const Variant *)p_value;
+
+		self->set(name, value);
+
+		// We have to return true so Godot doesn't try to call the real setter function.
+		return true;
+	}
+
+	static GDExtensionBool placeholder_instance_get(GDExtensionClassInstancePtr p_instance, GDExtensionConstStringNamePtr p_name, GDExtensionVariantPtr r_ret) {
+		PlaceholderExtensionInstance *self = (PlaceholderExtensionInstance *)p_instance;
+		const StringName &name = *(StringName *)p_name;
+		Variant *value = (Variant *)r_ret;
+
+		*value = self->get(name);
+
+		// We have to return true so Godot doesn't try to call the real getter function.
+		return true;
+	}
+
+	static const GDExtensionPropertyInfo *placeholder_instance_get_property_list(GDExtensionClassInstancePtr p_instance, uint32_t *r_count) {
+		*r_count = 0;
+		return nullptr;
+	}
+
+	static void placeholder_instance_free_property_list(GDExtensionClassInstancePtr p_instance, const GDExtensionPropertyInfo *p_list) {
+	}
+
+	static GDExtensionBool placeholder_instance_property_can_revert(GDExtensionClassInstancePtr p_instance, GDExtensionConstStringNamePtr p_name) {
+		return false;
+	}
+
+	static GDExtensionBool placeholder_instance_property_get_revert(GDExtensionClassInstancePtr p_instance, GDExtensionConstStringNamePtr p_name, GDExtensionVariantPtr r_ret) {
+		return false;
+	}
+
+	static GDExtensionBool placeholder_instance_validate_property(GDExtensionClassInstancePtr p_instance, GDExtensionPropertyInfo *p_property) {
+		return false;
+	}
+
+	static void placeholder_instance_notification(GDExtensionClassInstancePtr p_instance, int32_t p_what, GDExtensionBool p_reversed) {
+	}
+
+	static void placeholder_instance_to_string(GDExtensionClassInstancePtr p_instance, GDExtensionBool *r_is_valid, GDExtensionStringPtr p_out) {
+		*r_is_valid = true;
+	}
+
+	static void placeholder_instance_reference(GDExtensionClassInstancePtr p_instance) {
+	}
+
+	static void placeholder_instance_unreference(GDExtensionClassInstancePtr p_instance) {
+	}
+
+	static uint64_t placeholder_instance_get_rid(GDExtensionClassInstancePtr p_instance) {
+		return 0;
+	}
+
+	static GDExtensionObjectPtr placeholder_class_create_instance(void *p_class_userdata) {
+		ClassDB::ClassInfo *ti = (ClassDB::ClassInfo *)p_class_userdata;
+
+		// Find the closest native parent.
+		ClassDB::ClassInfo *native_parent = ti->inherits_ptr;
+		while (native_parent->gdextension) {
+			native_parent = native_parent->inherits_ptr;
+		}
+
+		// Construct a placeholder.
+		Object *obj = native_parent->creation_func();
+		obj->_extension = ClassDB::get_placeholder_extension(ti->name);
+		obj->_extension_instance = memnew(PlaceholderExtensionInstance(ti->name));
+		return obj;
+	}
+
+	static GDExtensionObjectPtr placeholder_class_recreate_instance(void *p_class_userdata, GDExtensionObjectPtr p_object) {
+		ClassDB::ClassInfo *ti = (ClassDB::ClassInfo *)p_class_userdata;
+		return memnew(PlaceholderExtensionInstance(ti->name));
+	}
+
+	static void placeholder_class_free_instance(void *p_class_userdata, GDExtensionClassInstancePtr p_instance) {
+		PlaceholderExtensionInstance *instance = (PlaceholderExtensionInstance *)p_instance;
+		memdelete(instance);
+	}
+
+	static GDExtensionClassCallVirtual placeholder_class_get_virtual(void *p_class_userdata, GDExtensionConstStringNamePtr p_name) {
+		return nullptr;
+	}
+};
+#endif
+
 bool ClassDB::_is_parent_class(const StringName &p_class, const StringName &p_inherits) {
 	if (!classes.has(p_class)) {
 		return false;
@@ -346,7 +478,7 @@ StringName ClassDB::get_compatibility_class(const StringName &p_class) {
 	return StringName();
 }
 
