Struct Computed 

pub struct Computed;

Marker type of a read-only R field computed from the Rust value (#1883).

Declare it as #[r_data(get = "Self::f")] pub f: Computed on a #[derive(ExternalPtr)] struct with an RSidecar selector. get names a plain Rust function fn(&Self) -> T with T: IntoR, resolved in the struct’s module (a leading Self is the struct); it needs no #[miniextendr]. The field holds no value: it is a zero-sized marker that gives the computed field its place among the type’s R fields, so every struct literal writes f: Computed once, as it writes _r: RSidecar.

#[derive(ExternalPtr)]
#[externalptr(s3)]
pub struct Engine {
    inner: Model,
    #[r_data] _r: RSidecar,
    #[r_data(get = "Self::group_by")] pub group_by: Computed,
    #[r_data(name = "keys")] pub r_keys: Sidecar<SEXP>,
    #[r_data(get = "Self::n_rows")] pub n_rows: Computed,
}

impl Engine {
    fn group_by(&self) -> Vec<String> { self.inner.group_by.clone() }
    fn n_rows(&self) -> i32 { self.inner.n_rows() }
}

What the derive generates for it:

• the standalone getter Engine_get_group_by(x), and an arm in the $ / [[ field methods of s3(r_data_accessors) & co., at the field’s declared place among the Sidecar and struct fields;
• no setter: no Engine_set_group_by(), and under the get/set form an assignment (x$group_by <- v) raises miniextendr_read_only_field.

The getter runs on R’s main thread with the live Rust value, so a computed field refuses a pointer restored from a saved session like a struct field does. Its errors are its own: a rust_error!(class = ...) in the getter reaches R with that class. The getter returns a value, not a Result; it takes &Self, so it can’t read Sidecar values (#1856), which are R fields already.

Only a pub Computed field reaches R, and only on S3, S4, env and vctrs types: R6 active bindings and S7 properties need a setter (#1888).

Trait Implementations

impl Clone for Computed

fn clone(&self) -> Self

Returns a duplicate of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl Copy for Computed

impl Debug for Computed

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl Default for Computed

fn default() -> Self

Returns the “default value” for a type.

impl Eq for Computed

#[doc(hidden)]

fn assert_receiver_is_total_eq(&self)

👎Deprecated since 1.95.0:

implementation detail of #[derive(Eq)]

#[doc(hidden)]

fn assert_fields_are_eq(&self)

🔬This is a nightly-only experimental API. (derive_eq_internals)

impl Hash for Computed

fn hash<__H: Hasher>(&self, state: &mut __H)

Feeds this value into the given Hasher.

fn hash_slice<H>(data: &[Self], state: &mut H)

where H: Hasher, Self: Sized,

Feeds a slice of this type into the given Hasher.

impl PartialEq for Computed

fn eq(&self, other: &Self) -> bool

Equality operator ==.

fn ne(&self, other: &Rhs) -> bool

Inequality operator !=.

impl StructuralPartialEq for Computed

impl TrivialClone for Computed

Layout

Note: Most layout information is completely unstable and may even differ between compilations. The only exception is types with certain repr(...) attributes. Please see the Rust Reference's “Type Layout” chapter for details on type layout guarantees.

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