godot-docs/contributing/development/core_and_modules/object_class.rst

.. _doc_object_class:

Object class
============

.. seealso::

    This page describes the C++ implementation of objects in Godot.
    Looking for the Object class reference? :ref:`Have a look here. <class_Object>`

General definition
------------------

:ref:`Object <class_object>` is the base class for almost everything. Most classes in Godot
inherit directly or indirectly from it. Objects provide reflection and
editable properties, and declaring them is a matter of using a single
macro like this:

.. code-block:: cpp

    class CustomObject : public Object {

        GDCLASS(CustomObject, Object); // this is required to inherit
    };

This adds a lot of functionality to Objects. For example:

.. code-block:: cpp

    obj = memnew(CustomObject);
    print_line("Object class: ", obj->get_class()); // print object class

    obj2 = Object::cast_to<OtherClass>(obj); // converting between classes, this also works without RTTI enabled.

References:
~~~~~~~~~~~

-  `core/object/object.h <https://github.com/godotengine/godot/blob/master/core/object/object.h>`__

Registering an Object
---------------------

ClassDB is a static class that holds the entire list of registered
classes that inherit from Object, as well as dynamic bindings to all
their methods properties and integer constants.

Classes are registered by calling:

.. code-block:: cpp

    ClassDB::register_class<MyCustomClass>()

Registering it will allow the class to be instanced by scripts, code, or
creating them again when deserializing.

Registering as virtual is the same but it can't be instanced.

.. code-block:: cpp

    ClassDB::register_virtual_class<MyCustomClass>()

Object-derived classes can override the static function
``static void _bind_methods()``. When one class is registered, this
static function is called to register all the object methods,
properties, constants, etc. It's only called once. If an Object derived
class is instanced but has not been registered, it will be registered as
virtual automatically.

Inside ``_bind_methods``, there are a couple of things that can be done.
Registering functions is one:

.. code-block:: cpp

    ClassDB::bind_method(D_METHOD("methodname", "arg1name", "arg2name", "arg3name"), &MyCustomType::method);

Default values for arguments can be passed as parameters at the end:

.. code-block:: cpp

    ClassDB::bind_method(D_METHOD("methodname", "arg1name", "arg2name", "arg3name"), &MyCustomType::method, DEFVAL(-1), DEFVAL(-2)); // Default values for arg2name (-1) and arg3name (-2).

Default values must be provided in the same order as they are declared,
skipping required arguments and then providing default values for the optional ones.
This matches the syntax for declaring methods in C++.

``D_METHOD`` is a macro that converts "methodname" to a StringName for more
efficiency. Argument names are used for introspection, but when
compiling on release, the macro ignores them, so the strings are unused
and optimized away.

Check ``_bind_methods`` of Control or Object for more examples.

If just adding modules and functionality that is not expected to be
documented as thoroughly, the ``D_METHOD()`` macro can safely be ignored and a
string passing the name can be passed for brevity.

References:
~~~~~~~~~~~

-  `core/object/class_db.h <https://github.com/godotengine/godot/blob/master/core/object/class_db.h>`__

Constants
---------

Classes often have enums such as:

.. code-block:: cpp

    enum SomeMode {
       MODE_FIRST,
       MODE_SECOND
    };

For these to work when binding to methods, the enum must be declared
convertible to int. A macro is provided to help with this:

.. code-block:: cpp

    VARIANT_ENUM_CAST(MyClass::SomeMode); // now functions that take SomeMode can be bound.

The constants can also be bound inside ``_bind_methods``, by using:

.. code-block:: cpp

    BIND_CONSTANT(MODE_FIRST);
    BIND_CONSTANT(MODE_SECOND);

Properties (set/get)
--------------------

Objects export properties, properties are useful for the following:

-  Serializing and deserializing the object.
-  Creating a list of editable values for the Object derived class.

Properties are usually defined by the PropertyInfo() class and
constructed as:

.. code-block:: cpp

    PropertyInfo(type, name, hint, hint_string, usage_flags)

For example:

.. code-block:: cpp

    PropertyInfo(Variant::INT, "amount", PROPERTY_HINT_RANGE, "0,49,1", PROPERTY_USAGE_EDITOR)

This is an integer property named "amount". The hint is a range, and the range
goes from 0 to 49 in steps of 1 (integers). It is only usable for the editor
(editing the value visually) but won't be serialized.

Another example:

.. code-block:: cpp

    PropertyInfo(Variant::STRING, "modes", PROPERTY_HINT_ENUM, "Enabled,Disabled,Turbo")

This is a string property, can take any string but the editor will only
allow the defined hint ones. Since no usage flags were specified, the
default ones are PROPERTY_USAGE_STORAGE and PROPERTY_USAGE_EDITOR.

