I am still being bamboozled by the borrow checker. Clearly I am missing something...
Given two similar programmes one that uses unwrap to access a value in a Option and one that uses match. The one that uses match has no problems whereas unwrap does.
I thought that both cases are equivalent, what am I missing?
I get a borrow checker error with this version:
use std::collections::HashMap;
struct S {
p:Option<HashMap<String, usize>>,
}
impl S {
#[allow(dead_code)]
fn cnt(&self) {
let c = &self.p;
let c = c.unwrap();
let _c = c.keys().count();
}
}
fn main() {
let _s = S{p:Some(HashMap::new())};
}
10 | let c = c.unwrap();
| ^ cannot move out of borrowed content
This one compiles
use std::collections::HashMap;
struct S {
p:Option<HashMap<String, usize>>,
}
impl S {
#[allow(dead_code)]
fn cnt(&self) {
match &self.p {
Some(c) => {
let _c = c.keys().count();
},
None => (),
}
}
}
fn main() {
let _s = S{p:Some(HashMap::new())};
}
unwrap() takes self which means it wants to move/consume it, but you have only a borrow of the Option field. as_ref() gives you an owned Option that has a ref to the value inside, and you can unwrap that.
The match statement is using match ergonomics and the Some arm is essentially:
Ok. I see that I only have a borrow of the option field (the &self in the signature of cnt). I do not see why unwrap needs to take ownership. On the LHS c is immutable so why does it not do a immutable borrow? What prevents it doing that? What is the reason for it?
I am happy, I learnt of Option::as_ref and I see what it does, unwrap returns what ever is in the option transferring ownership. Which is OK with a immutable reference but not a value. But I am unhappy because I do not see why.
c is &Option<HashMap<...>>. The signature of unwrap takes self, as mentioned. The dot operator, such as when calling a method, automatically dereferences through the reference. So you end up trying to consume/move a borrowed Option.
The learning curve for Rust I find, is shallow but very long! Or should that be short but very steep? Taking me a long time to get my head around a few concepts. This helped.
Try asking more questions here - someone might be able to phrase/formulate an answer that sits better with you than whatever other study material you’re using.
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