lifetimes are named regions of code that a reference must be valid for references are the ones that have lifetimes
fn main() {
let r;
{
let x = 5;
r = &x; <!--- lifetime of r starts here
}
println!("r: {}", r); <!--- lifetime of r ends here
} The above code is not valid because r crosses the curly braces to where x is not valid
fn main() {
let r;
{
let x = 5;
r = &x; <!--- lifetime of r starts here and ends here since
}
}
The above code is valid , look at the lifetime of r
// this code sample does *not* compile
fn f(s: &str, t: &str) -> &str {
if s.len() > 5 { s } else { t }
}In the above, We know that the returned reference must be one of the references we received as an input argument, but we don’t know which one.
fn f<'a>(s: &'a str, t: &'a str) -> &'a str {
if s.len() > 5 { s } else { t }
}The way to achieve this is to give both input parameters the same lifetime annotation. It’s how we tell the compiler that as long as both of these input parameters are valid, so is the returned value.
fn f<'a, 'b>(s: &'a str, _t: &'b str) -> &'a str {
s
}If you know what you are doing, you can do the above.