Ownership Basics

10 examples to get you started with ownership - 7 basic and 3 intermediate.

Prerequisites

• Rust Basics - variables and functions
• Ownership Preview - first look at moves

Basic Examples

1. Single Owner Rule

fn main() {
    let s = String::from("owner");
    consume(s);
    // println!("{s}"); // error: moved
}
 
fn consume(s: String) {
    println!("{s}");
}

• String owns heap bytes
• Passing s to consume moves ownership
• After a move, the original binding cannot be used
• consume drops s at end of scope

Related: Move Semantics & Copy - Copy types

2. Scope and Drop

fn main() {
    {
        let s = String::from("temp");
        println!("{s}");
    } // drop runs here
    println!("done");
}

• Owners go out of scope at end of block
• Drop::drop frees resources (heap for String)
• No GC - deterministic cleanup
• Order: last created, first dropped (stack unwind)

Related: Smart Pointers Basics - custom Drop

3. Move Between Variables

fn main() {
    let a = vec![1, 2, 3];
    let b = a;
    println!("{:?}", b);
}

• Vec is not Copy
• Assignment moves, not bitwise duplicate
• Only one binding owns the allocation
• Cheap move: copies pointer/length/capacity on stack

4. Function Return Transfers Ownership

fn make_greeting() -> String {
    String::from("hello")
}
 
fn main() {
    let g = make_greeting();
    println!("{g}");
}

• Caller becomes owner of returned value
• Return is a move (or NRVO-elided copy of stack value)
• Ownership can flow in and out of functions
• Return String when caller should own data

5. Partial Move from Structs

struct Pair { a: String, b: String }
 
fn main() {
    let p = Pair { a: "x".into(), b: "y".into() };
    let a = p.a; // partial move
    // println!("{}", p.b); // OK
    println!("{a}");
}

• Moving one field does not move the whole struct if other fields remain
• Entire struct is moved if all fields moved or struct is Copy
• Design structs to minimize partial-move confusion
• Prefer borrowing fields when possible

6. Borrowing Avoids Move

fn len(s: &String) -> usize { s.len() }
 
fn main() {
    let s = String::from("rust");
    println!("{}", len(&s));
    println!("still own: {s}");
}

• &String borrows without taking ownership
• Owner remains valid after borrow ends
• Many read-only APIs should take references
• See Borrowing section for &mut

Related: Borrowing & References

7. Ownership with Vec Growth

fn main() {
    let mut v = Vec::new();
    v.push(1);
    v.push(2);
    let last = v.pop().unwrap();
    println!("{last}, len={}", v.len());
}

• Vec owns a heap buffer
• push may reallocate and move elements
• pop removes ownership of last element to caller
• Iterators can borrow elements without moving the Vec

Intermediate Examples

8. Custom Drop Side Effects

struct LogOnDrop(&'static str);
 
impl Drop for LogOnDrop {
    fn drop(&mut self) {
        println!("dropping {}", self.0);
    }
}
 
fn main() {
    let _x = LogOnDrop("resource");
}

• Implement Drop for cleanup beyond memory
• drop called automatically, not recursively by value
• std::mem::drop(x) runs early drop explicitly
• Do not call Drop::drop manually on values

9. Rc for Shared Ownership (Preview)

use std::rc::Rc;
 
fn main() {
    let a = Rc::new(String::from("shared"));
    let b = Rc::clone(&a);
    println!("refs={}", Rc::strong_count(&a));
    drop(b);
    println!("{a}");
}

• Rc counts owners in single-threaded code
• Rc::clone increments count, not deep clone of string
• Value dropped when count hits zero
• Arc is the thread-safe variant

Related: Rc & Arc

10. Ownership in Collections

fn main() {
    let mut map = std::collections::HashMap::new();
    map.insert("key".to_string(), vec![1, 2]);
    if let Some(v) = map.remove("key") {
        println!("{:?}", v);
    }
}

• HashMap owns keys and values
• insert moves value in
• remove moves value out (leaves absent key)
• Borrow with get for read-only lookup

Related: Ownership Best Practices

Stack versions: This page was written for Rust 1.97.0 (edition 2024), Tokio 1.x, Axum 0.8, serde 1.0, sqlx 0.8, clap 4, and Polars 0.46+.