the first block of section 11.3.4 starts as follows :
// An Excerpt struct holding a reference to a part of a string ('str').
// The lifetime parameter 'a is declared on the struct name.
struct Excerpt<'a> {
// The 'part' field holds a reference tied to the lifetime 'a.
// This means the data referenced by 'part' must live at least as long as 'a.
part: &'a str,
}
// When implementing methods for a struct with lifetimes, declare them after 'impl'.
impl<'a> Excerpt<'a> {
// Method returning the held reference.
// Lifetime elision rule #3 applies because of '&self'.
// The return type implicitly gets the lifetime of '&self', which is 'a.
fn get_part(&self) -> &str { // Implicitly -> &'a str
self.part
}
}the explanation on get_part is incorrect.
it is correct that elision rule #3 applies, but that does not result in 'a at all.
the full explicit signature would be
fn get_part<'b>(self : &'b Excerpt<'a>) -> &'b str {
self.part
}in fact, writing
fn get_part_long(&self) -> &'a str {
self.part
}
// i.e. :
fn get_part_long<'b>(self : &'b Excerpt<'a>) -> &'a str {
self.part
}instead is possible (here part is simply copied), and leads to very different behavior.
fn main() {
let novel = String::from("Call me Ishmael. Some years ago...");
let mut i = Excerpt { part: &novel };
let part = i.get_part_long(); // fails to compile with `get_part`,
drop(i);
println!("Excerpt part: {part }");
}this is why i would always recommend to write get_part_long over get_part (as it is a strictly superior version) , unless you have a good reason not to. this of course only applies to immutable references which may be copied.
as to how to fix it, i'm not sure if the full explanation that i gave would be useful, but at leas saying that the lifetime of self and 'a are different would t be an improvement.
the first block of section 11.3.4 starts as follows :
the explanation on
get_partis incorrect.it is correct that elision rule
#3applies, but that does not result in'aat all.the full explicit signature would be
in fact, writing
instead is possible (here
partis simply copied), and leads to very different behavior.this is why i would always recommend to write
get_part_longoverget_part(as it is a strictly superior version) , unless you have a good reason not to. this of course only applies to immutable references which may be copied.as to how to fix it, i'm not sure if the full explanation that i gave would be useful, but at leas saying that the lifetime of
selfand'aare different would t be an improvement.