Ownership with smart pointers in Rust - Mini Project: Build & Apply

Ownership with smart pointers

📖 Scenario: You are building a simple program to manage a book's title using Rust's smart pointers. This will help you understand how ownership works with smart pointers in Rust.

🎯 Goal: Create a Rust program that uses a Box smart pointer to own a book title string, then change the title using ownership rules, and finally print the updated title.

📋 What You'll Learn

Create a Box smart pointer to hold a book title string

Create a mutable variable to hold the boxed title

Change the title by assigning a new boxed string

Print the updated book title

💡 Why This Matters

🌍 Real World

Smart pointers like <code>Box</code> help manage memory safely in Rust programs, especially when working with data on the heap.

💼 Career

Understanding ownership and smart pointers is essential for Rust developers to write safe and efficient code.

1

Create a boxed book title

Create a variable called book_title that uses Box::new to hold the string "Rust Programming".

Rust

# Your code here

Hint

Use Box::new to create a smart pointer that owns the string.

2

Make the boxed title mutable

Change the variable book_title to be mutable by adding mut so you can change its value later.

Rust

let mut book_title = Box::new("Rust Programming");
# Your code here

Hint

Add mut before the variable name to make it mutable.

3

Change the boxed book title

Assign a new boxed string "Advanced Rust" to the mutable variable book_title.

Rust

let mut book_title = Box::new("Rust Programming");
book_title = Box::new("Advanced Rust");  # Your code here

Hint

Assign a new Box::new string to book_title.

4

Print the updated book title

Use println! to print the value inside the book_title smart pointer. Use *book_title to dereference it.

Rust

let mut book_title = Box::new("Rust Programming");
book_title = Box::new("Advanced Rust");
println!("{}", *book_title);  # Your code here

Hint

Use println!("{}", *book_title); to print the string inside the box.

Practice

1. Which smart pointer in Rust allows multiple parts of your program to own the same data safely?

easy

A. RefCell

B. Box

C. Rc

D. Arc

Solution

1. Step 1: Understand ownership types

   Box provides unique ownership, Rc allows shared ownership in single-threaded contexts, RefCell allows interior mutability, and Arc is for shared ownership across threads.

2. Step 2: Identify shared ownership in single-threaded code

   Rc is the smart pointer designed for multiple owners in single-threaded programs.

3. Final Answer:

   Rc -> Option C

4. Quick Check:

   Shared ownership = Rc [OK]

Hint: Shared ownership in single-threaded Rust uses Rc [OK]

Common Mistakes:

• Confusing Box with shared ownership
• Thinking RefCell shares ownership
• Mixing Arc with single-threaded Rc

2. Which of the following is the correct way to create a Box smart pointer holding the integer 10?

easy

A. let b = Box::new(10);

B. let b = Box(10);

C. let b = new Box(10);

D. let b = Box::create(10);

Solution

1. Step 1: Recall Box creation syntax

   The correct way to create a Box is using Box::new(value).

2. Step 2: Check each option

   let b = Box::new(10); uses Box::new(10), which is correct. Options A, C, and D use invalid syntax.

3. Final Answer:

   let b = Box::new(10); -> Option A

4. Quick Check:

   Box creation = Box::new(value) [OK]

Hint: Use Box::new(value) to create a Box smart pointer [OK]

Common Mistakes:

• Using Box(10) instead of Box::new(10)
• Trying new Box(10) like other languages
• Using non-existent Box::create method

3. What will be the output of this Rust code?

use std::rc::Rc;

let a = Rc::new(5);
let b = Rc::clone(&a);
println!("{}", Rc::strong_count(&a));

medium

A. 1

B. 2

C. 5

D. Compilation error

Solution

1. Step 1: Understand Rc cloning

   Cloning an Rc pointer increases the reference count but does not clone the data.

2. Step 2: Count strong references

   Initially, a has a count of 1. After b = Rc::clone(&a), the count becomes 2.

3. Final Answer:

   2 -> Option B

4. Quick Check:

   Rc clone increments count = 2 [OK]

Hint: Rc::clone increases strong_count by 1 [OK]

Common Mistakes:

• Thinking Rc::clone copies data
• Expecting count to remain 1
• Confusing Rc with Box ownership

4. Identify the error in this Rust code using RefCell:

use std::cell::RefCell;

let data = RefCell::new(10);
let r1 = data.borrow_mut();
let r2 = data.borrow_mut();
println!("{} {}", r1, r2);

medium

A. Cannot borrow mutably twice at the same time

B. RefCell cannot hold integers

C. Missing semicolon after borrow_mut()

D. Cannot print RefCell values directly

Solution

1. Step 1: Understand RefCell borrowing rules

   RefCell allows mutable borrowing at runtime but only one mutable borrow at a time.

2. Step 2: Analyze the code

   The code tries to create two mutable borrows r1 and r2 simultaneously, which causes a runtime panic.

3. Final Answer:

   Cannot borrow mutably twice at the same time -> Option A

4. Quick Check:

   RefCell mutable borrow rule = one at a time [OK]

Hint: Only one mutable borrow allowed with RefCell at once [OK]

Common Mistakes:

• Thinking RefCell forbids integers
• Assuming multiple mutable borrows are allowed
• Confusing compile-time and runtime borrow checks

5. You want to create a tree structure where each node can have multiple children and each child can access its parent. Which combination of smart pointers is best to avoid ownership errors?

hard

A. Use Weak for children and Box for parent references

B. Use Box for both children and parent references

C. Use RefCell for children and Rc for parent references

D. Use Rc for children and Weak for parent references

Solution

1. Step 1: Understand ownership in tree structures

   Children own their data, so Rc is used for shared ownership of children nodes.

2. Step 2: Avoid reference cycles

   Parents are referenced by children, but to avoid cycles, parents should be referenced with Weak pointers.

3. Step 3: Combine smart pointers correctly

   Using Rc for children and Weak for parents prevents memory leaks and ownership errors.

4. Final Answer:

   Use Rc for children and Weak for parent references -> Option D

5. Quick Check:

   Tree ownership = Rc children + Weak parents [OK]

Hint: Use Weak for back references to avoid cycles [OK]

Common Mistakes:

• Using Box for parent causes ownership conflicts
• Using Rc for parents causes reference cycles
• Ignoring Weak pointers to break cycles