Rust for C-Programmers

9.3 Updating Struct Instances

Struct instances can be initialized using default values or updated by taking fields from existing instances, which can involve moving ownership.

9.3.1 Struct Update Syntax

You can build a new instance by reusing some fields from an existing instance:

let new_instance = StructName {
    field1: new_value,
    ..old_instance
};

Example:

struct Person {
    name: String,
    location: String,
    age: u8,
}
fn main() {
    let person1 = Person {
        name: String::from("Carol"),
        location: String::from("Berlin"),
        age: 22,
    };
    let person2 = Person {
        name: String::from("Dave"),
        age: 27,
        ..person1
    };

    println!("{} is {} years old and lives in {}.",
        person2.name, person2.age, person2.location);
    
    println!("{}", person1.name); // field was not used to initialize person2
    // println!("{}", person1.location); // value borrowed here after move
}

Because fields that do not implement Copy are moved, you can no longer access them from the original instance. However, Rust does allow continued access to fields that were not moved.

9.3.2 Field Init Shorthand

If a local variable’s name matches a struct field’s name:

let name = String::from("Eve");
let age = 28;

let person = Person { name, age };

This is shorthand for:

let person = Person {
    name: name,
    age: age,
};

9.3.3 Using Default Values

If a struct derives or implements the Default trait, you can create an instance with default values:

#![allow(unused)]
fn main() {
#[derive(Default)]
struct Person {
    name: String,
    age: u8,
}
}

Then:

let person1 = Person::default();
let person2: Person = Default::default();

Or override specific fields:

let person3 = Person {
    name: String::from("Eve"),
    ..Person::default()
};

9.3.4 Implementing the Default Trait Manually

If deriving the Default trait is insufficient, you can manually implement it:

impl Default for Person {
    fn default() -> Self {
        Person {
            name: String::from("Unknown"),
            age: 0,
        }
    }
}

Traits are discussed in detail in chapter 11.