I want to write a function that receives a &str argument and returns Option<&str>. I wrote this:
fn f(text: &str) -> Option<&str> {
if // some condition {
return None;
}
let mut res = String::new();
// write something into res.
// return res
Some(&res[..])
}
but I get an error:
resdoes not live long enough.
What is the best solution to fix this?
You cannot return a &str that points into a local String variable. Doing that would mean that you just returned a dangling pointer, as the String (res, in your code) is destroyed when the function returns.
The best way to fix your problem is probably to return a Option<String>, i.e. return the owned string instead of a string slice. Adapting your code, you might end up with something like this:
fn f(text: &str) -> Option<String> {
if // some condition {
return None;
}
let mut res = String::new();
// write something into res.
Some(res)
}
