4.1. Ownership
About 6 min
4.1. Ownership ๊ด๋ จ
Rust By Practice
Rust By Practice
Ownership
1. ๐๐
๐Problem
fn main() {
// Use as many approaches as you can to make it work
let x = String::from("hello, world");
let y = x;
println!("{},{}",x,y);
}
//
//
/*
Compiling playground v0.0.1 (/playground)
error[E0382]: borrow of moved value: `x`
--> src/main.rs:6:22
|
4 | let x = String::from("hello, world");
| - move occurs because `x` has type `String`, which does not implement the `Copy` trait
5 | let y = x;
| - value moved here
6 | println!("{},{}",x,y);
| ^ value borrowed here after move
|
= note: this error originates in the macro `$crate::format_args_nl` which comes from the expansion of the macro `println` (in Nightly builds, run with -Z macro-backtrace for more info)
help: consider cloning the value if the performance cost is acceptable
|
5 | let y = x.clone();
| ++++++++
For more information about this error, try `rustc --explain E0382`.
error: could not compile `playground` (bin "playground") due to previous error
*/
Solution
fn main() {
// Use as many approaches as you can to make it work
let x = String::from("hello, world");
let y = x;
println!("{},{}",x,y);
}
2. ๐๐
๐Problem
// Don't modify code in main!
fn main() {
let s1 = String::from("hello, world");
let s2 = take_ownership(s1);
println!("{}", s2);
}
// Only modify the code below!
fn take_ownership(s: String) {
println!("{}", s);
}
//
//
/*
Compiling playground v0.0.1 (/playground)
error[E0277]: `()` doesn't implement `std::fmt::Display`
--> src/main.rs:6:20
|
6 | println!("{}", s2);
| ^^ `()` cannot be formatted with the default formatter
|
= help: the trait `std::fmt::Display` is not implemented for `()`
note: in format strings you may be able to use `{:?}` (or {:#?} for pretty-print) instead
= note: this error originates in the macro `$crate::format_args_nl` which comes from the expansion of the macro `println` (in Nightly builds, run with -Z macro-backtrace for more info)
For more information about this error, try `rustc --explain E0277`.
error: could not compile `playground` (bin "playground") due to previous error
*/
Solution
// Don't modify code in main!
fn main() {
let s1 = String::from("hello, world");
let s2 = take_ownership(s1);
println!("{}", s2);
}
// Only modify the code below!
fn take_ownership(s: String) {
println!("{}", s);
}
3. ๐๐
๐Problem
fn main() {
let s = give_ownership();
println!("{}", s);
}
// Only modify the code below!
fn give_ownership() -> String {
let s = String::from("hello, world");
// Convert String to Vec
let _s = s.into_bytes();
s
}
//
//
/*
Compiling playground v0.0.1 (/playground)
error[E0382]: use of moved value: `s`
--> src/main.rs:12:5
|
9 | let s = String::from("hello, world");
| - move occurs because `s` has type `String`, which does not implement the `Copy` trait
10 | // Convert String to Vec
11 | let _s = s.into_bytes();
| ------------ `s` moved due to this method call
12 | s
| ^ value used here after move
|
note: `String::into_bytes` takes ownership of the receiver `self`, which moves `s`
--> /rustc/cc66ad468955717ab92600c770da8c1601a4ff33/library/alloc/src/string.rs:849:23
help: you can `clone` the value and consume it, but this might not be your desired behavior
|
11 | let _s = s.clone().into_bytes();
| ++++++++
For more information about this error, try `rustc --explain E0382`.
error: could not compile `playground` (bin "playground") due to previous error
*/
Solution
fn main() {
let s = give_ownership();
println!("{}", s);
}
// Only modify the code below!
