this post was submitted on 20 May 2024
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Learning Rust and Lemmy

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Reading Club: The Book Ch 4 "Understanding Ownership" [PROJECT] (rust-book.cs.brown.edu)
submitted 10 months ago by maegul@lemmy.ml to c/learningrustandlemmy@lemmy.ml
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This is supplementary/separate from the Twitch Streams (see sidebar for links), intended for discussion here on lemmy.

The idea being, now that both twitch streams have read Chapter 4, we can have a discussion here and those from the twitch streams can have a retrospective or re-cap on the topic.

This will be a regular occurrence for each discrete set of topics coming out of The Book as the twitch streams cover them

Ownership and the borrow checker are obviously a fundamental and unique topic to rust, so it's well worth getting a good grounding in AFAICT.

  1. Anyone up to trying to summarise or explain ownership/borrow-checker in rust?
    • it can be a good exercise to test your understanding and get feedback/clarification from others ... as well as probably a good way to teach others
  2. Any persistent gripes, difficulties or confusions?
  3. Any of the quizzes from The Book stump you?
  4. Any hard learnt lessons? Or tried and true tips?
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[–] maegul@lemmy.ml 1 points 10 months ago

2. Any persistent gripes, difficulties or confusions?

I'm not entirely sure why, but the whole Double-Free issue never quite sunk in from chapter 4. It's first covered, I think here, section 4.3: Fixing an Unsafe Program: Copying vs. Moving Out of a Collection

I think it was because the description of the issue kinda conflated ownership and the presence or absence of the Copy trait, which isn't covered until way after chapter 4. Additionally, it seems that the issue mechanically comes down to whether the value of a variable is actually a pointer to a heap allocation or not (??)

It was also a behaviour/issue that tripped me up in a later quiz, in an ownership recap quiz in chapter 6 where I didn't pick it up correctly.

Here's the first quiz question that touches on it (see Q2 in The Book here, by scrolling down).

Which of the following best describes the undefined behavior that could occur if this program were allowed to execute?

let s = String::from("Hello world");
let s_ref = &s;
let s2 = *s_ref;
println!("{s2}");

For those not clear, the issue, if this code were permitted to execute, is that s2 would be a pointer to the same String that s points too. Which means that when deallocations occur as the scope ends, both s and s2 would be deallocated, as well as their corresponding memory allocations on the heap. The second such deallocation would then be of undefined content.

I find this simple enough, but I feel like the issue can catch me whenever the code or syntax obscures that a pointer would be copied, not some other value, like in the re-cap quiz in chapter 6 that I got wrong and linked above.