How to specify custom constructor for C++ created classes

Andrej_Lapin · February 19, 2025, 7:40pm

Godot Version

4.3

Question

I have been trying to figure out if it is possible to create something akin to _init function inside of C++.
You can specify any kind of constructor for your class in C++, but I have not seen any way to set some constructor to be the default way to create your object.
Let’s say I have class defined in C++ called Component and in it I would define

void _init(StringName name, int number)

After binding such method I should be able to pass name and number in gdscript like so:

Component.new(name, number)

to construct this object.
But this does not happen. As a matter of fact if you try to bind a method with the name “_init”, it will make it not show up as this objects method at all…
Is it even possible to somehow specify custom construct from the C++ for GDScript?

space_jellyfish · March 8, 2025, 12:42pm

I found

GDExtension allows classes to be registered without a default constructor. They can either have custom constructors (static methods) that are invokable from GDScript, or non-exposed constructors only available in the binding language. This can be done by not providing create and recreate function pointers during class registration, and optionally marking a class as abstract.

from this issue.

Andrej_Lapin · March 17, 2025, 7:48pm

Thank you for the reply
I will try setting create_instance_func and recreate_instance_func, but it’s a little bit complicated and might not be very intended
Because I will have to define my custom _register_class function in my library

github.com/godotengine/godot-cpp

include/godot_cpp/core/class_db.hpp

4.4


      
          
          #define BIND_VIRTUAL_METHOD(m_class, m_method, m_hash)                                                                                        \
          	{                                                                                                                                         \
          		auto _call##m_method = [](GDExtensionObjectPtr p_instance, const GDExtensionConstTypePtr *p_args, GDExtensionTypePtr p_ret) -> void { \
          			call_with_ptr_args(reinterpret_cast<m_class *>(p_instance), &m_class::m_method, p_args, p_ret);                                   \
          		};                                                                                                                                    \
          		::godot::ClassDB::bind_virtual_method(m_class::get_class_static(), #m_method, _call##m_method, m_hash);                               \
          	}
          
          template <typename T, bool is_abstract>
          void ClassDB::_register_class(bool p_virtual, bool p_exposed, bool p_runtime) {
          	static_assert(TypesAreSame<typename T::self_type, T>::value, "Class not declared properly, please use GDCLASS.");
          	static_assert(!FunctionsAreSame<T::self_type::_bind_methods, T::parent_type::_bind_methods>::value, "Class must declare 'static void _bind_methods'.");
          	static_assert(!std::is_abstract_v<T> || is_abstract, "Class is abstract, please use GDREGISTER_ABSTRACT_CLASS.");
          	instance_binding_callbacks[T::get_class_static()] = &T::_gde_binding_callbacks;
          
          	// Register this class within our plugin
          	ClassInfo cl;
          	cl.name = T::get_class_static();
          	cl.parent_name = T::get_parent_class_static();
          	cl.level = current_level;

And then provide function pointers to create_instnace_function and recreate_instance_function for class info structure

github.com/godotengine/godot-cpp

include/godot_cpp/core/class_db.hpp

4.4


      
          	GDExtensionClassCreationInfo4 class_info = {
          		p_virtual, // GDExtensionBool is_virtual;
          		is_abstract, // GDExtensionBool is_abstract;
          		p_exposed, // GDExtensionBool is_exposed;
          		p_runtime, // GDExtensionBool is_runtime;
          		nullptr, // GDExtensionConstStringPtr icon_path;
          		T::set_bind, // GDExtensionClassSet set_func;
          		T::get_bind, // GDExtensionClassGet get_func;
          		T::has_get_property_list() ? T::get_property_list_bind : nullptr, // GDExtensionClassGetPropertyList get_property_list_func;
          		T::free_property_list_bind, // GDExtensionClassFreePropertyList2 free_property_list_func;
          		T::property_can_revert_bind, // GDExtensionClassPropertyCanRevert property_can_revert_func;
          		T::property_get_revert_bind, // GDExtensionClassPropertyGetRevert property_get_revert_func;
          		T::validate_property_bind, // GDExtensionClassValidateProperty validate_property_func;
          		T::notification_bind, // GDExtensionClassNotification2 notification_func;
          		T::to_string_bind, // GDExtensionClassToString to_string_func;
          		nullptr, // GDExtensionClassReference reference_func;
          		nullptr, // GDExtensionClassUnreference unreference_func;
          		&_create_instance_func<T>, // GDExtensionClassCreateInstance create_instance_func; /* this one is mandatory */
          		T::free, // GDExtensionClassFreeInstance free_instance_func; /* this one is mandatory */
          		&_recreate_instance_func<T>, // GDExtensionClassRecreateInstance recreate_instance_func;

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Even after this done, the create_instance_function is expecting void* data as input in any case, so we would need to find a way to tell the engine which arguments the Class.new() call is expecting, and currently I have no idea where that could be specified.