CODEWITHSUNDEEP
rustmove
akuiper
akuiper

Reputation: 215117

Why the value is not moved with Option unwrap?

New to rust here and trying to learn. Consider the following piece of code:

let x = Some("air");
println!("{}", x.unwrap());
println!("{}", x.unwrap());

Why does this work? Because the unwrap method has the following signature pub fn unwrap(self) -> T, so it should move self into the method, which means after the first call of x.unwrap, I shouldn't be able to access x anymore. However this still works? Can someone explain what misunderstanding I am taking here?

Upvotes: 1

Views: 1544

Answers (3)

ravi
ravi

Reputation: 10733

You are confusing &str with String, which rather is not Copy. 

fn main() {
    // This will work fine.
    let x = Some("data");
    println!("{:?}", x.unwrap());
    println!("{:?}", x.unwrap());

    // however, this will generate an error since value will be moved.
    let x = Some("data".to_string());
    println!("{:?}", x.unwrap());
    println!("{:?}", x.unwrap());
}

Upvotes: 9

akuiper
akuiper

Reputation: 215117

Just figured out, x in OP is not moved because Option<&str> implements the Copy trait. On the other hand if the inner type in the option is a more complex type for instance a struct, this wouldn't work:

struct Node<T> { elem: T }

let y = Some(Node { elem: 3});

println!("{}", y.unwrap().elem);
println!("{}", y.unwrap().elem);

will give the following error:

Line 33, Char 24: use of moved value: `y` (solution.rs)
   |
32 |         println!("{}", y.unwrap().elem);
   |                        - value moved here
33 |         println!("{}", y.unwrap().elem);
   |                        ^ value used here after move
   |

Upvotes: 2

Sreeram Nair
Sreeram Nair

Reputation: 2397

The unwrap() method is used to remove the parent element from the selected element and not the element itself.

Lets say Y is parent for 'X' and 'Z' is parent of 'Y' then when you say x.unwrap() X's parent 'Y' is removed and 'Z' becomes the parent of 'X' and now when you say x.unwrap() again then 'Z' is removed.

Upvotes: -2

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