5
Reading Club: The Book - Ownership Inventory Quiz no 1 [PROJECT] (rust-book.cs.brown.edu)
submitted 2 years ago* (last edited 2 years ago) by maegul@lemmy.ml to c/learningrustandlemmy@lemmy.ml
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After Chs 5 and 6 (see the reading club post here), we get a capstone quiz that covers ownership along with struts and enums.

So, lets do the quiz together! If you've done it already, revisiting might still be very instructive! I certainly thought these questions were useful "revision".

I'll post a comment for each question with the answer, along with my own personal notes (and quotes from The Book if helpful), behind spoiler tags.

Feel free to try to answer in a comment before checking (if you dare). But the main point is to understand the point the question is making, so share any confusions/difficulties too, and of course any corrections of my comments/notes!.

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[-] maegul@lemmy.ml 1 points 2 years ago

Q1

/// Makes a string to separate lines of text, 
/// returning a default if the provided string is blank
fn make_separator(user_str: &str) -> &str {
    if user_str == "" {
        let default = "=".repeat(10);
        &default
    } else {
        user_str
    }
}

When compiling, what's the best description of the compiler error?

  1. user_str does not live long enough
  2. function make_separator cannot return two different references
  3. function make_separator cannot return a reference of type &str
  4. cannot return reference to local variable default

Answer

Cannot return reference to a local variable

  • &default isn't allowed as default is local to the function.
    • What's the fix? Copy? Just return "=".repeat(10) directly?
    • How about just return an owned String (requires converting user_str to a String with to_string())

Context: Because default lives on the stack within make_separator, it will be deallocated once a call to make_separator ends. This leaves &default pointing to deallocated memory. Rust therefore complains that you cannot return a reference to a local variable.

this post was submitted on 21 Jun 2024
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