Bounds
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:
T: 'a
: All references in T
must outlive lifetime 'a
.
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);
}