## Keyboard shortcuts

Press `←` or `→` to navigate between chapters

Press `S` or `/` to search in the book

Press `?` to show this help

Press `Esc` to hide this help

# Error code E0261

An undeclared lifetime was used.

Erroneous code example:

    #![allow(unused)]
    fn main() {
    // error, use of undeclared lifetime name `'a`
    fn foo(x: &'a str) { }

    struct Foo {
        // error, use of undeclared lifetime name `'a`
        x: &'a str,
    }
    }

These can be fixed by declaring lifetime parameters:

    #![allow(unused)]
    fn main() {
    struct Foo<'a> {
        x: &'a str,
    }

    fn foo<'a>(x: &'a str) {}
    }

Impl blocks declare lifetime parameters separately. You need to add lifetime parameters to an impl block if you're implementing a type that has a lifetime parameter of its own. For example:

    #![allow(unused)]
    fn main() {
    struct Foo<'a> {
        x: &'a str,
    }

    // error,  use of undeclared lifetime name `'a`
    impl Foo<'a> {
        fn foo<'a>(x: &'a str) {}
    }
    }

This is fixed by declaring the impl block like this:

    #![allow(unused)]
    fn main() {
    struct Foo<'a> {
        x: &'a str,
    }

    // correct
    impl<'a> Foo<'a> {
        fn foo(x: &'a str) {}
    }
    }