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Object class

参见

This page describes the C++ implementation of objects in Godot. Looking for the Object class reference? Have a look here.

General definition

Object is the base class for almost everything. Most classes in Godot inherit directly or indirectly from it. Declaring them is a matter of using a single macro like this:

class CustomObject : public Object {
    GDCLASS(CustomObject, Object); // This is required to inherit from Object.
};

Objects come with a lot of built-in functionality, like reflection and editable properties:

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

OtherClass *obj2 = Object::cast_to<OtherClass>(obj); // Converting between classes, similar to dynamic_cast

References:

Registering Object classes

Most Object subclasses are registered by calling GDREGISTER_CLASS.

GDREGISTER_CLASS(MyCustomClass)

This will register it as a named, public class in the ClassDB, which will allow the class to be instantiated by scripts, code, or by deserialization. Note that classes registered as GDREGISTER_CLASS should expect to be instantiated or freed automatically, for example by the editor or the documentation system.

Besides GDREGISTER_CLASS, there are a few other modes of privateness:

// Registers the class publicly, but prevents automatic instantiation through ClassDB.
GDREGISTER_VIRTUAL_CLASS(MyCustomClass);

// Registers the class publicly, but prevents all instantiation through ClassDB.
GDREGISTER_ABSTRACT_CLASS(MyCustomClass);

// Registers the class in ClassDB, but marks it as private,
// such that it is not visible to scripts or extensions.
// This is the same as not registering the class explicitly at all
// - in this case, the class is registered as internal automatically
// when it is first constructed.
GDREGISTER_INTERNAL_CLASS(MyCustomClass);

// Registers the class such that it is only available at runtime (but not in the editor).
GDREGISTER_RUNTIME_CLASS(MyCustomClass);

It is also possible to use GDSOFTCLASS(MyCustomClass, SuperClass) instead of GDCLASS(MyCustomClass, SuperClass). Classes defined this way are not registered in the ClassDB at all. This is sometimes used for platform-specific subclasses.

Registering bindings

Object-derived classes can override the static function static void _bind_methods(). When the class is registered, this static function is called to register all the object methods, properties, constants, etc. It's only called once.

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

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

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

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:

Constants

Classes often have enums such as:

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:

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

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

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:

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

For example:

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:

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:

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

This creates the property using the setter and the getter.

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).

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.

Signals

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

// This is the function signature:
//
// Error connect(const StringName &p_signal, const Callable &p_callable, uint32_t p_flags = 0)
//
// For example:
obj->connect("signal_name_here", callable_mp(this, &MyCustomType::method), CONNECT_DEFERRED);

callable_mp is a macro to create a custom callable function pointer to member functions. For the values of p_flags, see ConnectFlags.

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

ADD_SIGNAL(MethodInfo("been_killed"))

Object ownership and casting

Objects are allocated on the heap. There are two different ownership models:

  • Objects derived from RefCounted are reference counted.

  • All other objects are manually memory managed.

The ownership models are fundamentally different. Refer to the section for each respectively to learn how to create, store, and free the object.

When you do not know whether an object passed to you (via Object *) is RefCounted, and you need to store it, you should store its ObjectID rather than a pointer (as explained below, in the manual memory management section).

When an object is passed to you via Variant, especially when using deferred callbacks, it is possible that the contained Object * was already freed by the time your function runs. Instead of converting directly to Object *, you should use get_validated_object:

void do_something(Variant p_variant) {
    Object *object = p_variant.get_validated_object();
    ERR_FAIL_NULL(object);
}

Manual memory management

Manually memory managed objects are created using memnew and freed using memdelete:

Node *node = memnew(Node);
// ...
memdelete(node);
node = nullptr;

When you are not the sole owner of an object, storing a pointer to it is dangerous: The object may at any point be freed through other references to it, causing your pointer to become a dangling pointer, which will eventually result in a crash.

When storing objects you are not the only owner of, you should store its ObjectID rather than a pointer:

Node *node = memnew(Node);
ObjectID node_id = node.get_instance_id();
// ...
Object *maybe_node = ObjectDB::get_instance(node_id);
ERR_FAIL_NULL(maybe_node); // The node may have been freed between calls.

RefCounted memory management

RefCounted subclasses are memory managed with reference counting semantics.

They are constructed using memnew, and should be stored in Ref instances. When the last Ref instance is dropped, the object automatically self-destructs.

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

Ref<MyRefCounted> my_ref = memnew(MyRefCounted);
// ...
// Ref holds shared ownership over the object, so the object
// will not be freed. As long as you have a valid, non-null
// Ref, it can be safely assumed the object is still valid.
my_ref->get_class_name();

You should never call memdelete for RefCounted subclasses, because there may be other owners of it.

You should also never store RefCounted subclasses using raw pointers, for example RefCounted *object = memnew(RefCounted). This is unsafe because other owners may destruct the object, leaving you with a dangling pointer, which will eventually result in a crash.

References:

Dynamic casting

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

void some_func(Object *p_object) {
     Button *button = Object::cast_to<Button>(p_object);
}

If the cast fails, nullptr is returned. This works the same as dynamic_cast, but does not use C++ RTTI.

Notifications

All objects in Godot have a _notification method that allows them to respond to engine-level callbacks that may relate to it. More information can be found on the Godot notifications page.

Resources

Resource inherits from RefCounted, 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:

Resource loading

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

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:

Resource saving

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

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: