Ownership Inventory #1 - The Rust Programming Language
rust-book.cs.brown.edu

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!.

  • maegul (he/they)OPMEnglish
    1·
    6 months ago

    Q6: Fixing

    Of the following fixes (highlighted in yellow), which fix best satisfies these three criteria:

    • The fixed function passes the Rust compiler,
    • The fixed function preserves the intention of the original code, and
    • The fixed function does not introduce unnecessary inefficiencies
    /// Gets the string out of an option if it exists,
/// returning a default otherwise
fn get_or_default(arg: &Option<String>) -> String {
    if arg.is_none() {
        return String::new();
    }
    let s = arg.unwrap();
    s.clone()
}

    1:

    fn get_or_default(arg: &Option<&str>) -> String {
    if arg.is_none() {
        return String::new();
    }
    let s = arg.unwrap();
    s.to_string()
}

    2:

    fn get_or_default(arg: &mut Option<String>) -> String {
    if arg.is_none() {
        return String::new();
    }
    let s = arg.as_mut().unwrap();
    s.clone()
}

    3:

    fn get_or_default(arg: Option<String>) -> String {
    if arg.is_none() {
        return String::new();
    }
    let s = arg.unwrap();
    s.clone()
}

    4:

    fn get_or_default(arg: &Option<String>) -> String {
    match arg {
        None => String::new(),
        Some(s) => s.clone()
    }
}
    Answer

    4

    • Really about best-practices here
    • 4 is a better, more idiomatic version of 3, especially because it requires ownership of arg which is restrictive and may not even be available
      • But I think 3 does fix the problem
    • 1 doesn’t fix the problem
    • 2 … I’m not sure about … but I don’t think having a mutable s helps with the problem either (?)

    Context: The combination of is_none and unwrap here is a Rust anti-pattern, since a match combines the two functionalities and automatically deals with pushing the reference &Option into the interior to produce &String. Therefore the match solution is the most idiomatic, and passes the compiler without changing the intended type signature of the function.

    The solution of changing &Option to Option is not desirable because it requires the caller to provide ownership of their option, which is a far more restrictive API.