-Object *ClassDB::instantiate(const StringName &p_class) {
+Object *ClassDB::_instantiate_internal(const StringName &p_class, bool p_require_real_class) {
 	ClassInfo *ti;
 	{
 		OBJTYPE_RLOCK;
@@ -367,10 +499,17 @@ Object *ClassDB::instantiate(const StringName &p_class) {
 	}
 #endif
 	if (ti->gdextension && ti->gdextension->create_instance) {
-		Object *obj = (Object *)ti->gdextension->create_instance(ti->gdextension->class_userdata);
+		ObjectGDExtension *extension = ti->gdextension;
 #ifdef TOOLS_ENABLED
-		if (ti->gdextension->track_instance) {
+		if (!p_require_real_class && ti->gdextension->is_runtime && Engine::get_singleton()->is_editor_hint()) {
-			ti->gdextension->track_instance(ti->gdextension->tracking_userdata, obj);
+			extension = get_placeholder_extension(ti->name);
+		}
+#endif
+		Object *obj = (Object *)extension->create_instance(extension->class_userdata);
+
+#ifdef TOOLS_ENABLED
+		if (extension->track_instance) {
+			extension->track_instance(extension->tracking_userdata, obj);
 		}
 #endif
 		return obj;
@@ -379,6 +518,82 @@ Object *ClassDB::instantiate(const StringName &p_class) {
 	}
 }
 
+Object *ClassDB::instantiate(const StringName &p_class) {
+	return _instantiate_internal(p_class);
+}
+
+Object *ClassDB::instantiate_no_placeholders(const StringName &p_class) {
+	return _instantiate_internal(p_class, true);
+}
+
+#ifdef TOOLS_ENABLED
+ObjectGDExtension *ClassDB::get_placeholder_extension(const StringName &p_class) {
+	ObjectGDExtension *placeholder_extension = placeholder_extensions.getptr(p_class);
+	if (placeholder_extension) {
+		return placeholder_extension;
+	}
+
+	ClassInfo *ti;
+	{
+		OBJTYPE_RLOCK;
+		ti = classes.getptr(p_class);
+		if (!ti || ti->disabled || !ti->creation_func || (ti->gdextension && !ti->gdextension->create_instance)) {
+			if (compat_classes.has(p_class)) {
+				ti = classes.getptr(compat_classes[p_class]);
+			}
+		}
+		ERR_FAIL_NULL_V_MSG(ti, nullptr, "Cannot get class '" + String(p_class) + "'.");
+		ERR_FAIL_COND_V_MSG(ti->disabled, nullptr, "Class '" + String(p_class) + "' is disabled.");
+		ERR_FAIL_NULL_V_MSG(ti->gdextension, nullptr, "Class '" + String(p_class) + "' has no native extension.");
+	}
+
+	placeholder_extensions[p_class] = ObjectGDExtension();
+	placeholder_extension = placeholder_extensions.getptr(p_class);
+
+	// Make a "fake" extension to act as a placeholder.
+	placeholder_extension->library = ti->gdextension->library;
+	placeholder_extension->parent = ti->gdextension->parent;
+	placeholder_extension->children = ti->gdextension->children;
+	placeholder_extension->parent_class_name = ti->gdextension->parent_class_name;
+	placeholder_extension->class_name = ti->gdextension->class_name;
+	placeholder_extension->editor_class = ti->gdextension->editor_class;
+	placeholder_extension->reloadable = ti->gdextension->reloadable;
+	placeholder_extension->is_virtual = ti->gdextension->is_virtual;
+	placeholder_extension->is_abstract = ti->gdextension->is_abstract;
+	placeholder_extension->is_exposed = ti->gdextension->is_exposed;
+	placeholder_extension->is_runtime = true;
+	placeholder_extension->is_placeholder = true;
+
+	placeholder_extension->set = &PlaceholderExtensionInstance::placeholder_instance_set;
+	placeholder_extension->get = &PlaceholderExtensionInstance::placeholder_instance_get;
+	placeholder_extension->get_property_list = &PlaceholderExtensionInstance::placeholder_instance_get_property_list;
+	placeholder_extension->free_property_list = &PlaceholderExtensionInstance::placeholder_instance_free_property_list;
+	placeholder_extension->property_can_revert = &PlaceholderExtensionInstance::placeholder_instance_property_can_revert;
+	placeholder_extension->property_get_revert = &PlaceholderExtensionInstance::placeholder_instance_property_get_revert;
+	placeholder_extension->validate_property = &PlaceholderExtensionInstance::placeholder_instance_validate_property;
+	placeholder_extension->notification = nullptr;
+	placeholder_extension->notification2 = &PlaceholderExtensionInstance::placeholder_instance_notification;
+	placeholder_extension->to_string = &PlaceholderExtensionInstance::placeholder_instance_to_string;
+	placeholder_extension->reference = &PlaceholderExtensionInstance::placeholder_instance_reference;
+	placeholder_extension->unreference = &PlaceholderExtensionInstance::placeholder_instance_unreference;
+	placeholder_extension->get_rid = &PlaceholderExtensionInstance::placeholder_instance_get_rid;
+
+	placeholder_extension->class_userdata = ti;
+	placeholder_extension->create_instance = &PlaceholderExtensionInstance::placeholder_class_create_instance;
+	placeholder_extension->free_instance = &PlaceholderExtensionInstance::placeholder_class_free_instance;
+	placeholder_extension->get_virtual = &PlaceholderExtensionInstance::placeholder_class_get_virtual;
+	placeholder_extension->get_virtual_call_data = nullptr;
+	placeholder_extension->call_virtual_with_data = nullptr;
+	placeholder_extension->recreate_instance = &PlaceholderExtensionInstance::placeholder_class_recreate_instance;
+
+	placeholder_extension->tracking_userdata = ti->gdextension->tracking_userdata;
+	placeholder_extension->track_instance = ti->gdextension->track_instance;
+	placeholder_extension->untrack_instance = ti->gdextension->untrack_instance;
+
+	return placeholder_extension;
+}
+#endif
+
 void ClassDB::set_object_extension_instance(Object *p_object, const StringName &p_class, GDExtensionClassInstancePtr p_instance) {
 	ERR_FAIL_NULL(p_object);
 	ClassInfo *ti;
@@ -1716,7 +1931,7 @@ Variant ClassDB::class_get_default_property_value(const StringName &p_class, con
 			c = Engine::get_singleton()->get_singleton_object(p_class);
 			cleanup_c = false;
 		} else if (ClassDB::can_instantiate(p_class) && !ClassDB::is_virtual(p_class)) { // Keep this condition in sync with doc_tools.cpp get_documentation_default_value.
-			c = ClassDB::instantiate(p_class);
+			c = ClassDB::instantiate_no_placeholders(p_class);
 			cleanup_c = true;
 		}
 
