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Annotation of lifetimes in structures are also similar to functions:
// A type `Borrowed` which houses a reference to an
// `i32`. The reference to `i32` must outlive `Borrowed`.
#[derive(Debug)]
struct Borrowed<'a>(&'a i32);
// Similarly, both references here must outlive this structure.
#[derive(Debug)]
struct NamedBorrowed<'a> {
x: &'a i32,
y: &'a i32,
}
// An enum which is either an `i32` or a reference to one.
#[derive(Debug)]
enum Either<'a> {
Num(i32),
Ref(&'a i32),
}
fn main() {
let x = 18;
let y = 15;
let single = Borrowed(&x);
let double = NamedBorrowed { x: &x, y: &y };
let reference = Either::Ref(&x);
let number = Either::Num(y);
println!("x is borrowed in {:?}", single);
println!("x and y are borrowed in {:?}", double);
println!("x is borrowed in {:?}", reference);
println!("y is *not* borrowed in {:?}", number);
}
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Annotation of lifetimes in trait methods basically are similar to functions. Note that impl may have annotation of lifetimes too.
// A struct with annotation of lifetimes.
#[derive(Debug)]
struct Borrowed<'a> {
x: &'a i32,
}
// Annotate lifetimes to impl.
impl<'a> Default for Borrowed<'a> {
fn default() -> Self {
Self {
x: &10,
}
}
}
fn main() {
let b: Borrowed = Default::default();
println!("b is {:?}", b);
}
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Just like generic types can be bounded, lifetimes (themselves generic) use bounds as well. The : character has a slightly different meaning here, but + is the same. Note how the following read:
1. T: 'a: All references in T must outlive lifetime 'a.
2. T: Trait + 'a: Type T must implement trait Trait and all references in T must outlive 'a.
The example below shows the above syntax in action used after keyword where:
use std::fmt::Debug; // Trait to bound with.
#[derive(Debug)]
struct Ref<'a, T: 'a>(&'a T);
// `Ref` contains a reference to a generic type `T` that has
// an unknown lifetime `'a`. `T` is bounded such that any
// *references* in `T` must outlive `'a`. Additionally, the lifetime
// of `Ref` may not exceed `'a`.
// A generic function which prints using the `Debug` trait.
fn print<T>(t: T) where
T: Debug {
println!("`print`: t is {:?}", t);
}
// Here a reference to `T` is taken where `T` implements
// `Debug` and all *references* in `T` outlive `'a`. In
// addition, `'a` must outlive the function.
fn print_ref<'a, T>(t: &'a T) where
T: Debug + 'a {
println!("`print_ref`: t is {:?}", t);
}
fn main() {
let x = 7;
let ref_x = Ref(&x);
print_ref(&ref_x);
print(ref_x);
}
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