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Rust Ownership: 50 Code Examples

Try them in the Rust Playground

Changelog:
2 Jan 2023 — Use Medium’s new code block for syntax highlighting

Running into ownership problems like borrowing a value that has already been moved? If you are, then I’ve got 50 code snippets (fine, there are 53) for us to practise together in the Rust Playground.

This article assumes you roughly know the basic types like String, integer types like i32, Vec, iterators, slices and structs. I assume you have read the The Rust Programming Language (esp. Chapter 4: Understanding Ownership) and Rust by Example books.

The examples start with simple types (starting with String) and move to slightly more ‘complex’ ones. ✅ means the code compiles and ❌ means it can’t. While some examples suggest alternatives to the code that cannot compile, note that the options are not exhaustive and meant for beginners. The examples do not include mutability (because I feel that they’re easier to grasp) and do not deal with ownership asynchronous programming.

Contents

1. String
2. i32
3. Struct with String field
4. Struct with u8 field
5. Struct with two String fields
6. Vec<String>
7. Iterator
8. &str

1. String

A String is a type that does not implement the Copy trait.

✅ Listing 1–1

Let’s create a String then call do_something with it.

Everything looks normal…

fn main() {
    let name = String::from("Rust");
    do_something(name);
}

fn do_something(name: String) {
    println!("Hello, {}!", name);
}

Hello, Rust!

Playground

❌ Listing 1–2

Let’s add a println! statement in Line 4…

fn main() {
    let name = String::from("Rust");
    do_something(name);
    println!("{}", name);
}

fn do_something(name: String) {
    println!("Hello, {}!", name);
}

error[E0382]: borrow of moved value: `name`…