@@ -1826,6 +2041,11 @@ void ClassDB::unregister_extension_class(const StringName &p_class, bool p_free_
 		}
 	}
 	classes.erase(p_class);
+	default_values_cached.erase(p_class);
+	default_values.erase(p_class);
+#ifdef TOOLS_ENABLED
+	placeholder_extensions.erase(p_class);
+#endif
 }
 
 HashMap<StringName, ClassDB::NativeStruct> ClassDB::native_structs;

core/object/class_db.h

@@ -149,6 +149,10 @@ public:
 	static HashMap<StringName, StringName> resource_base_extensions;
 	static HashMap<StringName, StringName> compat_classes;
 
+#ifdef TOOLS_ENABLED
+	static HashMap<StringName, ObjectGDExtension> placeholder_extensions;
+#endif
+
 #ifdef DEBUG_METHODS_ENABLED
 	static MethodBind *bind_methodfi(uint32_t p_flags, MethodBind *p_bind, bool p_compatibility, const MethodDefinition &method_name, const Variant **p_defs, int p_defcount);
 #else
@@ -178,6 +182,8 @@ private:
 	static MethodBind *_bind_vararg_method(MethodBind *p_bind, const StringName &p_name, const Vector<Variant> &p_default_args, bool p_compatibility);
 	static void _bind_method_custom(const StringName &p_class, MethodBind *p_method, bool p_compatibility);
 
+	static Object *_instantiate_internal(const StringName &p_class, bool p_require_real_class = false);
+
 public:
 	// DO NOT USE THIS!!!!!! NEEDS TO BE PUBLIC BUT DO NOT USE NO MATTER WHAT!!!
 	template <class T>
@@ -253,6 +259,7 @@ public:
 	static void get_class_list(List<StringName> *p_classes);
 #ifdef TOOLS_ENABLED
 	static void get_extensions_class_list(List<StringName> *p_classes);
+	static ObjectGDExtension *get_placeholder_extension(const StringName &p_class);
 #endif
 	static void get_inheriters_from_class(const StringName &p_class, List<StringName> *p_classes);
 	static void get_direct_inheriters_from_class(const StringName &p_class, List<StringName> *p_classes);
@@ -264,6 +271,7 @@ public:
 	static bool can_instantiate(const StringName &p_class);
 	static bool is_virtual(const StringName &p_class);
 	static Object *instantiate(const StringName &p_class);
+	static Object *instantiate_no_placeholders(const StringName &p_class);
 	static void set_object_extension_instance(Object *p_object, const StringName &p_class, GDExtensionClassInstancePtr p_instance);
 
