Trait WithBaseField 

pub trait WithBaseField: GodotClass<Declarer = DeclUser> + Bounds {
    // Required method
    fn to_gd(&self) -> Gd<Self>;

    // Provided methods
    fn base(&self) -> BaseRef<'_, Self> { ... }
    fn base_mut(&mut self) -> BaseMut<'_, Self> { ... }
}

Trait that is automatically implemented for user classes containing a Base<T> field.

Gives direct access to the containing Gd<Self> from self.

Usage as a bound

In order to call base() or base_mut() within a function or on a type you define, you need a WithBaseField<Base = T> bound, where T is the base class of your type.

fn some_fn<T>(value: &T)
where
    T: WithBaseField<Base = Node3D>,
{
    let base = value.base();
    let pos = base.get_position();
}

Required Methods

fn to_gd(&self) -> Gd<Self>

Returns the Gd pointer containing this object.

This is intended to be stored or passed to engine methods. You cannot call bind() or bind_mut() on it, while the method calling to_gd() is still running; that would lead to a double borrow panic.

Panics

If called during initialization (the init() function or Gd::from_init_fn()). Use Base::to_init_gd() instead.

Provided Methods

fn base(&self) -> BaseRef<'_, Self>

Returns a shared reference guard, suitable for calling &self engine methods on this object.

Holding a shared guard prevents other code paths from obtaining a mutable reference to self, as such it is recommended to drop the guard as soon as you no longer need it.

Examples

use godot::prelude::*;

#[derive(GodotClass)]
#[class(init, base=Node)]
struct MyClass {
    base: Base<Node>,
}

#[godot_api]
impl INode for MyClass {
    fn process(&mut self, _delta: f32) {
        let name = self.base().get_name();
        godot_print!("name is {name}");
    }
}

However, we cannot call methods that require &mut Base, such as Node::add_child().

use godot::prelude::*;

#[derive(GodotClass)]
#[class(init, base = Node)]
struct MyClass {
    ///     base: Base<Node>,
}

#[godot_api]
impl INode for MyClass {
    fn process(&mut self, _delta: f32) {
        let node = Node::new_alloc();
        // fails because `add_child` requires a mutable reference.
        self.base().add_child(&node);
    }
}

For this, use base_mut() instead.

fn base_mut(&mut self) -> BaseMut<'_, Self>

Returns an exclusive reference guard, suitable for calling &self/&mut self engine methods on this object.

This method will allow you to call back into the same object from Godot – something that to_gd() does not allow. You have to keep the BaseMut guard bound for the entire duration the engine might re-enter a function of your class. The guard temporarily absorbs the &mut self reference, which allows for an additional exclusive (mutable) reference to be acquired.

Holding an exclusive guard prevents other code paths from obtaining any reference to self, as such it is recommended to drop the guard as soon as you no longer need it.

Examples

#[derive(GodotClass)]
#[class(init, base = Node)]
struct MyClass {
    base: Base<Node>,
}

#[godot_api]
impl INode for MyClass {
    fn process(&mut self, _delta: f32) {
        let node = Node::new_alloc();
        self.base_mut().add_child(&node);
    }
}

We can call back into self through Godot:

#[derive(GodotClass)]
#[class(init, base=Node)]
struct MyClass {
    base: Base<Node>,
}

#[godot_api]
impl INode for MyClass {
    fn process(&mut self, _delta: f32) {
        self.base_mut().call("other_method", &[]);
    }
}

#[godot_api]
impl MyClass {
    #[func]
    fn other_method(&mut self) {}
}

Rust’s borrow checking rules are enforced if you try to overlap base_mut() calls:

// error[E0499]: cannot borrow `*self` as mutable more than once at a time

fn method(&mut self) {
    let mut a = self.base_mut();
    //          ---- first mutable borrow occurs here
    let mut b = self.base_mut();
    //          ^^^^ second mutable borrow occurs here
}

Dyn Compatibility

This trait is not dyn compatible.

In older versions of Rust, dyn compatibility was called "object safety", so this trait is not object safe.

Implementors