fn give_ownership() -> String {
let s = String::from("hello, world");
// Convert String to Vec
let _s = s.into_bytes();
s
}
4. ๐๐
๐Problem
// Fix the error without removing code line
fn main() {
let s = String::from("hello, world");
print_str(s);
println!("{}", s);
}
fn print_str(s: String) {
println!("{}",s)
}
//
//
/*
Compiling playground v0.0.1 (/playground)
error[E0382]: borrow of moved value: `s`
--> src/main.rs:7:20
|
3 | let s = String::from("hello, world");
| - move occurs because `s` has type `String`, which does not implement the `Copy` trait
4 |
5 | print_str(s);
| - value moved here
6 |
7 | println!("{}", s);
| ^ value borrowed here after move
|
note: consider changing this parameter type in function `print_str` to borrow instead if owning the value isn't necessary
--> src/main.rs:10:17
|
10 | fn print_str(s: String) {
| --------- ^^^^^^ this parameter takes ownership of the value
| |
| in this function
= note: this error originates in the macro `$crate::format_args_nl` which comes from the expansion of the macro `println` (in Nightly builds, run with -Z macro-backtrace for more info)
help: consider cloning the value if the performance cost is acceptable
|
5 | print_str(s.clone());
| ++++++++
For more information about this error, try `rustc --explain E0382`.
error: could not compile `playground` (bin "playground") due to previous error
*/
Solution
// Fix the error without removing code line
fn main() {
let s = String::from("hello, world");
print_str(s);
println!("{}", s);
}
fn print_str(s: String) {
println!("{}",s)
}
5. ๐๐
๐Problem
// Don't use clone ,use copy instead
fn main() {
let x = (1, 2, (), "hello".to_string());
let y = x.clone();
println!("{:?}, {:?}", x, y);
}
//
//
/*
(1, 2, (), "hello"), (1, 2, (), "hello")
*/
Solution
// Don't use clone ,use copy instead
fn main() {
let x = (1, 2, (), "hello".to_string());
let y = x.clone();
println!("{:?}, {:?}", x, y);
}
Mutability
Mutability can be changed when ownership is transferred.
6. ๐
๐Problem
fn main() {
let s = String::from("hello, ");
// Modify this line only !
let s1 = s;
s1.push_str("world");
println!("Success!");
}
//
//
/*
Compiling playground v0.0.1 (/playground)
error[E0596]: cannot borrow `s1` as mutable, as it is not declared as mutable
--> src/main.rs:8:5
|
8 | s1.push_str("world");
| ^^ cannot borrow as mutable
|
help: consider changing this to be mutable
|
6 | let mut s1 = s;
| +++
For more information about this error, try `rustc --explain E0596`.
error: could not compile `playground` (bin "playground") due to previous error
*/
Solution
fn main() {
let s = String::from("hello, ");
// Modify this line only !
let s1 = s;
s1.push_str("world");
println!("Success!");
}
7. ๐๐๐
๐Problem
fn main() {
let x = Box::new(5);
let ... // Implement this line, dont change other lines!
*y = 4;
assert_eq!(*x, 5);
println!("Success!");
}
//
//
/*
Compiling playground v0.0.1 (/playground)
error: unexpected `...`
--> src/main.rs:5:9
|
5 | let ... // Implement this line, dont change other lines!
| ^^^
| |
| not a valid pattern
| help: for a rest pattern, use `..` instead of `...`
error: expected `;`, found `*`
--> src/main.rs:5:12
|
5 | let ... // Implement this line, dont change other lines!
| ^ help: add `;` here
6 |
7 | *y = 4;
| - unexpected token
error[E0425]: cannot find value `y` in this scope
--> src/main.rs:7:6
|
7 | *y = 4;
| ^ help: a local variable with a similar name exists: `x`
error: `..` patterns are not allowed here
--> src/main.rs:5:9
|
5 | let ... // Implement this line, dont change other lines!
| ^^^
|
= note: only allowed in tuple, tuple struct, and slice patterns
For more information about this error, try `rustc --explain E0425`.
error: could not compile `playground` (bin "playground") due to 4 previous errors
*/
Solution
fn main() {
let x = Box::new(5);
let ... // Implement this line, dont change other lines!