There are plenty of hints and usage flags available in object.h, give them a
check.

Properties can also work like C# properties and be accessed from script
using indexing, but this usage is generally discouraged, as using
functions is preferred for legibility. Many properties are also bound
with categories, such as "animation/frame" which also make indexing
impossible unless using operator [].

From ``_bind_methods()``, properties can be created and bound as long as
set/get functions exist. Example:

.. code-block:: cpp

    ADD_PROPERTY(PropertyInfo(Variant::INT, "amount"), "set_amount", "get_amount")

This creates the property using the setter and the getter.

.. _doc_binding_properties_using_set_get_property_list:

Binding properties using ``_set``/``_get``/``_get_property_list``
-----------------------------------------------------------------

An additional method of creating properties exists when more flexibility
is desired (i.e. adding or removing properties on context).

The following functions can be overridden in an Object derived class,
they are NOT virtual, DO NOT make them virtual, they are called for
every override and the previous ones are not invalidated (multilevel
call).

.. code-block:: cpp

    protected:
         void _get_property_list(List<PropertyInfo> *r_props) const;      // return list of properties
         bool _get(const StringName &p_property, Variant &r_value) const; // return true if property was found
         bool _set(const StringName &p_property, const Variant &p_value); // return true if property was found

This is also a little less efficient since ``p_property`` must be
compared against the desired names in serial order.

Dynamic casting
---------------

Godot provides dynamic casting between Object-derived classes, for
example:

.. code-block:: cpp

    void somefunc(Object *some_obj) {

         Button *button = Object::cast_to<Button>(some_obj);
    }

If cast fails, NULL is returned. This system uses RTTI, but it also
works fine (although a bit slower) when RTTI is disabled. This is useful
on platforms where a small binary size is ideal, such as HTML5 or
consoles (with low memory footprint).

Signals
-------

Objects can have a set of signals defined (similar to Delegates in other
languages). This example shows how to connect to them:

.. code-block:: cpp

    obj->connect(<signal>, target_instance, target_method)
    // for example:
    obj->connect("enter_tree", this, "_node_entered_tree")

The method ``_node_entered_tree`` must be registered to the class using
``ClassDB::bind_method`` (explained before).

Adding signals to a class is done in ``_bind_methods``, using the
``ADD_SIGNAL`` macro, for example:

.. code-block:: cpp

    ADD_SIGNAL(MethodInfo("been_killed"))

Notifications
-------------

All objects in Godot have a :ref:`_notification <class_Object_private_method__notification>`
method that allows it to respond to engine level callbacks that may relate to it.
More information can be found on the :ref:`doc_godot_notifications` page.

References
----------

:ref:`RefCounted <class_RefCounted>` inherits from Object and holds a
reference count. It is the base for reference counted object types.
Declaring them must be done using Ref<> template. For example:

.. code-block:: cpp

    class MyReference: public RefCounted {
        GDCLASS(MyReference, RefCounted);
    };

    Ref<MyReference> myref(memnew(MyReference));

``myref`` is reference counted. It will be freed when no more Ref<>
templates point to it.

References:
~~~~~~~~~~~

-  `core/object/reference.h <https://github.com/godotengine/godot/blob/master/core/object/ref_counted.h>`__

Resources
----------

:ref:`Resource <class_resource>` inherits from Reference, so all resources
are reference counted. Resources can optionally contain a path, which
reference a file on disk. This can be set with ``resource.set_path(path)``,
though this is normally done by the resource loader. No two different
resources can have the same path; attempting to do so will result in an error.

Resources without a path are fine too.

References:
~~~~~~~~~~~

-  `core/io/resource.h <https://github.com/godotengine/godot/blob/master/core/io/resource.h>`__

Resource loading
----------------

Resources can be loaded with the ResourceLoader API, like this:

.. code-block:: cpp

    Ref<Resource> res = ResourceLoader::load("res://someresource.res")

If a reference to that resource has been loaded previously and is in
memory, the resource loader will return that reference. This means that
there can be only one resource loaded from a file referenced on disk at
the same time.

-  resourceinteractiveloader (TODO)

References:
~~~~~~~~~~~

-  `core/io/resource_loader.h <https://github.com/godotengine/godot/blob/master/core/io/resource_loader.h>`__

Resource saving
---------------

Saving a resource can be done with the resource saver API:

.. code-block:: cpp

    ResourceSaver::save("res://someresource.res", instance)

The instance will be saved, and sub resources that have a path to a file will
be saved as a reference to that resource. Sub resources without a path will
be bundled with the saved resource and assigned sub-IDs, like
``res://someresource.res::1``. This also helps to cache them when loaded.

References:
~~~~~~~~~~~

-  `core/io/resource_saver.h <https://github.com/godotengine/godot/blob/master/core/io/resource_saver.h>`__