Smart Pointers Basics

8 examples - 5 basic, 3 intermediate - for smart pointer types.

Prerequisites

Ownership Basics

Basic Examples

1. `Box<T>` Heap Allocation

fn main() {
    let b = Box::new(42);
    println!("{}", *b);
}

Single owner heap pointer.

2. `Rc<T>` Shared Ownership

use std::rc::Rc;
let a = Rc::new(String::from("shared"));
let b = Rc::clone(&a);

Reference counting - single thread.

3. `Arc<T>` Thread-Safe Shared

use std::sync::Arc;
let a = Arc::new(vec![1, 2]);

Send + Sync atomic refcount.

4. `RefCell<T>` Interior Mutability

use std::cell::RefCell;
let c = RefCell::new(0);
*c.borrow_mut() += 1;

Runtime borrow check - panics on violation.

5. `Cow<'a, T>` Clone-on-Write

use std::borrow::Cow;
let s: Cow<str> = Cow::Borrowed("hello");

Borrow until mutation forces owned copy.

Intermediate Examples

6. `Rc<RefCell<T>>`

use std::rc::{Rc, RefCell};
let v = Rc::new(RefCell::new(vec![1]));
v.borrow_mut().push(2);

Graphs in single-threaded code.

7. `Weak<T>` Breaking Cycles

use std::rc::{Rc, Weak};
// upgrade() returns Option<Rc<T>>

8. Custom `Deref`

use std::ops::Deref;
struct MyBox<T>(T);
impl<T> Deref for MyBox<T> {
    type Target = T;
    fn deref(&self) -> &T { &self.0 }
}

Related: Smart Pointers 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+.

Prerequisites

Ownership Basics

Basic Examples

1. `Box<T>` Heap Allocation

fn main() {
    let b = Box::new(42);
    println!("{}", *b);
}

Single owner heap pointer.

2. `Rc<T>` Shared Ownership

use std::rc::Rc;
let a = Rc::new(String::from("shared"));
let b = Rc::clone(&a);

Reference counting - single thread.

3. `Arc<T>` Thread-Safe Shared

use std::sync::Arc;
let a = Arc::new(vec![1, 2]);

Send + Sync atomic refcount.

4. `RefCell<T>` Interior Mutability

use std::cell::RefCell;
let c = RefCell::new(0);
*c.borrow_mut() += 1;

Runtime borrow check - panics on violation.

5. `Cow<'a, T>` Clone-on-Write

use std::borrow::Cow;
let s: Cow<str> = Cow::Borrowed("hello");

Borrow until mutation forces owned copy.

Intermediate Examples

6. `Rc<RefCell<T>>`

use std::rc::{Rc, RefCell};
let v = Rc::new(RefCell::new(vec![1]));
v.borrow_mut().push(2);

Graphs in single-threaded code.

7. `Weak<T>` Breaking Cycles

use std::rc::{Rc, Weak};
// upgrade() returns Option<Rc<T>>

8. Custom `Deref`

use std::ops::Deref;
struct MyBox<T>(T);
impl<T> Deref for MyBox<T> {
    type Target = T;
    fn deref(&self) -> &T { &self.0 }
}

Related: Smart Pointers 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+.

Smart Pointers Basics

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Smart Pointers Basics