 	static APIType get_api_type(const StringName &p_class);

core/object/object.cpp

@@ -492,6 +492,11 @@ void Object::get_property_list(List<PropertyInfo> *p_list, bool p_reversed) cons
 			ClassDB::get_property_list(current_extension->class_name, p_list, true, this);
 
 			if (current_extension->get_property_list) {
+#ifdef TOOLS_ENABLED
+				// If this is a placeholder, we can't call into the GDExtension on the parent class,
+				// because we don't have a real instance of the class to give it.
+				if (likely(!_extension->is_placeholder)) {
+#endif
 					uint32_t pcount;
 					const GDExtensionPropertyInfo *pinfo = current_extension->get_property_list(_extension_instance, &pcount);
 					for (uint32_t i = 0; i < pcount; i++) {
@@ -500,6 +505,9 @@ void Object::get_property_list(List<PropertyInfo> *p_list, bool p_reversed) cons
 					if (current_extension->free_property_list) {
 						current_extension->free_property_list(_extension_instance, pinfo);
 					}
+#ifdef TOOLS_ENABLED
+				}
+#endif
 			}
 
 			current_extension = current_extension->parent;
@@ -1792,6 +1800,16 @@ uint32_t Object::get_edited_version() const {
 #endif
 
 StringName Object::get_class_name_for_extension(const GDExtension *p_library) const {
+#ifdef TOOLS_ENABLED
+	// If this is the library this extension comes from and it's a placeholder, we
+	// have to return the closest native parent's class name, so that it doesn't try to
+	// use this like the real object.
+	if (unlikely(_extension && _extension->library == p_library && _extension->is_placeholder)) {
+		const StringName *class_name = _get_class_namev();
+		return *class_name;
+	}
+#endif
+
 	// Only return the class name per the extension if it matches the given p_library.
 	if (_extension && _extension->library == p_library) {
 		return _extension->class_name;
@@ -1919,6 +1937,7 @@ void Object::clear_internal_extension() {
 
 	// Clear the instance bindings.
 	_instance_binding_mutex.lock();
+	if (_instance_bindings) {
 		if (_instance_bindings[0].free_callback) {
 			_instance_bindings[0].free_callback(_instance_bindings[0].token, this, _instance_bindings[0].binding);
 		}
@@ -1926,6 +1945,7 @@ void Object::clear_internal_extension() {
 		_instance_bindings[0].token = nullptr;
 		_instance_bindings[0].free_callback = nullptr;
 		_instance_bindings[0].reference_callback = nullptr;
+	}
 	_instance_binding_mutex.unlock();
 
 	// Clear the virtual methods.

core/object/object.h

@@ -317,6 +317,10 @@ struct ObjectGDExtension {
 	bool is_virtual = false;
 	bool is_abstract = false;
 	bool is_exposed = true;
+#ifdef TOOLS_ENABLED
+	bool is_runtime = false;
+	bool is_placeholder = false;
+#endif
 	GDExtensionClassSet set;
 	GDExtensionClassGet get;
 	GDExtensionClassGetPropertyList get_property_list;
@@ -755,6 +759,7 @@ protected:
 	void _clear_internal_resource_paths(const Variant &p_var);
 
 	friend class ClassDB;
+	friend class PlaceholderExtensionInstance;
 
 	bool _disconnect(const StringName &p_signal, const Callable &p_callable, bool p_force = false);
 
@@ -977,6 +982,7 @@ public:
 #ifdef TOOLS_ENABLED
 	void clear_internal_extension();
 	void reset_internal_extension(ObjectGDExtension *p_extension);
+	bool is_extension_placeholder() const { return _extension && _extension->is_placeholder; }
 #endif
 
 	void clear_internal_resource_paths();

modules/gdscript/gdscript.cpp

@@ -95,7 +95,7 @@ Variant GDScriptNativeClass::_new() {
 }
 
 Object *GDScriptNativeClass::instantiate() {
-	return ClassDB::instantiate(name);
+	return ClassDB::instantiate_no_placeholders(name);
 }
 
 Variant GDScriptNativeClass::callp(const StringName &p_method, const Variant **p_args, int p_argcount, Callable::CallError &r_error) {