Lifetimes

This is a short note of this book

Lifetime

✅ borrowed value does live long enough.

fn main() {
    let my_money;

    // This is fine.
    let your_money = 5;
    my_money = your_money;

    println!("my_money: {}", my_money);
    println!("your_money: {}", your_money);
}

❌ borrowed value does not live long enough.

fn main() {
    let my_money;

    // 😱 This is not, because `your_money` is in your { } scope.
    {
        let your_money = 5;
        my_money = &your_money;
    }
    // 👆 `your_money` dropped here, it won't leave your { } scope.

    // borrow later used here 👇.
    println!("my_money: {}", my_money);
}

Lifetime Annotations

Lifetime Annotations in Function Signatures

fn main() {
    // Actually we need 'a 👇 lifetime annotations. 😱
    fn hello_with_lifetime<'a>(x: &'a str) -> &'a str {
        x
    }

    // Or this... 😱
    fn hello_with_any_lifetime(x: &'_ str) -> &'_ str {
        x
    }

    // Good news, we can do this instead (Thanks to compiler!) 🙏
    fn hello_str(x: &str) -> &str {
        x
    }

    // Anyway for multiple params, we not sure how long lifetime each one.
    // So this 👇 and  👇 also here 👇 and here 👇 will need. 😅
    fn longest<'a>(x: &'a str, y: &'a str) -> &'a str {
        // This └───────┴───────────┴───────────┘ have same lifetime
        // which defined as 'a (can be any e.g. 'foo, 'lol).

        if x.len() > y.len() {
            x // Maybe return this
        } else {
            y // Maybe return this
        }
    }

    println!("1️⃣ {:?}", hello_with_lifetime("world"));
    println!("2️⃣ {:?}", hello_with_any_lifetime("world"));
    println!("3️⃣ {:?}", hello_str("world"));

    let string1 = String::from("long string is long");

    {
        let string2 = String::from("xyz");
        let result = longest(string1.as_str(), string2.as_str());
        println!("3️⃣ The longest string is {}", result);
    }
}

Now we go deeper with outlive where clause.

fn main() {
    // Return 'a lifetime.
    fn longest_a<'a, 'b>(x: &'a str, y: &'b str) -> &'a str
    // where clause should look like this.
    where 'b: 'a, // 'b must outlive the lifetime of 👆 'a
    {
        if x.len() > y.len() {
            x // 'a
        } else {
            y // 'b
        }
    }

    println!("The longest string is {}", longest_a("foooooo", "bar"));
    println!("The longest string is {}", longest_a("foo", "barrrrr"));

    // Return 'b lifetime.
    fn longest_b<'a, 'b>(x: &'a str, y: &'b str) -> &'b str
    // where clause should look like this.
    where 'a: 'b, // 'a must outlive the lifetime of 👆 'b
    {
        if x.len() > y.len() {
            x // 'a
        } else {
            y // 'b
        }
    }

    println!("The longest string is {}", longest_b("foooooo", "bar"));
    println!("The longest string is {}", longest_b("foo", "barrrrr"));
}

And deeper!

Lifetime Annotations in Method Definitions

// We need👇 <'a> here.
struct Me<'a> {
    name: &'a str, // Because of this 'a.
    // Mean 👆 this str name is have a good life in this { } scope.
}

// So 👇 we will need <'a> here too when we impl! 🤷
impl<'a> Me<'a> {
    // Due to👆 this.
    fn say_my_name(&self) -> &str {
        self.name
    }
}

// But this don't.
struct You {
    name: String, // Because of no 'a here, why?
    // Because 👆 String, Vec, Box allocated on heap. Thanks heap!
}

// So this no need <'a>.
impl You {
    fn say_my_name(&self) -> String {
        self.name.to_owned()
    }
}

// And this also don't need <'a>
struct Cat {
    name: &'static str, // Because it's a long life static.
}

// So this no need <'a>.
impl Cat {
    fn say_my_name(&self) -> &str {
        self.name
    }
}

fn main() {
    // Say my name
    println!("{:?}", Me { name: "foo" }.say_my_name());

    // To &str → String You have to add 👇 to_owned.
    println!("{:?}", You { name: "bar".to_owned() }.say_my_name());

    // Say my name 🎵
    println!("{:?}", Cat { name: "baz" }.say_my_name());
}

Recap

• &'static str = lives the entire lifetime of your program = like book hotel for entire year = use it wisely.
• String = on heap.
• &'a str = named (as a) lifetime annotations = more specific lifetime scope = good (but noisy).
• to_owned() = more generic, can be any type.
• to_string() = more specific that we need String.

Consider read more about Common Rust Lifetime Misconceptions if you plan to use it properly.

Don't be surprise if this seem complicated at first, try start with why which will explain you again from error perspective.