Summary - Lifetimes

Write a function that returns the longest text using lifetime parameters.

Lifetimes summary

A lifetime is the region where a reference is valid; the compiler checks that a reference doesn't outlive its value. In most cases the three elision rules infer lifetimes. If several references go in and a reference comes out, the relationship is written explicitly with <'a>. Structs holding references take the form struct S<'a>. 'static is valid until the end of the program. A common fix for lifetime problems is returning an owned value.

Returning a piece of the text

Since there is a single reference parameter, the function below doesn't need a lifetime; but writing it explicitly is allowed too and improves readability by documenting the relationship:

fn en_uzun_kelime<'a>(metin: &'a str) -> &'a str {
    metin
        .split_whitespace()
        .max_by_key(|k| k.chars().count())
        .unwrap_or("")
}

The returned word is a slice of the source; nothing is copied.

Exercise Longest word

Write the en_uzun_kelime function above. Print the result for "Rust güvenli ve hızlı bir dildir".

Show example solution

fn en_uzun_kelime<'a>(metin: &'a str) -> &'a str {
    metin
        .split_whitespace()
        .max_by_key(|k| k.chars().count())
        .unwrap_or("")
}

fn main() {
    println!("{}", en_uzun_kelime("Rust güvenli ve hızlı bir dildir"));
}

Test yourself

Which element does <code>max_by_key</code> return when keys are equal?
- None
- The last one
- The first one
- A random one
Show answer

The last one — According to the docs, max_by_key returns the last element on ties and min_by_key the first.

Largest by key

metin.split_whitespace().____(|k| k.chars().count())

Show answer

metin.split_whitespace().max_by_key(|k| k.chars().count())

Group summary

'a

Lifetime parameter
'static

Whole program
Elision

Automatic inference
S<'a>

Struct holding a reference

Next lesson: Trait bound

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