*y = 4;
assert_eq!(*x, 5);
println!("Success!");
}
Partial move
Within the destructuring of a single variable, both by-move and by-reference pattern bindings can be used at the same time. Doing this will result in a partial move of the variable, which means that parts of the variable will be moved while other parts stay. In such a case, the parent variable cannot be used afterwards as a whole, however the parts that are only referenced (and not moved) can still be used.
Example
fn main() {
[derive(Debug)]
struct Person {
name: String,
age: Box<u8>,
}
let person = Person {
name: String::from("Alice"),
age: Box::new(20),
};
// `name` is moved out of person, but `age` is referenced
let Person { name, ref age } = person;
println!("The person's age is {}", age);
println!("The person's name is {}", name);
// Error! borrow of partially moved value: `person` partial move occurs
//println!("The person struct is {:?}", person);
// `person` cannot be used but `person.age` can be used as it is not moved
println!("The person's age from person struct is {}", person.age);
}
//
//
/*
The person's age is 20
The person's name is Alice
The person's age from person struct is 20
*/
Exercises
8. ๐
๐Problem
fn main() {
let t = (String::from("hello"), String::from("world"));
let _s = t.0;
// Modify this line only, don't use `_s`
println!("{:?}", t);
}
//
//
/*
Compiling playground v0.0.1 (/playground)
error[E0382]: borrow of partially moved value: `t`
--> src/main.rs:8:21
|
5 | let _s = t.0;
| --- value partially moved here
...
8 | println!("{:?}", t);
| ^ value borrowed here after partial move
|
= note: partial move occurs because `t.0` has type `String`, which does not implement the `Copy` trait
= note: this error originates in the macro `$crate::format_args_nl` which comes from the expansion of the macro `println` (in Nightly builds, run with -Z macro-backtrace for more info)
For more information about this error, try `rustc --explain E0382`.
error: could not compile `playground` (bin "playground") due to previous error
*/
Solution
fn main() {
let t = (String::from("hello"), String::from("world"));
let _s = t.0;
// Modify this line only, don't use `_s`
println!("{:?}", t);
}
9. ๐๐
๐Problem
fn main() {
let t = (String::from("hello"), String::from("world"));
// Fill the blanks
let (__, __) = __;
println!("{:?}, {:?}, {:?}", s1, s2, t); // -> "hello", "world", ("hello", "world")
}
//
//
/*
Compiling playground v0.0.1 (/playground)
error[E0416]: identifier `__` is bound more than once in the same pattern
--> src/main.rs:6:14
|
6 | let (__, __) = __;
| ^^ used in a pattern more than once
error[E0425]: cannot find value `__` in this scope
--> src/main.rs:6:20
|
6 | let (__, __) = __;
| ^^ not found in this scope
error[E0425]: cannot find value `s1` in this scope
--> src/main.rs:8:34
|
8 | println!("{:?}, {:?}, {:?}", s1, s2, t); // -> "hello", "world", ("hello", "world")
| ^^ not found in this scope
error[E0425]: cannot find value `s2` in this scope
--> src/main.rs:8:38
|
8 | println!("{:?}, {:?}, {:?}", s1, s2, t); // -> "hello", "world", ("hello", "world")
| ^^ not found in this scope
Some errors have detailed explanations: E0416, E0425.
For more information about an error, try `rustc --explain E0416`.
error: could not compile `playground` (bin "playground") due to 4 previous errors
*/
Solution
fn main() {
let t = (String::from("hello"), String::from("world"));
// Fill the blanks
let (__, __) = __;
println!("{:?}, {:?}, {:?}", s1, s2, t); // -> "hello", "world", ("hello", "world")
}
tips
You can find the solutions here (sunface/rust-by-practice) (under the solutions path), but only use it when you need it
