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Field update

Move fields from one struct to another with the .. syntax.

Struct update syntax

To copy most fields of an existing instance and change only some, ..kaynak is used; it's always written last:

let a = Oyuncu { ad: String::from("Ece"), puan: 10, aktif: true };
let b = Oyuncu { ad: String::from("Can"), ..a };
println!("{} {}", b.ad, b.puan);   // Can 10

Here puan and aktif are Copy types, so they are copied; a is still fully usable.

Partial moves

Only some fields of a struct can be moved; this is called a partial move. Access to the moved field and to the whole instance is forbidden; the remaining fields can be used:

let a = Oyuncu { ad: String::from("Ece"), puan: 10, aktif: true };
let b = Oyuncu { puan: 99, ..a };
println!("{}", a.puan);    // fine: Copy
// println!("{}", a.ad);   // error E0382
// let c = a;              // error: a was partially moved

To avoid the move, you can write ..a.clone() (the struct must derive Clone).

Practice Derive a new player

Define Oyuncu { ad: String, puan: u32, aktif: bool }. Let a be Ece, 10, true. Create b with ad: "Can" and ..a. Print b.ad, b.puan, b.aktif and a.ad separated by spaces.

Starter code

main.rs
fn main() {
    // buraya yaz
}

Sample solution

main.rs
struct Oyuncu {
    ad: String,
    puan: u32,
    aktif: bool,
}

fn main() {
    let a = Oyuncu {
        ad: String::from("Ece"),
        puan: 10,
        aktif: true,
    };
    let b = Oyuncu {
        ad: String::from("Can"),
        ..a
    };
    println!("{} {} {} {}", b.ad, b.puan, b.aktif, a.ad);
}

Expected output

Can 10 true Ece

Questions and answers

Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.

1 After <code>let b = Oyuncu { puan: 99, ..a };</code>, can <code>a.ad</code> be used?

  • Yes, it was copied
  • Yes, it was borrowed
  • b can't be created
  • No: ad (a String) was moved into b Correct answer

Explanation: ..a moves the fields not given. Since String isn't Copy, a.ad is no longer valid (a.puan and a.aktif can still be used).

2 Copy the rest

Question 
let b = Oyuncu {
    ad: String::from("Can"),
    ____
};
Answer 
let b = Oyuncu {
    ad: String::from("Can"),
    ..a
};

Explanation: ..a takes all fields not given from a and is written last.

3 Case and result

Term Meaning
{ ad: yeni, ..a } then a.ad Usable
{ puan: 1, ..a } then a.ad E0382
{ puan: 1, ..a } then a.puan Usable (Copy)
..a.clone() No move

All lessons in Beginner

1 Rust basics

  1. What is Rust?
  2. Project with Cargo
  3. Your first program
  4. println! and formatting
  5. Comments
  6. Summary - Rust basics

2 Variables

  1. Variables with let
  2. Mutability with mut
  3. Constants with const
  4. Static variables
  5. Shadowing
  6. Summary - Variables

3 Data types

  1. Integers
  2. Floating point
  3. Bool and char
  4. Tuple
  5. Array
  6. Summary - Data types

4 Strings

  1. &str vs String
  2. Creating a String
  3. Concatenating strings
  4. format! macro
  5. String length
  6. Summary - Strings

5 Project: Calculator

  1. Project skeleton
  2. Reading input
  3. Computing
  4. Error handling
  5. Loop and exit

6 Functions

  1. Defining with fn
  2. Parameters
  3. Return value
  4. Expressions and statements
  5. Early return
  6. Summary - Functions

7 Control flow

  1. if-else
  2. match basics
  3. loop
  4. while loop
  5. for loop
  6. Summary - Control flow

8 Ownership

  1. What is ownership?
  2. Move semantics
  3. Copy trait
  4. Functions and ownership
  5. Return and ownership
  6. Summary - Ownership

9 Borrowing

  1. References
  2. Mutable reference
  3. Borrowing rules
  4. Dangling reference
  5. References and functions
  6. Summary - Borrowing

10 Slices

  1. String slices
  2. Slice syntax
  3. Array slices
  4. Slices and functions
  5. Range types
  6. Summary - Slices

11 Project: Guessing game

  1. Game skeleton
  2. Reading input
  3. Comparing numbers
  4. Random number
  5. Loop and exit

12 Structs

  1. Struct definition
  2. Creating an instance
  3. Tuple struct
  4. Unit-like struct
  5. Field update
  6. Summary - Structs

13 Methods

  1. impl block
  2. self parameter
  3. &mut self
  4. Associated functions
  5. Multiple impl blocks
  6. Summary - Methods

14 Enums

  1. Enum definition
  2. Variants
  3. Variants with data
  4. Option<T>
  5. Option instead of null
  6. Summary - Enums

15 Pattern matching

  1. match patterns
  2. Value binding
  3. if let
  4. while let
  5. _ wildcard
  6. Summary - Pattern matching

16 Project: To-do CLI

  1. Task model
  2. Adding
  3. Listing
  4. Marking done
  5. Reading commands

17 Vectors

  1. Vec<T>
  2. Push and pop
  3. Index and get
  4. Looping a vector
  5. with_capacity
  6. Summary - Vectors

18 Advanced strings

  1. String and &str conversion
  2. chars and bytes
  3. Slicing strings
  4. contains and replace
  5. split and join
  6. Summary - Advanced strings

19 Error handling

  1. Result<T, E>
  2. panic!
  3. ? operator
  4. match and Result
  5. unwrap and expect
  6. Summary - Error handling

20 Beginner final

  1. Vowel counter
  2. Average calculator
  3. Bank account
  4. Temperature converter