Struct Update Syntax
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Basic Struct Update Syntax
Struct update syntax lets you create a new instance based on an existing one, explicitly overriding only the fields you want to change and copying the rest with ..base.
Example: Basic Struct Update Syntax
#[derive(Debug)]
struct Config {
width: u32,
height: u32,
fullscreen: bool,
}
fn main() {
let default_config = Config { width: 800, height: 600, fullscreen: false };
let custom = Config { fullscreen: true, ..default_config };
println!("{:?}", custom);
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Moving Non-Copy Fields
If the base struct contains non-Copy fields like String that are not overridden, those fields are moved into the new instance, making the original struct partially unusable afterward.
Example: Moving Non-Copy Fields
#[derive(Debug)]
struct Profile {
username: String,
active: bool,
}
fn main() {
let base = Profile { username: String::from("guest"), active: false };
let updated = Profile { active: true, ..base };
println!("{:?}", updated);
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..base Must Come Last
The ..base syntax must always appear as the final item in a struct literal, after every explicitly assigned field, since it fills in whatever remains.
Example: ..base Must Come Last
#[derive(Debug)]
struct Point3D {
x: i32,
y: i32,
z: i32,
}
fn main() {
let origin = Point3D { x: 0, y: 0, z: 0 };
let shifted = Point3D { x: 5, ..origin };
println!("{:?}", shifted);
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Combining With Copy Structs
When the base struct's fields are all Copy types, using struct update syntax leaves the original instance fully usable afterward, since nothing was moved out of it.
Example: Combining With Copy Structs
#[derive(Debug, Clone, Copy)]
struct Settings {
volume: u8,
brightness: u8,
}
fn main() {
let defaults = Settings { volume: 50, brightness: 70 };
let louder = Settings { volume: 80, ..defaults };
println!("defaults still usable: {:?}", defaults);
println!("louder: {:?}", louder);
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- Forgetting that struct update syntax
..othermoves non-Copyfields out ofother, potentially invalidating it afterward. - Placing
..otherbefore other field assignments -- it must come last in the struct literal. - Assuming struct update syntax mutates the original struct in place, when it actually creates a brand new instance.
..other_instanceat the end of a struct literal fills in any remaining fields from an existing instance.- Only the fields not explicitly listed are copied or moved from the base instance.
- If any moved (non-Copy) fields are taken from the base instance, that instance can no longer be used afterward.
- This syntax is a concise way to create variations of a struct without repeating every field.
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