CODEWITHSUNDEEP
rust
Karan Kumar
Karan Kumar

Reputation: 3176

Accessing rust reference after its scope has ended

I am learning rust, I am trying to understand references.

As per the rust's book -> https://doc.rust-lang.org/nightly/book/ch04-02-references-and-borrowing.html

It says

Note that a reference’s scope starts from where it is introduced 
and continues through the last time that reference is used.

And goes on showing (from the book):

    let mut s = String::from("hello");

    let r1 = &s; // no problem
    let r2 = &s; // no problem
    println!("{} and {}", r1, r2);
    // variables r1 and r2 will not be used after this point !! ... okay..?

BUT when I try to reproduce this, I get no error

My example (no error)

fn valid() {
    let s = String::from("hi");
    
    let r1 = &s; // valid immutable reference
    let r2 = &s; // valid immutable reference
    println!("{}, {}", r1, r2);
    println!("{}", r1); // Why am I able to use r1 again here??
}

Upvotes: 0

Views: 111

Answers (2)

Jared Smith
Jared Smith

Reputation: 21926

I think you misunderstood what you read. I'm assuming you're referencing this bit of code in the linked resource:

let mut s = String::from("hello");

let r1 = &s; // no problem
let r2 = &s; // no problem
println!("{} and {}", r1, r2);
// variables r1 and r2 will not be used after this point

let r3 = &mut s; // no problem
println!("{}", r3);

The point of the code is that since there is no further use of the shared references r1 and r2 it's okay to take the mutable reference r3. If you try to use one of them after doing so you'll get an error:

let mut s = String::from("hello");

let r1 = &s;
let r2 = &s;
println!("{} and {}", r1, r2);

let r3 = &mut s; // ERROR, no longer after last use of r1
println!("{}", r1);

The point of the example isn't that you can't use r1 and r2 after that line, it's that you cannot take a mutable reference &mut while something is still holding on to a shared reference &.

Playground

Upvotes: 2

Emoun
Emoun

Reputation: 2507

Because you have now changed when the "last use" of r1 is to be at the second print.

You decide when the reference should end and you do so by not using it after a given point. If you add a later use, then you have clearly decided on a new "last use" and so the compiler obliges.

Upvotes: 4

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